Most strains of E coli ferment lactose rapidly and produce indole. These and other biochemical reactions are sufficient to separate it from the other species. There are over 150 distinct O antigens and a large number of K and H antigens, all of which are designated by number. The antigenic formula for serotypes is described by linking the letter (O, K, or H) and the assigned number of the antigen(s) present (eg, O111:K76:H7). Alternatively, the emergence of multidrug-resistant clones in the sequencing era has led to the use of newer descriptive terminology (eg, E coli Sequence Type 131, Klebsiella pneumoniae Sequence Type 258).
✺ Serotypes use O, K, H antigens
Pili play a role in virulence as mediators of attachment to human epithelial surfaces. They show marked tropism for different epithelial cell types, which is determined by the availability of their specific receptor on the host cell surface. Most E coli express type 1 or common pili. Type 1 pili bind to the D-mannose residues commonly present on epithelial cell surfaces and thus mediate binding to a wide variety of cell types. More specialized pili are found in select clones of E coli. P pili bind to digalactoside (Gal–Gal) moieties on kidney cells and erythrocytes of the P blood group. Pili that mediate binding to enterocytes are found among the diarrhea-causing E coli and are specific to the pathogenic type, as shown in Figure 33–2 and listed in Table 33–1. Escherichia coli also causes diarrhea in animals, and different sets of pili exist with host-specific tropism for their enterocytes. The receptor(s) for the enteric pili are not known in detail, but include glycolipids and glycoproteins on the enterocyte surface.
Type 1 pili bind mannose
P pili bind kidney cells
Pili of diarrhea strains bind enterocytes
Antigenic structure of Escherichia coli. The O antigen is contained in the repeating polysaccharide units of the lipopolysaccharide (LPS) in the outer membrane of the cell wall. The H antigen is the flagellar protein. The K antigen is the polysaccharide capsule present in some strains. Most E coli have type 1 (common) hair-like pili extending from the surface. Some E coli have specialized P, colonization factor antigens (CFAs), or bundle-forming pili (Bfp), as well as type 1 pili.
TABLE 33–1Characteristics of Pathogenic Enterobacteriaceae ||Download (.pdf) TABLE 33–1 Characteristics of Pathogenic Enterobacteriaceae
| ||DIAGNOSTIC ANTIGENS ||PILI ||ADHESIN OR CAPSULE ||EXOTOXIN ||PATHOGENIC LESIONS ||SECRETED PROTEINSa ||GENETICS ||TRANSMISSION ||DISEASE |
|Escherichia coli ||O, H, K || || || || || || || || |
|Common ||>150 types ||Type 1b ||K1 polysaccharide ||α-Hemolysin ||Inflammation || || ||Intestinal flora ||Opportunistic, newborn meningitis |
|Uropathogenic (UPEC) || ||Type 1b, P (Gal–Gal) || ||α-Hemolysin ||Inflammation || ||PAI ||Fecal flora, ascending ||UTI |
|Enterotoxigenic (ETEC) || ||CFs || ||LT, ST ||Hypersecretion || ||Plasmid (CF, LT, ST) ||Fecal–oral ||Watery diarrhea (travelers) |
|Enteropathogenic (EPEC) || ||Bfp ||Intimin || ||A/E, small intestine ||Esps ||PAI ||Fecal–oral ||Watery diarrhea |
|Enteroinvasive (EIEC) || || ||Ipas || ||Invasion, inflammation, ulcers ||Ipas ||Large plasmid, PAI ||Fecal–oral ||Dysentery |
|Enterohemorrhagic (EHEC) ||O157:H7 ||Lpf ||Intimin ||Stx ||A/E, colon, hemorrhage ||Esps ||Prophage, PAI ||Fecal–oral direct, low dose, cattle ||Bloody diarrhea, HUS |
|Enteroaggregative (EAEC) || ||AAFs || ||Stxd ||Adherent biofilm || || || ||Watery diarrhea; bloody diarrhea and HUSd |
|Shigella ||O serogroups || || || || || || || || |
|S dysenteriae ||A (10 types) || ||Ipas ||Stx (serotype Al most potent) ||Invasion, inflammation, colonic ulcers ||Ipas ||Large plasmid, PAI ||Fecal–oral, direct, low dose ||Dysentery (severe), HUS |
|S flexneri ||B (6 types) || ||Ipas ||Stx (variable) ||Invasion, inflammation, colonic ulcers ||Ipas ||Large plasmid, PAI ||Fecal–oral, direct, low dose ||Dysentery, HUS |
|S boydii ||C (15 types) || ||Ipas ||Stx (variable) ||Invasion, inflammation, colonic ulcers ||Ipas ||Large plasmid, PAI ||Fecal–oral, direct, low dose ||Dysentery, HUS |
|S sonnei ||D || ||Ipas ||Stx (variable) ||Invasion, inflammation, colonic ulcers ||Ipas ||Large plasmid, PAI ||Fecal–oral, direct, low dose ||Dysentery, HUS |
|Salmonella enterica ||O, H1, H2, K || || || || || || || || |
|Serotypes ||>2000 serovars ||Type 1b || || ||Ruffles, invasion, inflammation ||Inv, Spa, others ||PAI ||Fecal–oral, animals, and humans ||Gastroenteritis, sepsis |
|Typhi ||O group D ||Type 1b ||Vi polysaccharide || ||Macrophage survival, RES growth ||As in serotypesc ||PAI ||Fecal–oral, moderate dose, humans only ||Enteric (typhoid) fever |
|Yersinia ||O, H || || || || || || || || |
|Y pestis || || ||Invasin ||Protease, fibrinolysin ||RES growth, bacteremia, pneumonia ||Yops ||PAI ||Rats, flea bite, aerosol (human) ||Plague |
|Y pseudotuberculosis ||10 types || ||Invasin || ||RES growth, microabscesses ||Yops ||PAI ||Fecal–oral, animal ||Mesenteric adenitis |
|Y enterocolitica ||>50 types || ||Invasin || ||RES growth, microabscesses ||Yops ||PAI ||Fecal–oral, animals ||Mesenteric adenitis, enteric fever |
|Klebsiella ||70 capsular types ||Pili ||Polysaccharide || || || || ||Intestinal flora ||Opportunistic, pneumonia, UTI |
|K pneumoniae || ||Pili ||K1, K2 polysaccharide || ||Abscesses || || ||Intestinal flora ||Liver abscess, endophthalmitis |
|Enterobacter, Serratia, Citrobacter || || || || || || || ||Intestinal flora ||Opportunistic, UTI |
|Proteus || || || || || ||Urease || ||Intestinal flora ||UTI |
The genetics of pilin expression are complex. The genes are organized into multicistronic clusters that encode structural pilin subunits and regulatory functions. Pili of different types may coexist on the same bacterium, and their expression may vary under different environmental conditions. Type 1 pilin expression can be turned “on” or “off” by inversion of a chromosomal DNA sequence containing the promoter responsible for initiating transcription of the pilin gene. Other genes control the orientation of this switch.
As a single species, E coli can produce every kind of protein exotoxin found among the Enterobacteriaceae. These include a pore-forming cytotoxin, inhibitors of protein synthesis, and a number of toxins that alter messenger pathways in host cells. The α-hemolysin is a pore-forming cytotoxin that inserts into the plasma membrane of a wide range of host cells in a manner similar to streptolysin O (Chapter 25) and Staphylococcus aureus α-toxin (Chapter 24). The toxin causes leakage of cytoplasmic contents and eventually cell death. The more recently discovered cytotoxic necrotizing factor (CNF) is often produced in concert with α-hemolysin. CNF is an A–B toxin that disrupts G proteins regulating signaling pathways in the cell cytoplasm with multiple effects including cytoskeleton rearrangement and apoptosis.
α-Hemolysin is pore-forming cytotoxin
CNF disrupts intracellular signaling
Shiga toxin (Stx) is named for the microbiologist who discovered Shigella dysenteriae, and this toxin was once believed to be limited to that species. It is now recognized to exist in at least two molecular forms released upon bacterial lysis by multiple E coli and Shigella strains. In the years after the discovery of this toxin, the term Shiga toxin was reserved for the original toxin, while others were called Shiga-like. In this book, the term Stx is used for all molecular variants that have the same mode of action regardless of the species under consideration. Stx is an A–B type toxin. The B unit directs binding to a specific glycolipid receptor (Gb3) present on eukaryotic cells and leads to internalization by an endocytotic vacuole. Inside the cell, the A subunit crosses the vacuolar membrane in the trans-Golgi network, exits to the cytoplasm, and enzymatically modifies the ribosome site (28S-RNA of 60S subunit) where amino acyl tRNA binds. This alteration blocks protein synthesis, leading to cell death (Figure 33–3). This action is very similar to the plant toxin ricin.
✺ Shiga toxin is produced by Shigella and E coli
✺ Inhibits protein synthesis by ribosomal modification
Stx (Shiga) toxin. The A–B toxin binds to the cytoplasmic membrane, enters in an endocytotic vacuole, and enters the Golgi network. Exiting to the cytoplasm, it combines at ribosome sites involved with tRNA binding. The result is interference with protein synthesis.
Labile toxin (LT) is also an A–B toxin. Its name relates to the physical property of heat lability, which was important in its discovery, and contrasts with the heat-stable toxin (ST) also produced by E coli. The B subunit binds to the cell membrane, and the A subunit catalyzes the ADP-ribosylation of a regulatory G protein located in the membrane of the intestinal epithelial cell. This inactivation of part of the G protein complex causes permanent activation of the membrane-associated adenylate cyclase system and a cascade of events, the net effect of which depends on the biologic function of the stimulated cell. If the cell is an enterocyte, the result is the stimulation of chloride secretion out of the cell and the blockage of NaCl absorption. The net effect is the secretion of water and electrolytes into the bowel lumen. The structure and action of LT are nearly identical with that already described for cholera toxin (CT), but LT is less potent than CT.
✺ LT ADP-ribosylates G protein
✺ Adenylate cyclase stimulation similar to cholera
Stable toxin is a small peptide that binds to a glycoprotein receptor, resulting in the activation of a membrane-bound guanylate cyclase. The subsequent increase in cyclic GMP concentration causes an LT-like net secretion of fluid and electrolytes into the bowel lumen.
✺ ST stimulates guanylate cyclase
E COLI OPPORTUNISTIC INFECTIONS
Escherichia coli accounts for more than 90% of the more than 7 million cases of cystitis and 250 000 of pyelonephritis estimated to occur in otherwise healthy individuals every year in the United States. Urinary tract infections are much more common in women, 40% of whom have an episode in their lifetime, usually when they are sexually active. The reservoir for these infections is the patient’s own intestinal E coli microbiota, which contaminate the perineal and urethral area. In individuals with urinary tract obstruction or instrumentation, exogenous sources assume greater importance.
Perineal flora is reservoir of common cystitis
Relatively minor trauma or mechanical disruptions can allow bacteria colonizing the periurethral area brief access to the urinary bladder. These bacteria originally derived from the fecal microbiota are frequently present in the bladder of women immediately after sexual intercourse. In most instances, they are purged by the flushing action of voiding, but may persist to cause a UTI, depending on host and bacterial factors. Although most UTIs are in otherwise healthy women, host situations that violate bladder integrity (urinary catheters) or that obstruct urine outflow (in men, enlarged prostate) may allow the bacteria more time to attach, multiply, and cause injury. Here, bacterial virulence factors are important, and E coli is the prototype UTI pathogen. Fewer than 10 E coli clones (whether characterized as serotypes or sequence types) account for the majority of UTI cases, and these UTI clones are not the most common ones in the microbiota. These E coli with enhanced potential to produce UTI are called uropathogenic E coli (UPEC).
Minor trauma admits E coli to the bladder
The ability of UPEC to produce UTI begins with type 1 pili, which are the most important for both periurethral and bladder colonization. The tips of such pili attach to mannose moieties presented by membrane proteins (uroplakins) in the transitional epithelium of the bladder. Other pili such as P pili may add to the strength of this attachment; however, because their cognate Gal–Gal receptor is most abundant in the renal pelvis and kidney, P pili are more important for upper urinary tract disease. E coli possessing P pili are a minor percentage of the fecal flora (less than 20%), but the proportion of P+ strains progressively rises with the severity of UTI, up to 70% in pyelonephritis isolates. Motility driven by flagellar motors also plays a role both in access to the bladder and swimming up the ureter to the kidney. Obviously, adherence and motility are at cross-purposes, but UPEC are able to reciprocally regulate these features with on/off switching of pilin expression; but even with type 1 pili switched on, UPEC can alternate between swimming and adherent phases due to the catch-bond properties of the tip adhesin. Another feature is the ability of UPEC to invade superficial epithelial cells. The raft-like clusters formed by this maneuver are felt to aid persistence against the periodic flushing of the bladder. Once established, LPS and the production of other virulence factors such as α-hemolysin and CNF cause injury. Spread to the bloodstream leads to LPS-induced septic shock. The adherence aspects of UPEC are illustrated in Figure 33–4.
✺ Type 1 pili adhere to periurethral and bladder cells
P pili prominent in pyelonephritis
Urinary tract infection due to Escherichia coli. The urinary bladder, perineal mucosa, and short female urethra are shown. E coli from the nearby rectal flora have colonized the perineum, utilizing binding by type 1 (common) pili. Also present are E coli with P pili though these adhesins are of no use at this site. A. A few E coli have gained access to the bladder owing to mechanical disruptions such as sexual intercourse or instrumentation (catheters). Note that receptors for the P pili not present on the perineal mucosa are found on the surface of bladder mucosal cells. B. During voiding, the bladder has expelled the E coli, which have only type 1 pili. The P pili-containing bacteria remain behind due to the strong binding to the P (Gal–Gal) receptor. C. The remaining E coli have multiplied and are causing a UTI (cystitis) with inflammation and hemorrhage. In some cases, the bacteria ascend the ureter to cause pyelonephritis in the kidney where the P (Gal–Gal) receptor is most abundant. WBCs, white blood cells.
OTHER OPPORTUNISTIC INFECTIONS
Escherichia coli is one of the most common causes of neonatal meningitis, producing many features similar to group B streptococcal disease. The pathogenesis involves colonization of the infant with maternal E coli via ruptured amniotic membranes or during childbirth. Failure of protective maternal IgM antibodies to cross the placenta and the immunologic immaturity of newborns surely play a role. Fully 75% of cases of newborn meningitis are caused by strains possessing the sialic acid-containing K1 capsular polysaccharide that is structurally identical to the group B polysaccharide of Neisseria meningitidis, another cause of meningitis.
✺ Infection from vaginal flora as with group B streptococcus
✺ K1 capsule identical to meningococcus
With the exception of UTIs, extraintestinal E coli infections are uncommon unless there is a significant breach in host defenses. Opportunistic infection may follow mechanical damage such as trauma or a ruptured intestinal diverticulum, or involve a generalized impairment of immune function. The virulence factors involved are likely the same as with UTI (eg, pili, α-hemolysin), but have been less specifically studied. Failure of local control of infection can lead to spread and eventually gram-negative septic shock. A significant proportion of blood isolates have the K1 surface polysaccharide. The particular diseases that result depend on the sites involved.
✺ Non-UTI infections require some breach of defenses
E COLI INTESTINAL INFECTIONS
Diarrheal illnesses continue to produce a tremendous mortality burden worldwide, particularly in children under 5 years of age, with the largest numbers of deaths occurring in sub-Saharan Africa and South Asia. Escherichia coli and Shigella (which are specialized E coli) are among the top causes of moderate to severe diarrhea among children in these areas. Diarrhea-causing E coli are classified according to their virulence properties as enterotoxigenic (ETEC), enteropathogenic (EPEC), enteroinvasive (EIEC), enterohemorrhagic (EHEC), or enteroaggregative (EAEC). Each group causes disease by a different mechanism, and the resulting syndromes usually differ clinically and epidemiologically. For example, ETEC and EIEC strains infect only humans. Food and water contaminated with human waste and person-to-person contact are the principal means of infection. A summary of the pathogenesis of infection, clinical syndromes, and epidemiology of infection for each enteropathogen is shown in Table 33–1.
✺ Several pathogenic mechanisms have distinctive epidemiologic and clinical features
ENTEROTOXIGENIC E COLI (ETEC)
Enterotoxigenic E coli produce diarrhea in infants in developing countries, where they are a leading cause of morbidity and mortality during the first 2 years of life. ETEC is also the most important cause of traveler’s diarrhea in visitors to these countries. Repeated bouts of diarrhea caused by ETEC and other infectious agents are an important cause of growth retardation, malnutrition, and developmental delay in third-world countries where ETEC are endemic. ETEC disease is rare in industrialized nations, although recent outbreaks suggest that it may be underestimated.
✺ Traveler’s diarrhea affects children in developing countries
Transmission is by consumption of food and water contaminated by infected or convalescent human carriers. Uncooked foods such as salads or marinated meats and vegetables are associated with the greatest risk. Direct person-to-person transmission is unusual, because the infecting dose is high. Animals are not involved in ETEC disease.
High dose in uncooked foods required
ETEC diarrhea is caused by strains of E coli that produce LT and/or ST enterotoxins in the proximal small intestine. ST seems to be more potent than LT, and strains that elaborate both cause the most severe illness. Adherence to surface microvilli, mediated by multiple variants of colonizing factor (CF) pili, is essential for the efficient delivery of toxin to the target enterocytes. The genes encoding the ST, LT, and the CF pili are borne on plasmids; a single plasmid can carry all three sets of genes. The bacteria remain on the epithelial surface, where the adenylate cyclase-stimulating action of the toxin(s) creates the flow of water and electrolytes from the enterocyte into the intestinal lumen. The mucosa becomes hyperemic but is not injured in the process. There is no invasion or inflammation.
✺ LT and/or ST cause fluid outpouring in small intestine
Although individuals may experience more than one episode of ETEC diarrhea, infections with ETEC do stimulate immunity. Travelers from industrialized nations have a much higher attack rate than adults living in the endemic area. This natural immunity is presumably mediated by secretory IgA specific for LT and CFs; the small ST peptides are nonimmunogenic. The disease is of very low incidence in breastfed infants, underscoring the protective effect of maternal antibody and the importance of transmission by contaminated food and water.
sIgA to LT and CFs may provide some protection
ENTEROPATHOGENIC E COLI (EPEC)
Enteropathogenic E coli strains were first identified as the cause of explosive outbreaks of diarrhea in hospital nurseries in the United States and Great Britain during the 1950s. The link to E coli was established on epidemiologic grounds alone, using serotyping of stool isolates—no small task. The World Health Organization still recognizes a group of 12 EPEC serotypes. The disease seems to have disappeared in industrialized nations, although it may be underestimated because of the difficulty of diagnosis. In developing countries throughout the world, EPEC account for up to 20% of diarrheal illnesses in bottle-fed infants younger than 1 year of age. The reservoir is infant cases and adult carriers, with transmission by the fecal–oral route. Nursery outbreaks demonstrate the importance of spread by fomites, suggesting that the infecting dose for infants is low. Adult cases are felt to require a very high infecting dose (108 to 1010 bacteria).
Nursery outbreaks and endemic diarrheas occur in developing world
Enteropathogenic E coli initially attach to small intestine enterocytes using bundle-forming pili (Bfp) to form clustered microcolonies on the enterocyte cell surface. The lesion then progresses with localized degeneration of the brush border, loss of the microvilli, and changes in the cell morphology including the production of dramatic “pedestals” with the EPEC bacterium at their apex. The combination of these actions is called the attaching and effacing (A/E) lesion (Figure 33–5). The many steps involved in the formation of the A/E lesion are genetically controlled in a PAI, which includes the genes for the major EPEC attachment protein, intimin, and an injection (type III) secretion system. The secretion system injects over 30 E coli secretion proteins (Esps) into the host cell cytoplasm including—remarkably—the surface receptor (Tir) for intimin, which migrates to the surface after its injection. The other E coli secretion proteins perturb intracellular signal transduction pathways, one effect of which is the induction of modifications in enterocyte cytoskeleton proteins (actin, talin). The cytoskeleton accumulates beneath the attached bacteria to form the pedestals and complete the actin-rich A/E lesion (Figure 33–6). The Esps cause a host of other intracellular disruptions, including mitochondrial injury and induction of apoptosis. The link between these morphologic changes of the A/E and diarrhea is not known, but the injected Esps have also been shown to change electrolyte transport across the luminal membrane.
✺ Intimin receptor and Esps are injected
✺ Cytoskeleton modification produces A/E lesion
Enteropathogenic Escherichia coli (EPEC) attachment to epithelial cells. The EPEC are attaching to and effacing the microvilli on the epithelial cell surface. The cell’s filamentous actin is rearranged at the attachment point. Note the pedestal below the EPEC cell.
Enteropathogenic Escherichia coli (EPEC) contact secretion system. (Left) An enterocyte is shown with a microvillus border and a delicate supporting cytoskeleton. (Middle) An EPEC has attached to the cell surface by binding of the bundle-forming pili to receptors on the host cell surface. A type III secretion system apparatus has been inserted into the cell and is exporting secretion proteins (Esps) into the cytoplasm. One of these is the receptor for intimin. (Right) The intimin receptor has been inserted below the host cell membrane and is now mediating tight binding to the surface. The other Esps have disrupted multiple cellular functions, including the structure of the cytoskeleton. Cytoskeleton elements have been concentrated to form a pedestal cradling the EPEC (Figure 33–5). Bfp, bundle-forming pili.
In endemic areas, EPEC can be isolated often from the stool of asymptomatic adults, but unlike ETEC, these strains do not seem to cause traveler’s diarrhea in individuals new to the area. This observation obscures whether adults have acquired immunity or resistance based on physiologic factors.
Little evidence for immunity
ENTEROHEMORRHAGIC E COLI (EHEC)
Enterohemorrhagic E coli disease and the associated hemolytic uremic syndrome (HUS) are the result of consumption of products from animals colonized with EHEC strains. It is also clear from secondary cases in families during outbreaks that person-to-person transmission also occurs. This disease occurs more in developed than developing countries.
Consumption of contaminated animal products is the main source
EHEC was first recognized when outbreaks of HUS (hemolytic anemia, renal failure, and thrombocytopenia) were linked to a single E coli serotype, O157:H7. Since then, EHEC disease has emerged as an important cause of bloody diarrhea in industrialized nations and retained a remarkable but not exclusive relationship with the O157:H7 serotype. Regional and national outbreaks associated with ground beef, unpasteurized juices, and fresh vegetables often catch the attention of the public, the press, and the government, and at times bring renewed scrutiny to food safety policies and practices in public and private sectors.
✺ Bloody diarrhea and HUS linked to O157:H7
The emergence of EHEC is related to its virulence, low infecting dose, common reservoir (cattle), and changes in the modern food processing industry that provide fresher meat (and bacteria) over wider distribution networks. The infecting dose, estimated to be as low as 100 organisms, is particularly important. This is a level at which food need not come directly from the infected animal, but only be contaminated by it. For example, large modern meat-processing plants can mix EHEC from colonized cattle at one ranch into beef from hundreds of other farms and quickly ship it all over the country. Therefore, the worst outbreaks have been seen in countries with the most advanced food production and distribution systems. If the organisms are ground into hamburger meat, an infecting dose of EHEC may remain even after cooking if the meat is left rare in the middle. Unpasteurized milk carries an obvious risk, but fruits and vegetables have also been the source for EHEC infection. In these instances, the EHEC from the manure of cattle grazing nearby has contaminated fruit in the field. The bacterial dose from a few “drop” apples (those picked up from the ground) included in a batch of cider has been enough to cause disease.
✺ Low infecting dose facilitates transmission
✺ Modern meat-processing facilitates widespread outbreaks
✺ Unpasteurized beverages are another risk
EHEC strains cause the A/E lesions previously described for EPEC, but also produce the Stx toxin. The EHEC pathotype, which first appeared in O157:H7 strains in 1982, is felt to have evolved by an EPEC acquiring the genes for Stx via prophage. Apparently the injection secretion system which creates the A/E pedestals also facilitates delivery of Stx to the enterocyte. Stx secretion is regulated through a quorum-sensing system which waits until there is a critical EHEC population to activate. The interaction of EHEC with enterocytes is much the same as that of EPEC, except that EHEC strains do not form localized microcolonies on the mucosa and have their own adhesive pili (long polar fimbriae [Lpf]), which mediate attachment in the colon rather than the small intestine. The outer membrane protein intimin mediates tight adherence, and the injection secretion system infuses the E coli secretion proteins, which cause alterations in the host cytoskeleton. The genes for these properties are also found in a PAI. The multiple extraintestinal features such as HUS are the result of circulating Stx.
✺ Produce both A/E lesions and Stx
✺ Quorum-sensing regulates Stx
The A/E features alone are sufficient to cause nonbloody diarrhea. On top of this, Stx production causes capillary thrombosis and inflammation of the colonic mucosa, leading to a hemorrhagic colitis. (The distinctive association between shiga toxin and bloody diarrhea has given rise to the term STEC, or shiga toxin-producing E coli, as an alternate pathotype descriptor for EHEC.) Although it has not been detected in the blood of human cases, Stx is presumed to be absorbed across the denuded intestinal mucosa. Circulating Stx binds to renal tissue, where its glycoprotein receptor (globotriaosylceramide, Gb3) is particularly abundant, causing glomerular swelling and the deposition of fibrin and platelets in the microvasculature. How Stx causes hemolysis is less clear; perhaps the erythrocytes are simply damaged as they attempt to traverse the occluded capillaries. Cases and outbreaks caused by Stx-producing E coli of other serotypes are common in many countries.
✺ Stx causes capillary thrombosis and inflammation
✺ Circulating Stx leads to HUS
ENTEROINVASIVE E COLI (EIEC)
The biochemistry, genetics, and pathogenesis of enteroinvasive E coli strains are so close to those of Shigella that our understanding of EIEC disease is generally extrapolated from that genus—EIEC disease is essentially a mild version of shigellosis. Epidemiologically, EIEC infections are primarily seen in children younger than 5 years living in developing nations. The occasional documented outbreaks in industrialized nations are usually linked to contaminated food or water. There is a lower incidence of person-to-person transmission of EIEC, which correlates with the observation that the infecting dose is higher than it is for Shigella. Humans are the only known reservoir.
✺ EIEC closely resemble Shigella
ENTEROAGGREGATIVE E COLI (EAEC)
Enteroaggregative E coli is associated with a protracted (more than 14 days) watery diarrhea that occasionally features blood and mucus. First recognized in infants and children in developing countries, EAEC is increasingly diagnosed in a variety of community settings. EAEC strains are identified by the “stacked-brick” pattern the bacteria make when adhering to cultured mammalian cells. The EAEC pili (aggregative adherence fimbriae [AAF]) mediate tight adherence to the intestinal mucosa, but the A/E lesions of the EPEC and EHEC are not present. The pathogenesis of diarrhea involves formation of a thick mucus–bacteria biofilm on the intestinal surface.
This view of EAEC was dramatically altered by a 2011 German outbreak of serotype O104:H4 initially thought to be caused by EHEC based on clinical features. There were a thousand cases of bloody diarrhea and 53 deaths due to HUS, but the rate of HUS development was twice that typical for EHEC disease. It turned out that the responsible strain had all the features of EAEC with the addition of Stx genes. There was no injection secretion system or A/E lesions. Apparently, the tight adherence of EAEC provided a particularly effective mechanism for delivery of Stx to the intestinal mucosa.
Adherence and biofilm cause diarrhea
✺ Outbreak strain acquired Stx genes
E COLI INFECTIONS: CLINICAL ASPECTS
The most common symptoms of E coli UTI are dysuria and urinary frequency and do not differ significantly in character from those produced by the other less common gram-negative urinary pathogens. If the infection ascends the ureters to produce pyelonephritis, fever and flank pain are common and bacteremia may develop. Although E coli may have enhanced virulence in the production of pneumonia as well as soft tissue and other infections, no clinical features distinguish these cases from those caused by other members of the Enterobacteriaceae.
✺ Dysuria and frequency are features of UTIs
Infections caused by all of the E coli virulence types usually begin with a mild watery diarrhea starting 2 to 4 days after ingestion of an infectious dose. In most instances, the duration of diarrhea is limited to a few days, with the exception of EAEC diarrhea, which can last for weeks. With ETEC and EPEC, the diarrhea remains watery, but with EIEC and EHEC, a dysenteric illness follows. Some EPEC cases may also become chronic. Enterohemorrhagic E coli disease begins like the others but often also includes vomiting. In 90% of cases, this is followed in 1 to 2 days by intense abdominal pain and bloody diarrhea, but fever is not prominent.
If the EHEC diarrhea is bloody, why would fever not be more prominent?
Some EHEC cases develop into a dysentery illness that is less severe than that seen in shigellosis. Colonoscopy reveals edema, hemorrhage, and pseudomembrane formation. Resolution usually takes place over a 3- to 10-day period, with few residual effects on the bowel mucosa.
✺ ETEC and EPEC diarrhea is watery
✺ EIEC and EHEC diarrhea is bloody
Hemolytic uremic syndrome develops as a complication in 5% to 10% of cases of EHEC hemorrhagic colitis, primarily in children under 10 years of age. The disease begins with oliguria, edema, and pallor, progressing to the triad of microangiopathic hemolytic anemia, thrombocytopenia, and renal failure. The systemic effects are often life-threatening, requiring transfusion and hemodialysis for survival. The mortality rate is 5%, and up to 30% of those who survive suffer sequelae such as renal impairment or hypertension.
✺ HUS begins as oliguria and may progress to renal failure
Like the rest of the Enterobacteriaceae, E coli is readily isolated in culture. In UTIs, the bacteria typically reach high numbers (greater than 105/mL), which makes them readily detectable by Gram stain even in an unspun urine specimen (Figure 33–7). For the diagnosis of intestinal disease, separating the virulent E coli from those universally found in stool presents a special problem. A myriad of immunoassay and nucleic acid amplification methods have been described that are able to detect the toxins (LT, ST, Stx) or genes associated with virulence. These methods work but their clinical use is hampered by limited positive predictive value (healthy persons may also have positive test results with these methods) and high cost, especially in developing countries where ETEC, EIEC, EPEC, and EAEC are prevalent. A screening test for EHEC takes advantage of the observation that the O157:H7 serotype typically fails to ferment sorbitol. Incorporating sorbitol in place of lactose in MacConkey agar provides an indicator medium from which suspect (colorless) colonies can be selected and then confirmed with O157 antisera. This procedure has become routine in areas where EHEC is endemic but does not detect the non-O157 EHEC strains.
Bacterial counts in urine are high
Diarrhea requires immunoassay or gene probe
✺ Sorbitol agar screens for O157:H7
Escherichia coli urinary tract infection. The ready observation of the large gram-negative bacilli (arrow) and WBCs in a drop of unspun urine indicates the number of bacteria in the urine is high. (Image contributed by Professor Shirley Lowe, University of California, San Francisco School of Medicine, with permission.)
Think ➱ Apply 33-1. The blood in the stool in EHEC is due to the action of Stx and not due to destruction of enterocytes. It usually takes cellular destruction to cause inflammation and thus fever. This is a feature of shigellosis (see below).
Acute uncomplicated UTIs are often treated empirically. Because of widespread resistance to earlier agents like ampicillin, use of trimethoprim/sulfamethoxazole (TMP-SMX) and fluoroquinolones for this purpose rose steadily. In turn, use of these agents set the stage for the rise of international multidrug-resistant clones like E coli Sequence Type 131, which boasts chromosome-encoded fluoroquinolone resistance and plasmid-borne resistance to TMP-SMX, gentamicin and, not infrequently, extended-spectrum cephalosporins. Thus, in many clinical settings domestically and abroad, E coli resistance to these latter agents has now exceeded the 20% level used to indicate the suitability of antibiotics for empiric use. In cases of empiric treatment failure, selection of other antimicrobials must be guided by antimicrobial susceptibility testing of the patient’s isolate.
Resistance patterns influence antimicrobial selection
Because most E coli diarrheas are mild and self-limiting, treatment is usually not required. When it is, rehydration and supportive measures are the mainstays of therapy, regardless of the causative agent. In the case of EHEC with hemorrhagic colitis and HUS, heroic supportive measures such as hemodialysis or hemapheresis may be required. Treatment with TMP-SMX or fluoroquinolones reduces the duration of diarrhea in ETEC, EIEC, and EPEC infection. Because the risk of HUS may be increased by the use of antimicrobial agents, their use is contraindicated when EHEC is even suspected. Antimotility agents are not helpful and are contraindicated when EIEC or EHEC could be the etiologic agent.
✺ Antibiotics may shorten symptoms for ETEC, EIEC, and EPEC
✺ Antibiotics may increase risk of HUS in EHEC
Traveler’s diarrhea is usually little more than an inconvenience. Because the infecting dose is high, the incidence of the disease can be greatly reduced by eating only cooked foods and peeled fruits and drinking hot or carbonated beverages. Avoiding uncertain water, ice, salads, and raw vegetables is a wise precaution when traveling in developing countries. High-priced hotel accommodations have no protective effect. Chemoprophylaxis against traveler’s diarrhea is not routinely recommended, and in fact may increase risk of intestinal colonization with multidrug-resistant Enterobacteriaceae encountered abroad. Short courses of TMP-SMX or ciprofloxacin (less than 2 weeks) have been recommended for those at high risk for disease resulting from such chronic conditions as achlorhydria, gastric resection, prolonged use of H2 blockers or antacids, and underlying immunosuppressive diseases.
✺ Avoid uncooked foods
✺ Chemoprophylaxis works for defined periods
These public health measures apply equally to EHEC, but here prevention is more difficult because the infecting dose is so low. Cooking hamburgers all the way through is sensible, but no one is recommending abstinence from salads when at home. Recent US recommendations for the irradiation of meats and the extension of pasteurization requirements to fruit juices are designed largely to stem the spread of EHEC.
✺ Rare hamburgers carry risk for EHEC