These organisms are usually opportunistic pathogens that cause nosocomial infections, especially pneumonia and urinary tract infections. K. pneumoniae is an important respiratory tract pathogen outside hospitals as well.
K. pneumoniae, Enterobacter cloacae, and Serratia marcescens are the species most often involved in human infections. They are frequently found in the large intestine but are also present in soil and water. These organisms have very similar properties and are usually distinguished on the basis of several biochemical reactions and motility. K. pneumoniae has a very large polysaccharide capsule, which gives its colonies a striking mucoid appearance. S. marcescens produces red-pigmented colonies (Figure 18–3).
Serratia marcescens—red-pigmented colonies. Arrow points to a red-pigmented colony of S. marcescens. (Used with permission from Professor Shirley Lowe, University of California, San Francisco School of Medicine.)
Pathogenesis & Epidemiology
Of the three organisms, K. pneumoniae is most likely to be a primary, nonopportunistic pathogen; this property is related to its antiphagocytic capsule. Although this organism is a primary pathogen, patients with K. pneumoniae infections frequently have predisposing conditions such as advanced age, chronic respiratory disease, diabetes, or alcoholism. The organism is carried in the respiratory tract of about 10% of healthy people, who are prone to pneumonia if host defenses are lowered.
Enterobacter and Serratia infections are clearly related to hospitalization, especially to invasive procedures such as intravenous catheterization, respiratory intubation, and urinary tract manipulations. In addition, outbreaks of Serratia pneumonia have been associated with contamination of the water in respiratory therapy devices. Prior to the extensive use of these procedures, S. marcescens was a harmless organism most frequently isolated from environmental sources such as water.
Serratia also causes endocarditis in users of injection drugs. As with many other gram-negative rods, the pathogenesis of septic shock caused by these organisms is related to the endotoxins in their cell walls.
Urinary tract infections and pneumonia are the usual clinical entities associated with these three bacteria, but bacteremia and secondary spread to other areas such as the meninges and liver occur. It is difficult to distinguish infections caused by these organisms on clinical grounds, with the exception of pneumonia caused by Klebsiella, which produces a thick, mucoid, bloody sputum (“currant-jelly” sputum) and can progress to necrosis and abscess formation.
There are two other species of Klebsiella that cause unusual human infections rarely seen in the United States. Klebsiella ozaenae is associated with atrophic rhinitis, and Klebsiella rhinoscleromatis causes a destructive granuloma of the nose and pharynx.
Organisms of this group produce lactose-fermenting (colored) colonies on differential agar such as MacConkey’s or EMB, although Serratia, which is a late lactose fermenter, can produce a negative reaction. These organisms are differentiated by the use of biochemical tests.
Because the antibiotic resistance of these organisms can vary greatly, the choice of drug depends on the results of sensitivity testing. Isolates from hospital-acquired infections are frequently resistant to multiple antibiotics. Strains of K. pneumoniae that produce extended-spectrum beta-lactamases (ESBL) are an important cause of hospital-acquired infections and are resistant to almost all known antibiotics. An aminoglycoside (e.g., gentamicin) and a cephalosporin (e.g., cefotaxime) are used empirically until the results of testing are known. In severe Enterobacter infections, a combination of imipenem and gentamicin is often used.
Some hospital-acquired infections caused by gram-negative rods can be prevented by such general measures as changing the site of intravenous catheters, removing urinary catheters when they are no longer needed, and taking proper care of respiratory therapy devices. There is no vaccine.
These organisms primarily cause urinary tract infections, both community- and hospital-acquired.
These gram-negative rods are distinguished from other members of the Enterobacteriaceae by their ability to produce the enzyme phenylalanine deaminase. In addition, they produce the enzyme urease, which cleaves urea to form NH3 and CO2. Certain species are very motile and produce a striking swarming effect on blood agar, characterized by expanding rings (waves) of organisms over the surface of the agar (Figure 18–4).
Proteus species—swarming motility on blood agar. Arrowhead points to the site where Proteus bacteria were placed on the blood agar. Short arrow points to the edge of the first ring of swarming motility. Long arrow points to the edge of the second ring of swarming motility. (Used with permission from Professor Shirley Lowe, University of California, San Francisco School of Medicine.)
The cell wall O antigens of certain strains of Proteus, such as OX-2, OX-19, and OX-K, cross-react with antigens of several species of rickettsiae. These Proteus antigens can be used in laboratory tests to detect the presence of antibodies against certain rickettsiae in patients’ serum. This test, called the Weil-Felix reaction after its originators, is being used less frequently as more specific procedures are developed.
In the past, there were four medically important species of Proteus. However, molecular studies of DNA relatedness showed that two of the four were significantly different. These species have been renamed: Proteus morganii is now Morganella morganii, and Proteus rettgeri is now Providencia rettgeri. In the clinical laboratory, these organisms are distinguished from Proteus vulgaris and Proteus mirabilis on the basis of several biochemical tests.
Pathogenesis & Epidemiology
The organisms are present in the human colon as well as in soil and water. Their tendency to cause urinary tract infections is probably due to their presence in the colon and to colonization of the urethra, especially in women. The vigorous motility of Proteus organisms may contribute to their ability to invade the urinary tract.
Production of the enzyme urease is an important feature of the pathogenesis of urinary tract infections by this group. Urease hydrolyzes the urea in urine to form ammonia, which raises the pH, producing an alkaline urine. This encourages the formation of stones (calculi) called “struvite” composed of magnesium ammonium phosphate. Struvite stones often manifest as staghorn calculi in the renal pelvis. They obstruct urine flow, damage urinary epithelium, and serve as a nidus for recurrent infection by trapping bacteria within the stone. Because alkaline urine also favors growth of the organisms and more extensive renal damage, treatment involves keeping the urine at a low pH.
The signs and symptoms of urinary tract infections caused by these organisms cannot be distinguished from those caused by E. coli or other members of the Enterobacteriaceae. Proteus species can also cause pneumonia, wound infections, and septicemia. P. mirabilis is the species of Proteus that causes most community- and hospital-acquired infections, but P. rettgeri is emerging as an important agent of nosocomial infections.
These organisms usually are highly motile and produce a “swarming” overgrowth on blood agar, which can frustrate efforts to recover pure cultures of other organisms. Growth on blood agar containing phenylethyl alcohol inhibits swarming, thus allowing isolated colonies of Proteus and other organisms to be obtained. They produce non–lactose-fermenting (colorless) colonies on MacConkey’s or EMB agar. P. vulgaris and P. mirabilis produce H2S, which blackens the butt of TSI agar, whereas neither M. morganii nor P. rettgeri does. P. mirabilis is indole-negative, whereas the other three species are indole-positive—a distinction that can be used clinically to guide the choice of antibiotics. These four medically important species are urease-positive. Identification of these organisms in the clinical laboratory is based on a variety of biochemical reactions.
Most strains are sensitive to aminoglycosides and trimethoprim-sulfamethoxazole, but because individual isolates can vary, antibiotic sensitivity tests should be performed. P. mirabilis is the species most frequently sensitive to ampicillin. The indole-positive species (P. vulgaris, M. morganii, and P. rettgeri) are more resistant to antibiotics than is P. mirabilis, which is indole-negative. The treatment of choice for the indole-positive species is a cephalosporin (e.g., cefotaxime). P. rettgeri is frequently resistant to multiple antibiotics.
There are no specific preventive measures, but many hospital-acquired urinary tract infections can be prevented by prompt removal of urinary catheters.
P. aeruginosa causes infections (e.g., sepsis, pneumonia, and urinary tract infections) primarily in patients with lowered host defenses. It also causes chronic lower respiratory tract infections in patients with cystic fibrosis, wound infections (cellulitis) in burn patients (Figure 18–5), and malignant otitis externa in diabetic patients. It is the most common cause of ventilator-associated pneumonia. (P. aeruginosa is also known as Burkholderia aeruginosa.) Pseudomonas cepacia (renamed Burkholderia cepacia) and Pseudomonas maltophilia (renamed Xanthomonas maltophilia and now called Stenotrophomonas maltophilia) also cause these infections, but much less frequently. Pseudomonas pseudomallei (also known as Burkholderia pseudomallei), the cause of melioidosis, is described in Chapter 27.
Cellulitis caused by Pseudomonas aeruginosa. Note the blue-green color of the pus in the burn wound infection. (Used with permission from Dr. Robert L. Sheridan.)
Pseudomonads are gram-negative rods that resemble the members of the Enterobacteriaceae but differ in that they are strict aerobes (i.e., they derive their energy only by oxidation of sugars rather than by fermentation). Because they do not ferment glucose, they are called nonfermenters, in contrast to the members of the Enterobacteriaceae, which do ferment glucose. Oxidation involves electron transport by cytochrome c (i.e., they are oxidase-positive).
Pseudomonads are able to grow in water containing only traces of nutrients (e.g., tap water), and this favors their persistence in the hospital environment. P. aeruginosa and B. cepacia have a remarkable ability to withstand disinfectants; this accounts in part for their role in hospital-acquired infections. They have been found growing in hexachlorophene-containing soap solutions, in antiseptics, and in detergents.
P. aeruginosa produces two pigments useful in clinical and laboratory diagnosis: (1) pyocyanin, which can color the pus in a wound blue and (2) pyoverdin (fluorescein), a yellow-green pigment that fluoresces under ultraviolet light, a property that can be used in the early detection of skin infection in burn patients. In the laboratory, these pigments diffuse into the agar, imparting a blue-green color that is useful in identification. P aeruginosa is the only species of Pseudomonas that synthesizes pyocyanin (Figure 18–6).
Pseudomonas aeruginosa—blue-green pigment. Blue-green pigment (pyocyanin) produced by P. aeruginosa diffuses into the agar. (Used with permission from Professor Shirley Lowe, University of California, San Francisco School of Medicine.)
Strains of P. aeruginosa isolated from cystic fibrosis patients have a prominent slime layer (glycocalyx), which gives their colonies a very mucoid appearance. The slime layer mediates adherence of the organism to mucous membranes of the respiratory tract and prevents antibody from binding to the organism.
Pathogenesis & Epidemiology
P. aeruginosa is found chiefly in soil and water, although approximately 10% of people carry it in the normal flora of the colon. It is found on the skin in moist areas and can colonize the upper respiratory tract of hospitalized patients. Its ability to grow in simple aqueous solutions has resulted in contamination of respiratory therapy and anesthesia equipment, intravenous fluids, and even distilled water.
P. aeruginosa is primarily an opportunistic pathogen that causes infections in hospitalized patients (e.g., those with extensive burns), in whom the skin host defenses are destroyed; in those with chronic respiratory disease (e.g., cystic fibrosis), in whom the normal clearance mechanisms are impaired; in those who are immunosuppressed; in those with neutrophil counts of less than 500/µL; and in those with indwelling catheters. It causes 10% to 20% of hospital-acquired infections and, in many hospitals, is the most common cause of gram-negative nosocomial pneumonia, especially ventilator-associated pneumonia.
Pathogenesis is based on multiple virulence factors: endotoxin, exotoxins, and enzymes. Its endotoxin, like that of other gram-negative bacteria, causes the symptoms of sepsis and septic shock. The best known of the exotoxins is exotoxin A, which causes tissue necrosis. It inhibits eukaryotic protein synthesis by the same mechanism as diphtheria exotoxin, namely, ADP-ribosylation of elongation factor-2. It also produces enzymes, such as elastase and proteases, that are histotoxic and facilitate invasion of the organism into the bloodstream. Pyocyanin damages the cilia and mucosal cells of the respiratory tract.
Strains of P. aeruginosa that have a “type III secretion system” are significantly more virulent than those that do not. This secretion system transfers the exotoxin from the bacterium directly into the adjacent human cell, which allows the toxin to avoid neutralizing antibody. Type III secretion systems are mediated by transport pumps in the bacterial cell membrane. Of the four exoenzymes known to be transported by this secretion system, Exo S is the one most clearly associated with virulence. Exo S has several modes of action, the most important of which is ADP-ribosylation of a Ras protein, leading to damage to the cytoskeleton.
P. aeruginosa can cause infections virtually anywhere in the body, but urinary tract infections, pneumonia (especially in cystic fibrosis patients), and wound infections (especially burns) (see Figure 18–5) predominate. It is an important cause of hospital-acquired pneumonia, especially in those undergoing mechanical ventilation (ventilator-associated pneumonia). From these sites, the organism can enter the blood, causing sepsis. The bacteria can spread to the skin, where they cause black, necrotic lesions called ecthyma gangrenosum (Figure 18–7). Patients with P. aeruginosa sepsis have a mortality rate of greater than 50%. It is an important cause of endocarditis in intravenous drug users.
Ecthyma gangrenosum. Necrotic skin lesion caused by Pseudomonas aeruginosa. (Reproduced with permission from Wolff K, Johnson R, Saavedra A: Fitzpatrick’s Color Atlas & Synopsis of Clinical Dermatology, 8th ed. New York, NY: McGraw-Hill Education; 2017.)
Severe external otitis (malignant otitis externa) and other skin lesions (e.g., folliculitis) occur in users of swimming pools and hot tubs (hot tub folliculitis) in which the chlorination is inadequate. P. aeruginosa is the most common cause of osteomyelitis of the foot in those who sustain puncture wounds through the soles of gym shoes. Corneal infections caused by P. aeruginosa are seen in contact lens users.
In addition to P. aeruginosa, Stenotrophomonas and Burkholderia also cause chronic lung infections in patients with cystic fibrosis.
P. aeruginosa grows as non–lactose-fermenting (colorless) colonies on MacConkey’s or EMB agar. It is oxidase-positive. A typical metallic sheen of the growth on TSI agar, coupled with the blue-green pigment on ordinary nutrient agar (see Figure 18–6), and a fruity aroma are sufficient to make a presumptive diagnosis. The diagnosis is confirmed by biochemical reactions. Identification for epidemiologic purposes is done by bacteriophage or pyocin6 typing.
Because P. aeruginosa is resistant to many antibiotics, treatment must be tailored to the sensitivity of each isolate and monitored frequently; resistant strains can emerge during therapy. The treatment of choice is an antipseudomonal penicillin (e.g., piperacillin/tazobactam or ticarcillin/clavulanate) plus an aminoglycoside (e.g., gentamicin or amikacin). Ceftazidime is also effective. For infections caused by highly resistant strains, colistin (polymyxin E) is useful. The drug of choice for urinary tract infections is ciprofloxacin. The drug of choice for infections caused by B. cepacia and S. maltophilia is trimethoprim-sulfamethoxazole.
Prevention of P. aeruginosa infections involves keeping neutrophil counts above 500/µL, removing indwelling catheters promptly, taking special care of burned skin, and taking other similar measures to limit infection in patients with reduced host defenses.
Members of the genus Bacteroides are the most common cause of serious anaerobic infections (e.g., sepsis, peritonitis, and abscesses). Bacteroides fragilis is the most frequent pathogen. Prevotella melaninogenica is also an important pathogen. It was formerly known as Bacteroides melaninogenicus, and both names are still encountered.
Bacteroides and Prevotella organisms are anaerobic, non–spore-forming, gram-negative rods. Of the many species of Bacteroides, two are human pathogens: B. fragilis7 and Bacteroides corrodens.
Members of the B. fragilis group are the predominant organisms in the human colon, numbering approximately 1011/g of feces and are found in the vagina of approximately 60% of women. P. melaninogenica and B. corrodens occur primarily in the mouth.
Pathogenesis & Epidemiology
Because Bacteroides and Prevotella species are part of the normal flora, infections are endogenous, usually arising from a break in a mucosal surface, and are not communicable. These organisms cause a variety of infections, such as local abscesses at the site of a mucosal break, metastatic abscesses by hematogenous spread to distant organs, or lung abscesses by aspiration of oral flora.
Predisposing factors such as surgery, trauma, and chronic disease play an important role in pathogenesis. Local tissue necrosis, impaired blood supply, and growth of facultative anaerobes at the site contribute to anaerobic infections. The facultative anaerobes, such as E. coli, utilize the oxygen, thereby reducing it to a level that allows the anaerobic Bacteroides and Prevotella strains to grow. As a result, many anaerobic infections contain a mixed facultative and anaerobic flora. This has important implications for therapy; both the facultative anaerobes and the anaerobes should be treated.
The polysaccharide capsule of B. fragilis is an important virulence factor. The host response to the capsule plays a major role in abscess formation. Note also that the endotoxin of B. fragilis contains a variant lipid A that is missing one of the fatty acids and consequently is 1000-fold less active than the typical endotoxin of bacteria such as Neisseria meningitidis.
Enzymes such as hyaluronidase, collagenase, and phospholipase are produced and contribute to tissue damage. Enterotoxin-producing strain of B. fragilis can cause diarrhea in both children and adults.
The B. fragilis group of organisms is most frequently associated with intra-abdominal infections, either peritonitis or localized abscesses. Pelvic abscesses, necrotizing fasciitis, and bacteremia occur as well. Abscesses of the mouth, pharynx, brain, and lung are more commonly caused by P. melaninogenica, a member of the normal oral flora, but B. fragilis is found in about 25% of lung abscesses. In general, B. fragilis causes disease below the diaphragm, whereas P. melaninogenica causes disease above the diaphragm. Prevotella intermedia is an important cause of gingivitis, periodontitis, and dental abscess.
Bacteroides species can be isolated anaerobically on blood agar plates containing kanamycin and vancomycin to inhibit unwanted organisms. They are identified by biochemical reactions (e.g., sugar fermentations) and by production of certain organic acids (e.g., formic, acetic, and propionic acids), which are detected by gas chromatography. P. melaninogenica produces characteristic black colonies (Figure 18–8).
Prevotella melaninogenica—black pigmented colonies. Arrow points to a black pigmented colony of P. melaninogenica. (Used with permission from Professor Shirley Lowe, University of California, San Francisco School of Medicine.)
Members of the B. fragilis group are resistant to penicillins, first-generation cephalosporins, and aminoglycosides, making them among the most antibiotic-resistant of the anaerobic bacteria. Penicillin resistance is the result of β-lactamase production. Metronidazole is the drug of choice, with cefoxitin, clindamycin, and chloramphenicol as alternatives. Aminoglycosides are frequently combined to treat the facultative gram-negative rods in mixed infections. The drug of choice for P. melaninogenica infections is either metronidazole or clindamycin. β-Lactamase-producing strains of P. melaninogenica have been isolated from patients. Surgical drainage of abscesses usually accompanies antibiotic therapy, but lung abscesses often heal without drainage.
Prevention of Bacteroides and Prevotella infections centers on perioperative administration of a cephalosporin, frequently cefoxitin, for abdominal or pelvic surgery. There is no vaccine.
Fusobacterium species are long, anaerobic, gram-negative rods with pointed ends (Figure 18–9). They are part of the human normal flora of the mouth, colon, and female genital tract and are isolated from brain, pulmonary, intra-abdominal, and pelvic abscesses. They are frequently found in mixed infections with other anaerobes and facultative anaerobes.
Fusobacterium nucleatum—Gram stain. Note the long, thin gram-negative rods with pointed ends. (Used with permission from Dr. V.R. Dowell, Jr. Public Health Image Library, Centers for Disease Control and Prevention.)
Fusobacterium nucleatum occurs, along with various spirochetes, in cases of Vincent’s angina (trench mouth), which is characterized by a necrotizing ulcerative gingivitis. Fusobacterium necrophorum causes Lemierre’s disease, which is an anaerobic infection of the posterior pharyngeal space accompanied by thrombophlebitis of the internal jugular vein and metastatic infectious emboli to the lung.
The laboratory diagnosis is made by culturing the organism anaerobically. The drug of choice for Fusobacterium infections is either penicillin G, clindamycin, or metronidazole. There is no vaccine.