Chapter 15: Enteric Gram-Negative Rods (Enterobacteriaceae)

The Enterobacteriaceae are a large, heterogeneous group of gram-negative rods whose natural habitat is the intestinal tract of humans and animals. The family includes many genera (Escherichia, Shigella, Salmonella, Enterobacter, Klebsiella, Serratia, Proteus, and others). Some enteric organisms, such as Escherichia coli, are part of the normal microbiota and incidentally cause disease, but others, the salmonellae and shigellae, are regularly pathogenic for humans. The Enterobacteriaceae are facultative anaerobes or aerobes, ferment a wide range of carbohydrates, possess a complex antigenic structure, and produce a variety of toxins and other virulence factors. Enterobacteriaceae, enteric gram-negative rods, and enteric bacteria are the terms used in this chapter, but these bacteria may also be called coliforms.

The Enterobacteriaceae are the most common group of gram-negative rods cultured in clinical laboratories and along with staphylococci and streptococci are among the most common bacteria that cause disease. The taxonomy of the Enterobacteriaceae is complex and rapidly changing since the introduction of techniques that measure evolutionary distance, such as nucleic acid hybridization and nucleic acid sequencing. According to the National Library of Medicine’s Internet Taxonomy database (available at http://www.ncbi.nlm.nih.gov//Taxonomy//Browser/wwwtax.cgl?id=543), 63 genera have been defined; however, the clinically significant Enterobacteriaceae comprise 20–25 species, and other species are encountered infrequently. In this chapter, taxonomic refinements will be minimized, and the names commonly used in the medical literature are generally used. A comprehensive approach to identification of the Enterobacteriaceae is presented in Chapters 33, 37, and 38 of Jorgensen et al, 2015.

Members of the family Enterobacteriaceae have the following characteristics: They are gram-negative rods, either motile with peritrichous flagella or nonmotile; grow on peptone or meat extract media without the addition of sodium chloride or other supplements; grow well on MacConkey agar; grow aerobically and anaerobically (are facultative anaerobes); ferment rather than oxidize glucose, often with gas production; are catalase positive, oxidase negative (except for Plesiomonas) and reduce nitrate to nitrite; and have a 39–59% G + C DNA content. They can be differentiated to species level by a vast array of biochemical tests. In the United States, commercially prepared kits or automated systems are used to a large extent for this purpose. However, these are largely being replaced by other methods. The implementation of matrix-assisted laser desorption ionization time of flight mass spectroscopy (MALDI-TOF MS) for identification of culture isolates is replacing the more traditional panels of biochemicals currently in use in most clinical microbiology laboratories. This new technology seems to work quite well for identification of most of the common Enterobacteriaceae encountered in clinical material except for Shigella species. This technology is unable to differentiate Shigella from E coli.

The major groups of Enterobacteriaceae are described and discussed briefly in the following paragraphs. Specific characteristics of salmonellae, shigellae, and the other medically important enteric gram-negative rods and the diseases they cause are discussed separately later in this chapter.

Morphology and Identification

A. Typical Organisms

The Enterobacteriaceae are short gram-negative rods (Figure 15-1A). Typical morphology is seen in growth on solid media in vitro, but morphology is highly variable in clinical specimens. Capsules are large and regular in Klebsiella species, less so in Enterobacter species, and uncommon in the other species.

B. Culture

E coli and most of the other enteric bacteria form circular, convex, smooth colonies with distinct edges. Enterobacter colonies are similar but somewhat more mucoid. Klebsiella colonies are large and very mucoid and tend to coalesce with prolonged incubation. The salmonellae and shigellae produce colonies similar to E coli but do not ferment lactose. Some strains of E coli produce hemolysis on blood agar.

C. Growth Characteristics

Carbohydrate fermentation patterns and the activity of amino acid decarboxylases and other enzymes are used in biochemical differentiation. Some tests, such as the production of indole from tryptophan, are commonly used in rapid identification systems, but others, such as the Voges-Proskauer reaction (production of acetylmethylcarbinol from dextrose), are used less often. Culture on “differential” media that contain special dyes and carbohydrates (eg, eosin-methylene blue [EMB], MacConkey, or deoxycholate medium) distinguishes lactose-fermenting (colored) from non–lactose-fermenting colonies (nonpigmented) and may allow rapid presumptive identification of enteric bacteria (Table 15-1).

Many complex media have been devised to help in identification of the enteric bacteria. One such medium is triple sugar iron (TSI) agar, which is often used to help differentiate salmonellae and shigellae from other enteric gram-negative rods in stool cultures. The medium contains 0.1% glucose, 1% sucrose, 1% lactose, ferrous sulfate (for detection of H₂S production), tissue extracts (protein growth substrate), and a pH indicator (phenol red). It is poured into a test tube to produce a slant with a deep butt and is inoculated by stabbing bacterial growth into the butt. If only glucose is fermented, the slant and the butt initially turn yellow from the small amount of acid produced; as the fermentation products are subsequently oxidized to CO₂ and H₂O and released from the slant and as oxidative decarboxylation of proteins continues with formation of amines, the slant turns alkaline (red). If lactose or sucrose is fermented, so much acid is produced that the slant and butt remain yellow (acid). Salmonellae and shigellae typically yield an alkaline slant and an acid butt. Although Proteus, Providencia, and Morganella species produce an alkaline slant and acid butt, they can be identified by their rapid formation of red color in Christensen’s urea medium. Organisms producing acid on the slant and acid and gas (bubbles) in the butt are other enteric bacteria.

1. Escherichia—E coli typically produces positive test results for indole, lysine decarboxylase, and mannitol fermentation and produces gas from glucose. An isolate from urine can be quickly identified as E coli by its hemolysis on blood agar, typical colonial morphology with an iridescent “sheen” on differential media such as EMB agar, and a positive spot indole test result. More than 90% of E coli isolates are positive for β-glucuronidase using the substrate 4-methylumbelliferyl-β-glucuronide (MUG). Isolates from anatomic sites other than urine, with characteristic properties (above plus negative oxidase test results) often can be confirmed as E coli with a positive MUG test result.

2. Klebsiella–Enterobacter–Serratia group—Klebsiella species exhibit mucoid growth, large polysaccharide capsules, and lack of motility, and they usually give positive test results for lysine decarboxylase and citrate. Most Enterobacter species give positive test results for motility, citrate, and ornithine decarboxylase and produce gas from glucose. Enterobacter aerogenes has small capsules. Some species of Enterobacter have been moved into the genus Cronobacter. Serratia species produces DNase, lipase, and gelatinase. Klebsiella, Enterobacter, and Serratia species usually give positive Voges-Proskauer reactions.

3. Proteus–Morganella–Providencia group—The members of this group deaminate phenylalanine, are motile, grow on potassium cyanide medium (KCN), and ferment xylose. Proteus species move very actively by means of peritrichous flagella, resulting in “swarming” on solid media unless the swarming is inhibited by chemicals, such as phenylethyl alcohol or CLED (cystine-lactose-electrolyte-deficient) medium. Whereas Proteus species and Morganella morganii are urease positive, Providencia species usually are urease negative. The Proteus–Providencia group ferments lactose very slowly or not at all.

4. Citrobacter—These bacteria typically are citrate positive and differ from the salmonellae in that they do not decarboxylate lysine. They ferment lactose very slowly if at all.

5. Shigella—Shigellae are nonmotile and usually do not ferment lactose but do ferment other carbohydrates, producing acid but not gas. They do not produce H₂S. The four Shigella species are closely related to E coli. Many share common antigens with one another and with other enteric bacteria (eg, Hafnia alvei and Plesiomonas shigelloides).

6. Salmonella—Salmonellae are motile rods that characteristically ferment glucose and mannose without producing gas but do not ferment lactose or sucrose. Most salmonellae produce H₂S. They are often pathogenic for humans or animals when ingested. Organisms originally described in the genus Arizona are included as subspecies in the Salmonella group.

7. Other Enterobacteriaceae—Yersinia species are discussed in Chapter 19. Other genera occasionally found in human infections include Cronobacter, Edwardsiella, Ewingella, Hafnia, Cedecea, Plesiomonas, and Kluyvera.

Antigenic Structure

Enterobacteriaceae have a complex antigenic structure. They are classified by more than 150 different heat-stable somatic O (lipopolysaccharide) antigens, more than 100 heat-labile K (capsular) antigens, and more than 50 H (flagellar) antigens (Figure 15-1B). In Salmonella serotype Typhi, the capsular antigens are called Vi antigens. The antigenic classification of Enterobacteriaceae often indicates the presence of each specific antigen; for example, the antigenic formula of an E coli may be O55:K5:H21.

O antigens are the most external part of the cell wall lipopolysaccharide and consist of repeating units of polysaccharide. Some O-specific polysaccharides contain unique sugars. O antigens are resistant to heat and alcohol and usually are detected by bacterial agglutination. Antibodies to O antigens are predominantly IgM.

Although each genus of Enterobacteriaceae is associated with specific O groups, a single organism may carry several O antigens. Thus, most shigellae share one or more O antigens with E coli. E coli may cross-react with some Providencia, Klebsiella, and Salmonella species. Occasionally, O antigens may be associated with specific human diseases (eg, specific O types of E coli are found in diarrhea and in urinary tract infections).

K antigens are external to O antigens on some but not all Enterobacteriaceae. Some are polysaccharides, including the K antigens of E coli; others are proteins. K antigens may interfere with agglutination by O antisera, and they may be associated with virulence (eg, E coli strains producing K1 antigen are prominent in neonatal meningitis, and K antigens of E coli cause attachment of the bacteria to epithelial cells before gastrointestinal or urinary tract invasion).

Klebsiellae form large capsules consisting of polysaccharides (K antigens) covering the somatic (O or H) antigens and can be identified by capsular swelling tests with specific antisera. Human infections of the respiratory tract are caused particularly by capsular types 1 and 2 and those of the urinary tract by types 8, 9, 10, and 24.

H antigens are located on flagella and are denatured or removed by heat or alcohol. They are preserved by treating motile bacterial variants with formalin. Such H antigens agglutinate with anti-H antibodies, mainly IgG. The determinants in H antigens are a function of the amino acid sequence in flagellar protein (flagellin). Within a single serotype, flagellar antigens may be present in either or both of two forms, called phase 1 (conventionally designated by lowercase letters) and phase 2 (conventionally designated by Arabic numerals), as shown in 15-3. The organism tends to change from one phase to the other; this is called phase variation. H antigens on the bacterial surface may interfere with agglutination by anti-O antibody.

There are many examples of overlapping antigenic structures between Enterobacteriaceae and other bacteria. Most Enterobacteriaceae share the O14 antigen of E coli. The type 2 capsular polysaccharide of Klebsiella is very similar to the polysaccharide of type 2 pneumococci. Some K antigens cross-react with capsular polysaccharides of Haemophilus influenzae or Neisseria meningitidis. Thus, E coli O75:K100:H5 can induce antibodies that react with H influenzae type b.

Toxins and Enzymes

Most gram-negative bacteria possess complex lipopolysaccharides in their cell walls. These substances, cell envelope (cytoplasmic membrane, peptidoglycan, outer membrane) endotoxins, have a variety of pathophysiologic effects that are summarized in Chapter 9. Many gram-negative enteric bacteria also produce exotoxins of clinical importance. Some specific toxins are discussed in subsequent sections.

Causative Organisms

E coli are members of the normal intestinal microbiota (see Chapter 10). Other enteric bacteria (Proteus, Enterobacter, Klebsiella, Morganella, Providencia, Citrobacter, and Serratia species) are also found as members of the normal intestinal microbiota but are considerably less common than E coli. The enteric bacteria are sometimes found in small numbers as part of the normal microbiota of the upper respiratory and genital tracts. The enteric bacteria generally do not cause disease, and in the intestine, they may even contribute to normal function and nutrition. When clinically important infections occur, they are usually caused by E coli, but the other enteric bacteria are causes of hospital-acquired infections and occasionally cause community-acquired infections. The bacteria become pathogenic only when they reach tissues outside of their normal intestinal or other less common normal microbiota sites. The most frequent sites of clinically important infection are the urinary tract, biliary tract, and other sites in the abdominal cavity, but any anatomic site (eg, bloodstream, prostate gland, lung, bone, meninges) can be the site of disease. Some of the enteric bacteria (eg, Serratia marcescens, E aerogenes) are opportunistic pathogens. When normal host defenses are inadequate—particularly in infancy or old age, in the terminal stages of other diseases, after immunosuppression, or with indwelling venous or urethral catheters—localized clinically important infections can result, and the bacteria may reach the bloodstream and cause sepsis.

Pathogenesis and Clinical Findings

The clinical manifestations of infections with E coli and the other enteric bacteria depend on the site of the infection and cannot be differentiated by symptoms or signs from processes caused by other bacteria.

A. E coli

1. Urinary tract infection—E coli is the most common cause of urinary tract infection and accounts for approximately 90% of first urinary tract infections in young women (see Chapter 48). The symptoms and signs include urinary frequency, dysuria, hematuria, and pyuria. Flank pain is associated with upper tract infection. None of these symptoms or signs is specific for E coli infection. Urinary tract infection can result in bacteremia with clinical signs of sepsis.

Most of the urinary tract infections that involve the bladder or kidney in an otherwise healthy host are caused by a small number of O antigen types that have specifically elaborated virulence factors that facilitate colonization and subsequent clinical infections. These organisms are designated as uropathogenic E coli. Typically, these organisms produce hemolysin, which is cytotoxic and facilitates tissue invasion. Strains that cause pyelonephritis express K antigen and elaborate a specific type of pilus, P fimbriae, which binds to the P blood group antigen.

Over the last decade, a pandemic clone, E coli O25b/ST131, has emerged as a significant pathogen. This organism has been successful largely as a result of its acquisition of plasmid-mediated resistance factors that encode resistance to β-lactam antibiotics (elaboration of extended spectrum β-lactamases), fluoroquinolones, and aminoglycosides (see the review by Johnson et al, 2010).

2. E coli–associated diarrheal diseases—E coli that cause diarrhea are extremely common worldwide. These E coli are classified by the characteristics of their virulence properties (see later discussion), and each group causes disease by a different mechanism—at least six of which have been characterized. The small or large bowel epithelial cell adherence properties are encoded by genes on plasmids. Similarly, the toxins often are plasmid or phage mediated. Some clinical aspects of diarrheal diseases are discussed in Chapter 48.

Enteropathogenic E coli (EPEC) are an important cause of diarrhea in infants, especially in developing countries. EPEC adhere to the mucosal cells of the small bowel. Pathogenicity requires two important factors, the bundle forming pilus encoded by a plasmid EPEC adherence factor (EAF) and the chromosomal locus of enterocyte effacement (LEE) pathogenicity island that promote the tight adherence characteristic of EPEC (attachment and effacement). After attachment, there is loss of microvilli (effacement); formation of filamentous actin pedestals or cuplike structures; and, occasionally, entry of the EPEC into the mucosal cells. Characteristic lesions can be seen on electron micrographs of small bowel biopsy lesions. The result of EPEC infection in infants is characterized by severe, watery diarrhea, vomiting, and fever, which are usually self-limited but can be prolonged or chronic. EPEC diarrhea has been associated with multiple specific serotypes of E coli; strains are identified by O antigen and occasionally by H antigen typing. A two-stage infection model using HEp-2 or HeLa cells also can be performed. Tests to identify EPEC are performed in reference laboratories. The duration of the EPEC diarrhea can be shortened and the chronic diarrhea cured by antibiotic treatment.

Enterotoxigenic E coli (ETEC) are a common cause of “traveler’s diarrhea” and a very important cause of diarrhea in children less than 5 years of age in developing countries. ETEC colonization factors (pili known as colonization factor antigens [CFAs]) specific for humans promote adherence of ETEC to epithelial cells of the small bowel. Some strains of ETEC produce a heat-labile enterotoxin (LT) (molecular weight [MW], 80,000) that is under the genetic control of a plasmid and is closely related to cholera toxin. Its subunit B attaches to the GM₁ ganglioside in the apical membrane of enterocytes and facilitates the entry of subunit A (MW, 26,000) into the cell, where the latter activates adenylyl cyclase. This markedly increases the local concentration of cyclic adenosine monophosphate (cAMP) after which ensues a complex cascade that involves the cystic fibrosis transmembrane conductance regulator. The end result is an intense and prolonged hypersecretion of water and chlorides and inhibition of the reabsorption of sodium. The gut lumen is distended with fluid, and hypermotility and diarrhea ensue, lasting for several days. LT is antigenic and cross-reacts with the enterotoxin of Vibrio cholerae, which has an identical mechanism of action. LT stimulates the production of neutralizing antibodies in the serum (and perhaps on the gut surface) of persons previously infected with enterotoxigenic E coli. Persons residing in areas where such organisms are highly prevalent (eg, in some developing countries) are likely to possess antibodies and are less prone to develop diarrhea on reexposure to the LT-producing E coli. Assays for LT include (1) fluid accumulation in the intestines of laboratory animals, (2) typical cytologic changes in cultured Chinese hamster ovary cells or other cell lines, (3) stimulation of steroid production in cultured adrenal tumor cells, (4) binding and immunologic assays with standardized antisera to LT, and (5) detection of the genes that encode the toxins. These assays are done only in reference laboratories.

Some strains of ETEC produce the heat-stable enterotoxin ST_a (MW, 1500–4000), which is under the genetic control of a heterogeneous group of plasmids. ST_a activates guanylyl cyclase in enteric epithelial cells and stimulates fluid secretion. Many ST_a-positive strains also produce LT. The strains with both toxins produce a more severe diarrhea.

The plasmids carrying the genes for enterotoxins (LT, ST) also may carry genes for the CFAs that facilitate the attachment of E coli strains to intestinal epithelium. Recognized colonization factors occur with particular frequency in some serotypes. Certain serotypes of ETEC occur worldwide; others have a limited recognized distribution. It is possible that virtually any E coli may acquire a plasmid encoding for enterotoxins. There is no definite association of ETEC with the EPEC strains causing diarrhea in children. Likewise, there is no association between enterotoxigenic strains and those able to invade intestinal epithelial cells.

Care in the selection and consumption of foods potentially contaminated with ETEC is highly recommended to help prevent traveler’s diarrhea. Antimicrobial prophylaxis can be effective but may result in increased antibiotic resistance in the bacteria and probably should not be uniformly recommended. When diarrhea develops, antibiotic treatment effectively shortens the duration of disease.

Shiga toxin-producing E coli (STEC) are named for the cytotoxic toxins they produce. There are at least two antigenic forms of the toxin referred to as Shiga-like toxin 1 and Shiga-like toxin 2. STEC has been associated with mild non-bloody diarrhea, hemorrhagic colitis, a severe form of diarrhea, and with hemolytic uremic syndrome, a disease resulting in acute renal failure, microangiopathic hemolytic anemia, and thrombocytopenia. Shiga-like toxin 1 is identical to the Shiga toxin of Shigella dysenteriae type 1, and Shiga-like toxin 2 also has many properties that are similar to the Shiga toxin; however, the two toxins are antigenically and genetically distinct. A low infectious dose (< 200 CFU) is associated with infection. Of the more than 150 E coli serotypes that produce Shiga toxin, O157:H7 is the most common and is the one that can be identified most readily in clinical specimens. STEC O157:H7 does not use sorbitol, unlike most other E coli, and is negative (clear colonies) on sorbitol MacConkey agar (sorbitol is used instead of lactose); O157:H7 strains also are negative on MUG tests (see earlier discussion). Many of the non-O157 serotypes may be sorbitol positive when grown in culture. Specific antisera are used to identify the O157:H7 strains. Tests for the detection of both Shiga toxins using commercially available enzyme immunoassays (EIAs) are done in many laboratories. Other sensitive test methods include cell culture cytotoxin testing using Vero cells and polymerase chain reaction for the direct detection of toxin genes directly from stool samples. Many cases of hemorrhagic colitis and its associated complications can be prevented by thoroughly cooking ground beef and by avoiding unpasteurized products such as apple cider. In 2011, the largest outbreak of hemorrhagic colitis attributed to a non-O157 serotype—namely, E coli O104:H4—was related to consumption of contaminated sprouts in Germany. This organism had increased virulence characterized by enhanced adherence as well as the production of shiga-like toxins (see reference by Buchholz et al, 2011).

Enteroinvasive E coli (EIEC) produce a disease very similar to shigellosis. The disease occurs most commonly in children in developing countries and in travelers to these countries. Similar to Shigella, EIEC strains are nonlactose or late lactose fermenters and are nonmotile. EIEC produce disease by invading intestinal mucosal epithelial cells.

Enteroaggregative E coli (EAEC) causes acute and chronic diarrhea (>14 days in duration) in persons in developing countries. These organisms also are the cause of foodborne illnesses in industrialized countries and have been associated with traveler’s diarrhea and persistent diarrhea in patients with HIV. They are characterized by their specific patterns of adherence to human cells. This group of diarrheagenic E coli is quite heterogeneous, and the exact pathogenic mechanisms are still not completely elucidated. Some strains of EAEC produce ST-like toxin (see earlier discussion on E coli O104:H11); others a plasmid-encoded enterotoxin that produces cellular damage; and still others, a hemolysin. Diagnosis can be suspected clinically but requires confirmation by tissue culture adhesion assays not readily available in most clinical laboratories.

3. Sepsis—When normal host defenses are inadequate, E coli may reach the bloodstream and cause sepsis. Newborns may be highly susceptible to E coli sepsis because they lack IgM antibodies. Sepsis may occur secondary to urinary tract infection and often the major clone associated with invasion is E coli O25b/ST131.

4. Meningitis—E coli and group B streptococci are the leading causes of meningitis in infants. Approximately 80% of E coli from meningitis cases have the K1 antigen. This antigen cross-reacts with the group B capsular polysaccharide of N meningitidis. The mechanisms of virulence associated with the K1 antigen are reviewed in the reference by Kim et al (2005).

B. Klebsiella–Enterobacter–Serratia; Proteus–Morganella–Providencia; and Citrobacter

The pathogenesis of disease caused by these groups of enteric gram-negative rods is similar to that of the nonspecific factors in disease caused by E coli.

1. Klebsiella—Klebsiella pneumoniae is present in the respiratory tract and feces of about 5% of normal individuals. It causes a small proportion (~1%) of bacterial pneumonias. K pneumoniae can produce extensive hemorrhagic necrotizing consolidation of the lung. It produces urinary tract infection and bacteremia with focal lesions in debilitated patients. Other enterics also may produce pneumonia. Recently a particular clone of K pneumoniae has emerged as a cause of community acquired pyogenic liver abscess that is seen mostly among Asian males worldwide. This particular K1 encapsulated strain phenotypically appears hypermucoviscous when grown in culture. Klebsiella species rank among the top 10 bacterial pathogens responsible for hospital-acquired infections. Multilocus sequencing typing has identified global emergence of two particularly important clones. Sequence type 16 has elaborated extended spectrum β-lactamases resulting in resistance to a broad range of penicillins and cephalosporins (but not carbapenem antibiotics). ST 258 is a multidrug resistant strain called a “carbapenamase producer” because it is resistant to all β-lactam antibiotics including the broad spectrum carbapenem agents. Typically it is resistant to other antimicrobial agents as a result of acquisition of plasmids that carry multiple resistance genes. Two other Klebsielleae are associated with inflammatory conditions of the upper respiratory tract: K pneumoniae subspecies ozaenae has been isolated from the nasal mucosa in ozena, a fetid, progressive atrophy of mucous membranes; and K pneumoniae subspecies rhinoscleromatis form rhinoscleroma, a destructive granuloma of the nose and pharynx. Klebsiella granulomatis (formerly Calymmatobacterium granulomatis) causes a chronic genital ulcerative disease, granuloma inguinale, an uncommon sexually transmitted disease. The organism grows with difficulty on media containing egg yolk. Ampicillin or tetracycline is effective treatment.

2. Enterobacter—Three species/complexes of Enterobacter—Enterobacter cloacae complex, E aerogenes complex, and Enterobacter sakazakii (now in the genus Cronobacter)—cause the majority of Enterobacter infections. These bacteria ferment lactose, may contain capsules that produce mucoid colonies, and are motile. These organisms cause a broad range of hospital-acquired infections such as pneumonia, urinary tract infections, and wound and device infections. Most strains possess a chromosomal β-lactamase called ampC, which renders them intrinsically resistant to ampicillin and first- and second-generation cephalosporins. Mutants may hyperproduce β-lactamase, conferring resistance to third-generation cephalosporins. Like K pneumoniae, some hospital-acquired strains have plasmids that make them multidrug resistant including the carbapenem class of antimicrobial agents.

3. Serratia—Serratia marcescens is a common opportunistic pathogen in hospitalized patients. Serratia (usually nonpigmented) causes pneumonia, bacteremia, and endocarditis (especially in narcotics addicts) and in hospitalized patients. Only about 10% of isolates form the red pigment (prodigiosin) that has long characterized S marcescens. S marcescens is often multiply resistant to aminoglycosides and penicillins; infections can be treated with third-generation cephalosporins.

4. Proteus—Proteus species produce infections in humans only when the bacteria leave the intestinal tract. They are found in urinary tract infections and produce bacteremia, pneumonia, and focal lesions in debilitated patients or those receiving contaminated intravenous infusions. P mirabilis causes urinary tract infections and occasionally other infections. Proteus vulgaris and M morganii are also important nosocomial pathogens.

Proteus species produce urease, resulting in rapid hydrolysis of urea with liberation of ammonia. Thus, in urinary tract infections with Proteus species, the urine becomes alkaline, promoting stone formation and making acidification virtually impossible. The rapid motility of Proteus may contribute to its invasion of the urinary tract.

Strains of Proteus vary greatly in antibiotic susceptibility. P mirabilis is often inhibited by penicillins; the most active antibiotics for other members of the group are aminoglycosides and cephalosporins.

5. Providencia—Providencia species (Providencia rettgeri, Providencia alcalifaciens, and Providencia stuartii) are members of the normal intestinal microbiota. All cause urinary tract infections and occasionally other infections and are often resistant to antimicrobial therapy.

6. Citrobacter—Citrobacter species can cause urinary tract infections and sepsis principally among debilitated hospitalized patients. In addition, Citrobacter koseri has been associated with meningitis in infants less than 2 months of age.

Diagnostic Laboratory Tests

A. Specimens

Specimens include urine, blood, pus, spinal fluid, sputum, or other material, as indicated by the localization of the disease process.

B. Smears

The Enterobacteriaceae resemble each other morphologically. The presence of large capsules is suggestive of Klebsiella species.

C. Culture

Specimens are plated on both blood agar and differential media. With differential media, rapid preliminary identification of gram-negative enteric bacteria is often possible (see Chapter 47).

D. Nucleic Acid Amplification Tests (NAATs)

A variety of multiplex NAATs designed to detect the most common pathogens responsible for particular syndromes, are currently available and many more are entering clinical trials. These panel tests detect members of the Enterobacteriaceae in specimens such as positive blood cultures, cerebrospinal fluid, respiratory specimens, and stool. In some of these assays resistance markers are also detected. The reader should consult the literature for the most up to date information on these assays.

Immunity

Specific antibodies develop in systemic infections, but it is uncertain whether significant immunity to the organisms follows.

Treatment

No single specific therapy is available. The sulfonamides, ampicillin, cephalosporins, fluoroquinolones, and aminoglycosides have marked antibacterial effects against the enterics, but variation in susceptibility is great, and laboratory tests for antibiotic susceptibility are essential. Multiple drug resistance is common and is under the control of transmissible plasmids.

Certain conditions predisposing to infection by these organisms require surgical correction, such as relief of urinary tract obstruction, closure of a perforation in an abdominal organ, or resection of a bronchiectatic portion of lung.

Treatment of gram-negative bacteremia and impending septic shock requires rapid institution of antimicrobial therapy, restoration of fluid and electrolyte balance, and treatment of disseminated intravascular coagulation.

Various means have been proposed for the prevention of traveler’s diarrhea, including daily ingestion of bismuth subsalicylate suspension (bismuth subsalicylate can inactivate E coli enterotoxin in vitro) and regular doses of tetracyclines or other antimicrobial drugs for limited periods. Because none of these methods are entirely successful or lacking in adverse effects, it is widely recommended that caution be observed in regard to food and drink in areas where environmental sanitation is poor and that early and brief treatment (eg, with ciprofloxacin or trimethoprim–sulfamethoxazole) be substituted for prophylaxis.

Epidemiology, Prevention, and Control

The enteric bacteria establish themselves in the normal intestinal tract within a few days after birth and from then on constitute a main portion of the normal aerobic (facultative anaerobic) microbial flora. E coli is the prototype. Enterics found in water or milk are accepted as proof of fecal contamination from sewage or other sources.

Control measures are not feasible as far as the normal endogenous microbiota is concerned. Enteropathogenic E coli serotypes should be controlled like salmonellae (see below). Some of the enterics constitute a major problem in hospital infection. It is particularly important to recognize that many enteric bacteria are “opportunists” that cause illness when they are introduced into debilitated patients. Within hospitals or other institutions, these bacteria commonly are transmitted by personnel, instruments, or parenteral medications. Their control depends on handwashing, rigorous asepsis, sterilization of equipment, disinfection, restraint in intravenous therapy, and strict precautions in keeping the urinary tract sterile (ie, closed drainage). For control of the multidrug-resistant pathogens, especially carbapenamase producers, surveillance of hospitalized patients with prompt implementation of contact precautions for colonized patients is often employed.

The natural habitat of shigellae is limited to the intestinal tracts of humans and other primates, where they produce bacillary dysentery.

Morphology and Identification

A. Typical Organisms

Shigellae are slender gram-negative rods; coccobacillary forms occur in young cultures.

B. Culture

Shigellae are facultative anaerobes but grow best aerobically. Convex, circular, transparent colonies with intact edges reach a diameter of about 2 mm in 24 hours.

C. Growth Characteristics

All shigellae ferment glucose. With the exception of Shigella sonnei, they do not ferment lactose. The inability to ferment lactose distinguishes shigellae on differential media. Shigellae form acid from carbohydrates but rarely produce gas. They may also be divided into those organisms that ferment mannitol and those that do not (Table 15-2).

Antigenic Structure

Shigellae have a complex antigenic pattern. There is great overlapping in the serologic behavior of different species, and most of them share O antigens with other enteric bacilli.

The somatic O antigens of shigellae are lipopolysaccharides. Their serologic specificity depends on the polysaccharide. There are more than 40 serotypes. The classification of shigellae relies on biochemical and antigenic characteristics. The pathogenic species are S sonnei, Shigella flexneri, S dysenteriae, and Shigella boydii (Table 15-2).

Pathogenesis and Pathology

Shigella infections are almost always limited to the gastrointestinal tract; bloodstream invasion is quite rare. Shigellae are highly communicable; the infective dose is on the order of 10³ organisms (it usually is 10⁵–10⁸ for salmonellae and vibrios). The essential pathologic process is invasion of the mucosal epithelial cells (eg, M cells) by induced phagocytosis, escape from the phagocytic vacuole, multiplication and spread within the epithelial cell cytoplasm, and passage to adjacent cells. Microabscesses in the wall of the large intestine and terminal ileum lead to necrosis of the mucous membrane, superficial ulceration, bleeding, and formation of a “pseudomembrane” on the ulcerated area. This consists of fibrin, leukocytes, cell debris, a necrotic mucous membrane, and bacteria. As the process subsides, granulation tissue fills the ulcers, and scar tissue forms.

Toxins

A. Endotoxin

Upon autolysis, all shigellae release their toxic lipopolysaccharide. This endotoxin probably contributes to the irritation of the bowel wall.

B. Shigella Dysenteriae Exotoxin

S dysenteriae type 1 (Shiga bacillus) produces a heat-labile exotoxin that affects both the gut and the central nervous system. The exotoxin is a protein that is antigenic (stimulating production of antitoxin) and lethal for experimental animals. Acting as an enterotoxin, it produces diarrhea as does the E coli Shiga-like toxin, perhaps by the same mechanism. In humans, the exotoxin also inhibits sugar and amino acid absorption in the small intestine. Acting as a “neurotoxin,” this material may contribute to the extreme severity and fatal nature of S dysenteriae infections and to the central nervous system reactions observed in them (ie, meningismus, coma). Patients with S flexneri or S sonnei infections develop antitoxin that neutralizes S dysenteriae exotoxin in vitro. The toxic activity is distinct from the invasive property of shigellae in dysentery. The two may act in sequence, the toxin producing an early nonbloody, voluminous diarrhea and the invasion of the large intestine, resulting in later dysentery with blood and pus in stools.

Clinical Findings

After a short incubation period (1–2 days), there is a sudden onset of abdominal pain, fever, and watery diarrhea. A day or so later, as the infection involves the ileum and colon, the number of stools increases; they are less liquid but often contain mucus and blood. Each bowel movement is accompanied by straining and tenesmus (rectal spasms), with resulting lower abdominal pain. In more than half of adult cases, fever and diarrhea subside spontaneously in 2–5 days. However, in children and elderly adults, loss of water and electrolytes may lead to dehydration, acidosis, and even death. The illness caused by S dysenteriae may be particularly severe.

On recovery, most persons shed dysentery bacilli for only a short period. Upon recovery from the infection, most persons develop circulating antibodies to shigellae, but these do not protect against reinfection.

Diagnostic Laboratory Tests

A. Specimens

Specimens include fresh stool, mucus flecks, and rectal swabs for culture. Large numbers of fecal leukocytes and some red blood cells often are seen microscopically.

B. Culture

The materials are streaked on differential media (eg, MacConkey or EMB agar) and on selective media (Hektoen enteric agar or xylose-lysine-deoxycholate agar), which suppress other Enterobacteriaceae and gram-positive organisms. Colorless (lactose-negative) colonies are inoculated into TSI agar. Organisms that fail to produce H₂S, that produce acid but not gas in the butt and an alkaline slant in TSI agar medium, and that are nonmotile, should be subjected to slide agglutination by specific Shigella antisera. Shigella and E coli cannot be differentiated by MALDI-ToF MS.

C. Serology

Healthy persons often have agglutinins against several Shigella species. However, serial determinations of antibody titers may show a rise in specific antibody. Serology is not used to diagnose Shigella infections.

D. Nucleic Acid Amplification Tests

There are several commercial NAATs that directly detect shigellae in fecal samples along with some of the other major enteric pathogens.

Immunity

Infection is followed by a type-specific antibody response. Injection of killed shigellae stimulates production of antibodies in serum but fails to protect humans against infection. IgA antibodies in the gut may be important in limiting reinfection; these may be stimulated by live attenuated strains given orally as experimental vaccines. Serum antibodies to somatic Shigella antigens are IgM.

Treatment

Ciprofloxacin, ampicillin, doxycycline, and trimethoprim–sulfamethoxazole are most commonly inhibitory for Shigella isolates and can suppress acute clinical attacks of dysentery and shorten the duration of symptoms. Azithromycin is often used to treat children with shigellosis. Some antibiotics may fail to eradicate the organisms from the intestinal tract. Multiple drug resistance can be transmitted by plasmids, and resistant infections are widespread. Many cases are self-limited. Opioids should be avoided in Shigella dysentery.

Epidemiology, Prevention, and Control

Shigellae are transmitted by “food, fingers, feces, and flies” from person to person. A low infectious dose (1–100 organisms) is capable of causing disease. Most cases of Shigella infection occur in children younger than 10 years of age. Shigellosis, caused primarily by S sonnei, has become an important problem in daycare centers in the United States. S dysenteriae can spread widely. Because humans are the main recognized host of pathogenic shigellae, control efforts must be directed at eliminating the organisms from this reservoir by (1) sanitary control of water, food, and milk; sewage disposal and fly control; (2) isolation of patients and disinfection of excreta; (3) detection of subclinical cases and carriers, particularly food handlers; and (4) antibiotic treatment of infected individuals.

Salmonellae are often pathogenic for humans or animals when acquired by the oral route. They are transmitted from animals and animal products to humans, where they cause enteritis, systemic infection, and enteric fever.

Morphology and Identification

Salmonellae vary in length. Most isolates are motile with peritrichous flagella. Salmonellae grow readily on simple media, but they almost never ferment lactose or sucrose. They form acid and sometimes gas from glucose and mannose. They usually produce H₂S. They survive freezing in water for long periods. Salmonellae are resistant to certain chemicals (eg, brilliant green, sodium tetrathionate, sodium deoxycholate) that inhibit other enteric bacteria; such compounds are therefore useful for inclusion in media to isolate salmonellae from feces.

Classification

The classification of salmonellae is complex because the organisms are a continuum rather than a defined species. The members of the genus Salmonella were originally classified on the basis of epidemiology; host range; biochemical reactions; and structures of the O, H, and Vi (when present) antigens. The names (eg, S typhi, Salmonella typhimurium) were written as if they were genus and species; this form of the nomenclature remains in widespread but incorrect use. DNA–DNA hybridization studies have demonstrated that there are seven evolutionary groups. Currently, the genus Salmonella is divided into two species each with multiple subspecies and serotypes. The two species are Salmonella enterica and Salmonella bongori (formerly subspecies V). S enterica contains five subspecies, which are subspecies enterica (subspecies I), subspecies salamae (subspecies II), subspecies arizonae (subspecies IIIa), subspecies diarizonae (subspecies IIIb), subspecies houtenae (subspecies IV), and subspecies indica (subspecies VI). Most human illness is caused by the subspecies I strains, written as S enterica subspecies enterica. Rarely human infections may be caused by subspecies IIIa and IIIb or the other subspecies frequently found in cold-blooded animals. Frequently, these infections are associated with exotic pets such as reptiles. It seems probable that the widely accepted nomenclature for classification will be as follows: S enterica subspecies enterica serotype Typhimurium, which can be shortened to S Typhimurium with the genus name in italics and the serotype name in roman type. National and international reference laboratories may use the antigenic formulas following the subspecies name because they impart more precise information about the isolates (see Table 15-3).

There are more than 2500 serotypes of salmonellae, including more than 1400 in DNA hybridization group I that can infect humans. Four serotypes of salmonellae that cause enteric fever can be identified in the clinical laboratory by biochemical and serologic tests. These serotypes should be routinely identified because of their clinical significance. They are as follows: Salmonella paratyphi A (serogroup A), S paratyphi B (serogroup B), Salmonella choleraesuis (serogroup C1), and S typhi (serogroup D). Salmonella serotypes Enteritidis and Typhimurium are the two most common serotypes reported in the United States. The more than 1400 other salmonellae that are isolated in clinical laboratories are serogrouped by their O antigens as A, B, C₁, C₂, D, and E; some are nontypeable with this set of antisera. The isolates are then sent to reference laboratories for definitive serologic identification. This allows public health officials to monitor and assess the epidemiology of Salmonella infections on a statewide and nationwide basis.

Variation

Organisms may lose H antigens and become nonmotile. Loss of O antigen is associated with a change from smooth to rough colony form. Vi antigen may be lost partially or completely. Antigens may be acquired (or lost) in the process of transduction.

Pathogenesis and Clinical Findings

S serotype Typhi, S serotype, and perhaps S serotype Paratyphi and S serotype Paratyphi B are primarily infective for humans, and infection with these organisms implies acquisition from a human source. The vast majority of salmonellae, however, are chiefly pathogenic in animals that constitute the reservoir for human infection; these include poultry, pigs, rodents, cattle, pets (from turtles to parrots), and many others.

The organisms almost always enter via the oral route, usually with contaminated food or drink. The mean infective dose to produce clinical or subclinical infection in humans is 10⁵–10⁸ salmonellae (but perhaps as few as 10³ S serotype Typhi organisms). Among the host factors that contribute to resistance to salmonella infection are gastric acidity, normal intestinal microbiota, and local intestinal immunity (see later).

Salmonellae produce three main types of disease in humans, but mixed forms are frequent (Table 15-4).

A. The “Enteric Fevers” (Typhoid Fever)

This syndrome is produced by only a few of the salmonellae, of which S serotype Typhi (typhoid fever) is the most important. The ingested salmonellae reach the small intestine, from which they enter the lymphatics and then the bloodstream. They are carried by the blood to many organs, including the intestine. The organisms multiply in intestinal lymphoid tissue and are excreted in stools.

After an incubation period of 10–14 days, fever, malaise, headache, constipation, bradycardia, and myalgia occur. The fever rises to a high plateau, and the spleen and liver become enlarged. Rose spots, usually on the skin of the abdomen or chest, are seen briefly in rare cases. The white blood cell count is normal or low. In the preantibiotic era, the chief complications of enteric fever were intestinal hemorrhage and perforation, and the mortality rate was 10–15%. Treatment with antibiotics has reduced the mortality rate to less than 1%.

The principal lesions are hyperplasia and necrosis of lymphoid tissue (eg, Peyer’s patches); hepatitis; focal necrosis of the liver; and inflammation of the gallbladder, periosteum, lungs, and other organs.

B. Bacteremia With Focal Lesions

This is associated commonly with S serotype choleraesuis but may be caused by any salmonella serotype. After oral infection, there is early invasion of the bloodstream (with possible focal lesions in lungs, bones, meninges, etc), but intestinal manifestations are often absent. Blood culture results are positive.

C. Enterocolitis

This is the most common manifestation of salmonella infection. In the United States, S Typhimurium and Salmonella Enteritidis are prominent, but enterocolitis can be caused by any of the more than 1400 group I serotypes of salmonellae. Eight to 48 hours after ingestion of salmonellae, there is nausea, headache, vomiting, and profuse diarrhea, with few leukocytes in the stools. Low-grade fever is common, but the episode usually resolves in 2–3 days.

Inflammatory lesions of the small and large intestine are present. Bacteremia is rare (2–4%) except in immunodeficient persons. Blood culture results are usually negative, but stool culture results are positive for salmonellae and may remain positive for several weeks after clinical recovery.

Diagnostic Laboratory Tests

A. Specimens

Blood for culture must be taken repeatedly. In enteric fevers and septicemias, blood culture results are often positive in the first week of the disease. Bone marrow cultures may be useful. Urine culture results may be positive after the second week.

Stool specimens also must be taken repeatedly. In enteric fevers, the stools yield positive results from the second or third week on; in enterocolitis, the stools yield positive results during the first week.

A positive culture of duodenal drainage establishes the presence of salmonellae in the biliary tract in carriers.

B. Bacteriologic Methods for Isolation of Salmonellae

1. Differential medium cultures—EMB, MacConkey, or desoxycholate medium permits rapid detection of lactose nonfermenters (not only salmonellae and shigellae but also Proteus, Serratia, Pseudomonas, etc). Gram-positive organisms are somewhat inhibited. Bismuth sulfite medium permits rapid detection of salmonellae, which form black colonies because of H₂S production. Many salmonellae produce H₂S.

2. Selective medium cultures—The specimen is plated on salmonella-shigella (SS) agar, Hektoen enteric agar, xylose-lysine desoxycholate (XLD) agar, or desoxycholate-citrate agar, which favor growth of salmonellae and shigellae over other Enterobacteriaceae. Chromogenic agars specifically for salmonella recovery are also available.

3. Enrichment cultures—The specimen (usually stool) also is put into selenite F or tetrathionate broth, both of which inhibit replication of normal intestinal bacteria and permit multiplication of salmonellae. After incubation for 1–2 days, this is plated on differential and selective media.

4. Final identification—Suspect colonies from solid media are identified by biochemical reaction patterns and slide agglutination tests with specific sera.

C. Serologic Methods

Serologic techniques are used to identify unknown cultures with known sera (see later discussion) and may also be used to determine antibody titers in patients with unknown illness, although the latter is not very useful in the diagnosis of Salmonella infections.

1. Agglutination test—In this test, known sera and unknown culture are mixed on a slide. Clumping, when it occurs, can be observed within a few minutes. This test is particularly useful for rapid preliminary identification of cultures. There are commercial kits available to agglutinate and serogroup salmonellae by their O antigens: A, B, C₁, C₂, D, and E.

2. Tube dilution agglutination test (Widal test)—Serum agglutinins rise sharply during the second and third weeks of S serotype Typhi infection. The Widal test to detect these antibodies against the O and H antigens has been in use for decades. At least two serum specimens, obtained at intervals of 7–10 days, are needed to prove a rise in antibody titer. Serial dilutions of unknown sera are tested against antigens from representative salmonellae. False-positive and false-negative results occur. The interpretive criteria when single serum specimens are tested vary, but a titer against the O antigen of greater than 1:320 and against the H antigen of greater than 1:640 is considered positive. High titer of antibody to the Vi antigen occurs in some carriers. Alternatives to the Widal test include rapid colorimetric and EIA methods. There are conflicting reports in the literature regarding superiority of these methods to the Widal test. Results of serologic tests for Salmonella infection cannot be relied upon to establish a definitive diagnosis of typhoid fever and are most often used in resource poor areas of the world where blood cultures are not readily available.

D. Nucleic Acid Amplification Tests

As mentioned above for the shigellae, several commercial NAATs are available for direct detection of salmonellae in fecal samples of patients with acute diarrhea. Since these assays are new, performance characteristics of the assays and their impact on public health surveillance are still under investigation.

Immunity

Infections with S serotype Typhi or S Paratyphi usually confer a certain degree of immunity. Reinfection may occur but is often milder than the first infection. Circulating antibodies to O and Vi are related to resistance to infection and disease. However, relapses may occur in 2–3 weeks after recovery despite antibodies. Secretory IgA antibodies may prevent attachment of salmonellae to intestinal epithelium.

Persons with S/S hemoglobin (sickle cell disease) are exceedingly susceptible to Salmonella infections, particularly osteomyelitis. Persons with A/S hemoglobin (sickle cell trait) may be more susceptible than normal individuals (those with A/A hemoglobin).

Treatment

Although enteric fevers and bacteremias with focal lesions require antimicrobial treatment, the vast majority of cases of enterocolitis do not. Antimicrobial treatment of Salmonella enteritis in neonates is important. In enterocolitis, clinical symptoms and excretion of the salmonellae may be prolonged by antimicrobial therapy. In severe diarrhea, replacement of fluids and electrolytes is essential.

Antimicrobial therapy of invasive Salmonella infections is with ampicillin, trimethoprim–sulfamethoxazole, or a third-generation cephalosporin. Multiple drug resistance transmitted genetically by plasmids among enteric bacteria is a problem in Salmonella infections. Susceptibility testing is an important adjunct to selecting a proper antibiotic.

In most carriers, the organisms persist in the gallbladder (particularly if gallstones are present) and in the biliary tract. Some chronic carriers have been cured by ampicillin alone, but in most cases cholecystectomy must be combined with drug treatment.

Epidemiology

The feces of persons who have unsuspected subclinical disease or are carriers are a more important source of contamination than frank clinical cases that are promptly isolated, such as when carriers working as food handlers are “shedding” organisms. Many animals, including cattle, rodents, and fowl, are naturally infected with a variety of salmonellae and have the bacteria in their tissues (meat), excreta, or eggs. The high incidence of salmonellae in commercially prepared chickens has been widely publicized. The incidence of typhoid fever has decreased, but the incidence of other Salmonella infections has increased markedly in the United States. The problem probably is aggravated by the widespread use of animal feeds containing antimicrobial drugs that favor the proliferation of drug-resistant salmonellae and their potential transmission to humans.

A. Carriers

After manifest or subclinical infection, some individuals continue to harbor salmonellae in their tissues for variable lengths of time (ie, convalescent carriers or healthy permanent carriers). Three percent of survivors of typhoid become permanent carriers, harboring the organisms in the gallbladder, biliary tract, or, rarely, the intestine or urinary tract.

B. Sources of Infection

The sources of infection are food and drink that have been contaminated with salmonellae. The following sources are important:

1. Water—Contamination with feces often results in explosive epidemics

2. Milk and other dairy products (ice cream, cheese, custard)—Contamination with feces and inadequate pasteurization or improper handling; some outbreaks are traceable to the source of supply

3. Shellfish—From contaminated water

4. Dried or frozen eggs—From infected fowl or contaminated during processing

5. Meats and meat products—From infected animals (poultry) or contamination with feces by rodents or humans

6. “Recreational” drugs—Marijuana and other drugs

7. Animal dyes—Dyes (eg, carmine) used in drugs, foods, and cosmetics

8. Household pets—Turtles, dogs, cats, exotic pets such as reptiles, and so on

Prevention and Control

Sanitary measures must be taken to prevent contamination of food and water by rodents or other animals that excrete salmonellae. Infected poultry, meats, and eggs must be thoroughly cooked. Carriers must not be allowed to work as food handlers and should observe strict hygienic precautions.

Two typhoid vaccines are currently available in the United States: an oral live, attenuated vaccine (Ty 21a) and a Vi capsular polysaccharide vaccine (Vi CPS) for intramuscular use. Vaccination is recommended for travelers to endemic regions, especially if the traveler visits rural areas or small villages where food choices are limited. Both vaccines have an efficacy of 50–80%. The time required for primary vaccination and age limits for each vaccine varies, and individuals should consult the Centers for Disease Control and Prevention’s Web site or obtain advice from a travel clinic regarding the latest vaccine information.

• Members of the Enterobacteriaceae are short gram-negative rods that grow rapidly on standard laboratory media.

• Members of this group are catalase positive; nitrate positive and, with the exception of Plesiomonas, are cytochrome oxidase negative. Organisms can be identified readily by the ability to ferment lactose on MacConkey and by other biochemical reactions or newer technologies such as MALDI-TOF MS.

• Enterobacteriaceae express a variety of antigens that include somatic or O antigens (cell wall lipopolysaccharide), capsular or K antigens, and H or flagellar antigens. Salmonella express the Vi antigens. These antigens are virulence factors and can be used to serotype those organisms that possess them.

• Enterobacteriaceae cause a variety of human infections that can be broadly classified as either enteric diseases or extraintestinal infections such as urinary tract infections, bacteremia, and meningitis.

• Genera associated with enteric illnesses include Salmonella, Shigella, and diarrheagenic E coli, of which there are six types based on the mechanism of disease (eg, toxigenic or invasive or both).

• The most common extraintestinal infections caused by these organisms are urinary tract infections. E coli predominates, but the urea-positive organisms such as Proteus species can cause bladder and kidney stones.

• Enterobacteriaceae acquired in the hospital environment are often resistant to many antimicrobial agents usually mediated by plasmid-encoded resistance determinants.