Diarrhea is the most common outcome of gastrointestinal tract infection

Infections of the gastrointestinal tract range in their effects from a mild self-limiting attack of ‘the runs’ to severe, sometimes fatal, diarrhea. There may be associated vomiting, fever and malaise. Diarrhea is the result of an increase in fluid and electrolyte loss into the gut lumen, leading to the production of unformed or liquid faeces and can be thought of as the method by which the host forcibly expels the pathogen (and in doing so, aids its dissemination). However, diarrhea also occurs in many non-infectious conditions, and an infectious cause should not be assumed.

In the resource-poor world, diarrheal disease is a major cause of mortality in children

In the resource-poor world, diarrheal disease is a major cause of morbidity and mortality, particularly in young children. In the resource-rich world, it remains a very common complaint, but is usually mild and self-limiting except in the very young, the elderly and immunocompromised patients. Most of the pathogens listed in Table 22.1 are found throughout the world, but some, such as Vibrio cholerae, have a more limited geographic distribution. However, such infections can be acquired by travellers to these areas and imported into their home countries.

Many cases of diarrheal disease are not diagnosed, either because they are mild and self-limiting and the patient does not seek medical attention, or because medical and laboratory facilities are unavailable, particularly in resource-poor countries. It is generally impossible to distinguish on clinical grounds between infections caused by the different pathogens. However, information about the patient’s recent food and travel history, and macroscopic and microscopic examination of the faeces for blood and pus can provide helpful clues. A precise diagnosis can only be achieved by laboratory investigations. This is especially important in outbreaks, because of the need to instigate appropriate epidemiologic investigations and control measures.

There are six distinct groups of E. coli with different pathogenetic mechanisms

Initially, all diarrhea-associated Escherichia coli were termed enteropathogenic E. coli (EPEC). However, greater insight into mechanisms of pathogenicity has led to specific group designations: enteropathogenic E. coli (EPEC), enterotoxigenic E. coli (ETEC), enterohaemorrhagic E. coli (EHEC), enteroinvasive E. coli (EIEC), enteroaggregative E. coli (EAEC), and diffuse-aggregative E. coli (DAEC).

Enteropathogenic E. coli (EPEC) do not appear to make any toxins

They do produce bundle-forming pili (Bfp), intimin (an adhesin) and an associated protein (translocated intimin receptor, Tir). These virulence factors allow bacterial attachment to epithelial cells of the small intestine, leading to disruption of the microvillus (an ‘attaching–effacing’ mechanism of action; Table 22.2; Fig. 22.3) leading to diarrhea (Table 22.3).

Figure 22.3 Electron micrograph of enteropathogenic E. coli adhering to the brush border of intestinal mucosal cells with localized destruction of microvilli.

(Courtesy of S. Knutton.)

Table 22.3 The clinical features of bacterial diarrheal infection

Enterotoxigenic E. coli (ETEC) possess colonization factors (fimbrial adhesins)

These bind the bacteria to specific receptors on the cell membrane of the small intestine (Table 22.2; Fig. 22.4). These organisms produce powerful plasmid-associated enterotoxins which are characterized as being either heat labile (LT) or heat stable (ST):

Figure 22.4 Electron micrograph of enterotoxin E. coli, showing pili necessary for adherence to mucosal epithelial cells.

(Courtesy of S. Knutton.)

Enterohaemorrhagic E. coli (EHEC) isolates produce a verotoxin

The verotoxin (i.e. toxic to tissue cultures of ‘vero’ cells) is essentially identical to Shiga (Shigella) toxin. After attachment to the mucosa of the large intestine (by the ‘attaching– effacing’ mechanism also seen in EPEC), the produced toxin has a direct effect on intestinal epithelium, resulting in diarrhea (Table 22.3). EHEC cause haemorrhagic colitis (HC) and haemolytic-uraemic syndrome (HUS). In HC, there is destruction of the mucosa and consequent haemorrhage; this may be followed by HUS. Verotoxin receptors have been identified on renal epithelium and may account for kidney involvement. While there are many serotypes of EHEC, the most common is O157:H7.

Enteroinvasive E. coli (EIEC) attach specifically to the mucosa of the large intestine

They invade the cells by endocytosis by using plasmid-associated genes. Inside the cell, they lyse the endocytic vacuole, multiply and spread to adjacent cells, causing tissue destruction, inflammation, necrosis and ulceration, resulting in blood and mucus in stools (Tables 22.2, 22.3).

Enteroaggregative E. coli (EAEC) derive their name from their characteristic attachment pattern to tissue culture cells

The pattern is an aggregative or ‘stacked brick’ formation. These organisms act in the small intestine to cause persistent diarrhea, especially in children in resource-poor countries. Their aggregative adherence ability is due to plasmid-associated fimbriae. EAEC also produce heat-labile toxins (an enterotoxin and a toxin related to E. coli haemolysin) but their role in diarrheal disease is uncertain.

Diffuse-aggregative E. coli (DAEC) produce an alpha haemolysin and cytotoxic necrotizing factor 1

They are also known as diffuse-adherent or cell-detaching E. coli. Their role in diarrheal disease, especially in young children, is incompletely understood and somewhat controversial, with some studies reporting no association.

EPEC and ETEC are the most important contributors to global incidence of diarrhea, while EHEC is more important in resource-rich countries

The diarrhea produced by E. coli varies from mild to severe, depending upon the strain and the underlying health of the host. ETEC diarrhea in children in resource-poor countries may be clinically indistinguishable from cholera. EIEC and EHEC strains both cause bloody diarrhea (Table 22.3). Following EHEC infection, HUS is characterized by acute renal failure (Fig. 22.5), anaemia and thrombocytopenia, and there may be neurologic complications. HUS is the most common cause of acute renal failure in children in the UK and USA. Although E. coli O157:H7 is the most commonly recognized serotype involved in HUS, E. coli 0104:H4, that had not been reported as causing an outbreak previously, caused a significant outbreak of HUS and bloody diarrhea in 15 countries across Europe in 2011. Over several months starting in May 2011, 860 individuals with HUS and over 3000 with bloody diarrhea were reported in Germany, many of whom had laboratory confirmed E. coli 0104:H4 infection. More than 50 people died and the likely vehicle was sprouted beans imported from the Middle East.

Figure 22.5 Verotoxin-producing E. coli infection, showing fibrin ‘thrombi’ in glomerular capillaries in haemolytic–uraemic syndrome. (Weigert stain.)

(Courtesy of H.R. Powell.)

Specific tests are needed to identify strains of pathogenic E. coli

Because E. coli is a member of the normal gastrointestinal flora, specific tests are required to identify strains that may be responsible for diarrheal disease. These are summarized in Table 22.2. Infections are more common in children and are also often travel-associated, and these factors should be considered when samples are received in the laboratory. It is important to note that specialized tests beyond routine stool cultures are required to identify specific diarrhea-associated E. coli types. Such tests are not ordinarily performed with uncomplicated diarrhea, which is usually self-limiting. However, concern regarding EHEC (e.g. bloody diarrhea) has led most laboratories in resource-rich countries to screen for E. coli O157:H7.

Antibacterial therapy is not indicated for E. coli diarrhea

Specific antibacterial therapy is not indicated. Fluid replacement may be necessary, especially in young children. Treatment of HUS is urgent and may involve dialysis.

Provision of a clean water supply and adequate systems for sewage disposal are fundamental to the prevention of disease. Food and unpasteurized milk can be important vehicles of infection, especially for EIEC and EHEC, but there is no evidence of an animal or environmental reservoir.

Salmonellae are the most common cause of food-associated diarrhea in many resource-rich countries

However, in some countries such as the USA and UK, they have been relegated to second place by Campylobacter. Like E. coli, the salmonellae belong to the family Enterobacteriaceae. Historically, salmonella nomenclature has been somewhat confusing, with more than 2000 serotypes defined on the basis of differences in the cell wall (O) and flagellar (H) antigens (Kauffmann–White scheme). However, DNA hybridization studies indicate that there are only two species, the most important of which, for human infection, is Salmonella enterica. To simplify discussion and comparison, past convention has been to replace this species name with the serotype designation. While technically incorrect (the serotype is not a species), this practice is helpful when discussing interrelationships between different isolates, e.g. in epidemiologic analysis when tracing the source of an outbreak. This convention is thus followed here to maintain continuity with other scientific literature.

All salmonellae except for Salmonella typhi and S. paratyphi are found in animals as well as humans. There is a large animal reservoir of infection, which is transmitted to humans via contaminated food, especially poultry and dairy products (Fig. 22.6). Water-borne infection is less frequent. Salmonella infection is also transmitted from person to person, and secondary spread can therefore occur, for example, within a family after one member has become infected after consuming contaminated food.

Figure 22.6 The recycling of salmonellae. With the exception of Salmonella typhi, salmonellae are widely distributed in animals, providing a constant source of infection for humans. Excretion of large numbers of salmonellae from infected individuals and carriers allows the organisms to be ‘recycled’.

Salmonellae are almost always acquired orally in food or drink that is contaminated

Diarrhea is produced as a result of invasion by the salmonellae of epithelial cells in the terminal portion of the small intestine (Fig. 22.7). Initial entry is probably through uptake by M cells (the ‘antigenic samplers’ of the bowel) with subsequent spread to epithelial cells. A similar route of invasion occurs in Shigella, Yersinia and reovirus infections. The bacteria migrate to the lamina propria layer of the ileocaecal region, where their multiplication stimulates an inflammatory response, which both confines the infection to the gastrointestinal tract and mediates the release of prostaglandins. These in turn activate cyclic adenosine monophosphate (cAMP) and fluid secretion, resulting in diarrhea.

Figure 22.7 The passage of salmonellae through the body. The vast majority of salmonellae cause infection localized to the gastrointestinal tract and do not invade beyond the gut mucosa. cAMP, cyclic adenosine monophosphate.

Species of Salmonella that normally cause diarrhea (e.g. S. enteritidis, S. choleraesuis) may become invasive in patients with particular predispositions (e.g. children, immunocompromised patients or those with sickle cell anaemia). The organisms are not contained within the gastrointestinal tract, but invade the body to cause septicaemia; consequently, many organs become seeded with salmonellae, sometimes leading to osteomyelitis, pneumonia or meningitis.

In the vast majority of cases, Salmonella spp. cause an acute but self-limiting diarrhea, though in the young and the elderly the symptoms may be more severe. Vomiting is also common with enterocolitis, while fever is usually a sign of invasive disease (Table 22.3). S. typhi and S. paratyphi invade the body from the gastrointestinal tract to cause systemic illness and are discussed in a later section.

Salmonella diarrhea can be diagnosed by culture on selective media

The methods for culturing faecal specimens on selective media are summarized in the Appendix. The organisms are not fastidious and can usually be isolated within 24    h, although small numbers may require enrichment in selenite broth before culture. Preliminary identification can be made rapidly, but the complete result, including serotype, takes at least 48    h.

Fluid and electrolyte replacement may be needed for salmonella diarrhea

Diarrhea is usually self-limiting and resolves without treatment. Fluid and electrolyte replacement may be required, particularly in the very young and the elderly. Unless there is evidence of invasion and septicaemia, antibiotics should be positively discouraged because they do not reduce the symptoms or shorten the illness, and may prolong excretion of salmonellae in the faeces. There is some evidence that symptomatic treatment with drugs that reduce diarrhea has the same adverse effect.

Salmonellae may be excreted in the faeces for several weeks after a salmonella infection

Figure 22.6 illustrates the problems associated with the prevention of salmonella infections. The large animal reservoir makes it impossible to eliminate the organisms, and preventive measures must therefore be aimed at ‘breaking the chain’ between animals and humans, and from person to person. Such measures include:

Following an episode of salmonella diarrhea, an individual can continue to carry and excrete organisms in the faeces for several weeks. Although in the absence of symptoms, the organisms will not be dispersed so liberally into the environment, thorough handwashing before food handling is essential. People employed as food handlers are excluded from work until three specimens of faeces have failed to grow salmonella.

Campylobacter

Campylobacter infections are among the most common causes of diarrhea

Campylobacter spp. are curved or S-shaped Gram-negative rods (Fig. 22.8). They have long been known to cause diarrheal disease in animals, but are also one of the most common causes of diarrhea in humans. The delay in recognizing the importance of these organisms was due to their cultural requirements, which differ from those of the enterobacteria as they are microaerophilic and thermophilic (growing well at 42°C); they do not therefore grow on the media used for isolating E. coli and salmonellae. Several species of the genus Campylobacter are associated with human disease, but Campylobacter jejuni is by far the most common. Helicobacter pylori, previously classified as Campylobacter pylori, is an important cause of gastritis and gastric ulcers (see below).

Figure 22.8 Campylobacter jejuni infection. Gram stain showing Gram-negative, S-shaped bacilli.

(Courtesy of I. Farrell.)

As with salmonellae, there is a large animal reservoir of Campylobacter in cattle, sheep, rodents, poultry and wild birds. Infections are acquired by consumption of contaminated food, especially poultry, milk or water. Studies have shown an association between infection and consumption of milk from bottles with tops that have been pecked by wild birds. Household pets such as dogs and cats can become infected and provide a source for human infection, particularly for young children. Person-to-person spread by the faecal–oral route is rare, as is transmission from food handlers.

Campylobacter diarrhea is clinically similar to that caused by other bacteria such as salmonella and shigella

The gross pathology and histologic appearances of ulceration and inflamed bleeding mucosal surfaces in the jejunum, ileum and colon (Fig. 22.9) are compatible with invasion of the bacteria, but the production of cytotoxins by C. jejuni has also been demonstrated. Invasion and bacteraemia are not uncommon, particularly in neonates and debilitated adults.

Figure 22.9 Inflammatory enteritis caused by Campylobacter jejuni, involving the entire mucosa, with flattened atrophic villi, necrotic debris in the crypts and thickening of the basement membrane. (Cresyl-fast violet stain.)

(Courtesy of J. Newman.)

The clinical presentation is similar to that of diarrhea caused by salmonellae and shigella, although the disease may have a longer incubation period and a longer duration. The key features are summarized in Table 22.3.

Cultures for Campylobacter should be set up routinely in every investigation of a diarrheal illness

The methods are described in the Appendix, but it is important to note that the media and conditions for growth differ from those required for the enterobacteria. Growth is often somewhat slow compared with that of the enterobacteria, but a presumptive identification should be available within 48    h of culture.

Erythromycin is used for severe Campylobacter diarrhea

Macrolide antibiotics such as erythromycin can be used in diarrheal disease that is severe enough to warrant treatment. Invasive infections may require treatment with an additional antibiotic such as a quinolone or an aminoglycoside.

The preventive measures for salmonella infections described above are equally applicable to the prevention of Campylobacter infections, but there are no requirements for the screening of food handlers because contamination of food by this route is very uncommon.

Cholera flourishes in communities with inadequate clean drinking water and sewage disposal

The disease remains endemic in SE Asia and parts of Africa and South America. Unlike salmonellae and Campylobacter, V. cholerae is a free-living inhabitant of fresh water, but causes infection only in humans. Asymptomatic human carriers are believed to be a major reservoir. The disease is spread via contaminated food; shellfish grown in fresh and estuarine waters have also been implicated. Direct person-to-person spread is thought to be uncommon. Therefore, cholera continues to flourish in communities where there is absent or unreliable provision of clean drinking water and sewage disposal. Cases still occur in resource-rich countries (e.g. the Gulf Coast of Louisiana and Texas in the USA), but high standards of hygiene mean that secondary spread should not occur.

V. cholerae serotypes are based on somatic (O) antigens

Serotype O1 is the most important and is further divided into two biotypes: classical and El Tor (Fig. 22.11). The El Tor biotype, named after the quarantine camp where it was first isolated from pilgrims returning from Mecca, differs from classical V. cholerae in several ways. In particular, it causes only mild diarrhea and has a higher ratio of carriers to cases than classic cholera; carriage is also more prolonged, and the organisms survive better in the environment. The El Tor biotype, which was responsible for the seventh pandemic, has now spread throughout the world and has largely displaced the classic biotype.

Figure 22.11 Vibrio cholerae serotype O1, the cause of cholera, can be subdivided into different biotypes with different epidemiologic features, and into sero-subgroups and phage types for the purposes of investigating outbreaks of infection. Although V. cholerae is the most important pathogen of the genus, other species can also cause infections of both the gastrointestinal tract and other sites.

In 1992, a new non-O1 strain (O139) arose in south India. It spread rapidly, infected O1-immune individuals, caused epidemics, and was the eighth pandemic strain of cholera. V. cholerae O139 appeared to have originated from the El Tor O1 biotype when the latter acquired a new O (capsular) antigen by horizontal gene transfer from a non-O1 strain. This provided the recipient strain with a selective advantage in a region where a large part of the population was immune to O1 strains.

Other species of Vibrio cause a variety of infections in humans (Fig. 22.11). V. parahaemolyticus is another cause of diarrheal disease, but this is usually much less severe than cholera (see below).

The symptoms of cholera are caused by an enterotoxin

The symptoms of cholera are entirely due to the production of an enterotoxin in the gastrointestinal tract (see Ch. 17). However, the organism requires additional virulence factors to enable it to survive the host defences and adhere to the intestinal mucosa. These are illustrated in Figure 22.12 (see also Ch. 13).

Figure 22.12 The production of an enterotoxin is central to the pathogenesis of cholera, but the organisms must possess other virulence factors to allow them to reach the small intestine and to adhere to the mucosal cells.

The clinical features of cholera are summarized in Table 22.3. The severe watery non-bloody diarrhea is known as rice water stool because of its appearance (Fig. 22.13) and can result in the loss of 1    L of fluid every hour. It is this fluid loss and the consequent electrolyte imbalance that results in marked dehydration, metabolic acidosis (loss of bicarbonate), hypokalaemia (potassium loss) and hypovolaemic shock resulting in cardiac failure. Untreated, the mortality from cholera is 40–60%; rapidly instituted fluid and electrolyte replacement reduces the mortality to <    1%.

Figure 22.13 Rice water stool in cholera.

(Courtesy of A.M. Geddes.)

Culture is necessary to diagnose sporadic or imported cases of cholera and carriers

In countries where cholera is prevalent, diagnosis is based on clinical grounds, and laboratory confirmation is rarely sought. It is worth remembering that ETEC infection can resemble cholera in both its severity and the management of infected individuals, as fluid and electrolyte replacement are of paramount importance. The methods are given in the Appendix.

Prompt rehydration with fluids and electrolytes is central to the treatment of cholera

Oral or intravenous rehydration may be used. Antibiotics are not necessary, but tetracycline may be given, as some evidence indicates that this reduces the time of excretion of V. cholerae thereby reducing the risk of transmission. There have, however, been reports of tetracycline-resistant V. cholerae in some areas.

As with other diarrheal disease, a clean drinking water supply and adequate sewage disposal are fundamental to the prevention of cholera. As there is no animal reservoir, it should in theory be possible to eliminate the disease. However, carriage in humans, albeit for only a few weeks, occurs in 1–20% of previously infected patients, making eradication difficult to achieve.

Cholera vaccines are not recommended for most travellers

A killed whole-cell vaccine is available and is given parenterally, but is effective in only about 50% of those vaccinated, with protection lasting for only 3–6    months. It is no longer recommended by the World Health Organization (WHO) for travellers to cholera-endemic areas, although it may be required in certain countries. Oral vaccines (not available in the USA) appear to provide somewhat better protection.

Symptoms of Shigella infection range from mild to severe depending upon the infecting species

Shigellosis is also known as bacillary dysentery (in contrast to amoebic dysentery; see below) because in its more severe form it is characterized by an invasive infection of the mucosa of the large intestine, causing inflammation and resulting in the presence of pus and blood in the diarrheal stool. However, symptoms range from mild to severe depending upon the species of Shigella involved and on the underlying state of health of the host. There are four species:

Shigellosis is primarily a paediatric disease. When associated with severe malnutrition it may precipitate complications such as the protein deficiency syndrome ‘kwashiorkor’. Like V. cholerae, shigellae are human pathogens without an animal reservoir, but unlike the vibrios, they are not found in the environment, being spread from person to person by the faecal–oral route and less frequently by contaminated food and water. Shigellae appear to be able to initiate infection from a small infective dose (10–100 organisms) and therefore spread is easy in situations where sanitation or personal hygiene may be poor (e.g. refugee camps, nurseries, daycare centres and other residential institutions).

Shigella diarrhea is usually watery at first, but later contains mucus and blood

Shigellae attach to, and invade, the mucosal epithelium of the distal ileum and colon, causing inflammation and ulceration (Fig. 22.14). However, they rarely invade through the gut wall to the bloodstream. S. dysenteriae produce a (Shiga) toxin similar to that associated with enterohaemorrhagic E. coli (EHEC; see above), which can cause damage to the intestinal epithelium and glomerular endothelial cells, the latter leading to kidney failure (haemolytic-uraemic syndrome, HUS; see above).

Figure 22.14 Shigellosis. Histology of the colon showing disrupted epithelium covered by pseudomembrane and interstitial infiltration. Mucin glands have discharged their contents and the goblet cells are empty. E, epithelium; I, interstitial infiltration; M, mucin in glands; P, pseudomembrane (colloidal iron stain).

(Courtesy of R.H. Gilman.)

The main features of shigella infection are summarized in Table 22.3. Diarrhea is usually watery initially, but later contains mucus and blood. Lower abdominal cramps can be severe. The disease is usually self-limiting, but dehydration can occur, especially in the young and elderly. Complications can be associated with malnutrition (see above).

Antibiotics should only be given for severe shigella diarrhea

Rehydration may be indicated. Antibiotics, especially those that also decrease intestinal motility, should not be given except in severe cases. Plasmid-mediated resistance is common, and antibiotic susceptibility tests should be performed on shigella isolates if treatment is required.

Education in personal hygiene and proper sewage disposal are important. Infected individuals may continue to excrete shigellae for a few weeks, but longer-term carriage is unusual; therefore, with adequate public health measures and no animal reservoir, the disease is potentially eradicable.

Other bacterial causes of diarrheal disease

The pathogens described in the previous sections are the major bacterial causes of diarrheal disease. Salmonella and Campylobacter infections and some types of E. coli infections are most often food-associated, whereas cholera is more often water-borne and shigellosis is usually spread by direct faecal–oral contact. Other bacterial pathogens that cause food-associated infection or food poisoning are described below.

V. parahaemolyticus and Yersinia enterocolitica are food-borne Gram-negative causes of diarrhea

V. parahaemolyticus is a halophilic (salt-loving) vibrio that contaminates seafood and fish. If these foods are consumed uncooked, diarrheal disease can result. The mechanism of pathogenesis is still unclear. Most strains associated with infection are haemolytic due to production of a heat-stable cytotoxin and have been shown to invade intestinal cells (in contrast to V. cholerae, which is non-invasive and cholera toxin, which is not cytotoxic).

The clinical features of infection are summarized in Table 22.3. The methods used for the laboratory diagnosis of V. parahaemolyticus infection are given in the Appendix (e.g. special media for cultivation). Prevention of infection depends upon cooking fish and seafood properly.

Yersinia enterocolitica is a member of the Enterobacteriaceae and is a cause of food-associated infection especially among infants and particularly in colder parts of the world. The reason for this geographic distribution is unknown, but it has been speculated that it is because the organism prefers to grow at temperatures of 22–25    °C. Y. enterocolitica is found in a variety of animal hosts including rodents, rabbits, pigs, sheep, cattle, horses and domestic pets. Transmission to humans from household dogs has been reported. The organism survives and multiplies, albeit more slowly, at refrigeration temperatures (48    °C) and has been implicated in outbreaks of infection associated with contaminated milk as well as other foods.

The mechanism of pathogenesis is unknown, but the clinical features of the disease result from invasion of the terminal ileum, necrosis in Peyer’s patches and an associated inflammation of the mesenteric lymph nodes (Fig. 22.15). The presentation, with enterocolitis and often mesenteric adenitis, can easily be confused with acute appendicitis, particularly in children. The clinical features are summarized in Table 22.3. The laboratory diagnosis is outlined in the Appendix. As with V. parahaemolyticus, an indication of a suspicion of yersinia infection is useful so that the laboratory staff can process the specimen appropriately.

Figure 22.15 Yersinia enterocolitica infection of the ileum, showing superficial necrosis of the mucosa and ulceration.

(Courtesy of J. Newman.)

Clostridium perfringens and Bacillus cereus are spore-forming Gram-positive causes of diarrhea

The Gram-negative organisms described in the previous sections invade the intestinal mucosa or produce enterotoxins, which cause diarrhea. None of these organisms produces spores. Two Gram-positive species are important causes of diarrheal disease, particularly in association with spore-contaminated food. These are Clostridium perfringens and Bacillus cereus.

Cl. perfringens is associated with diarrheal diseases in different circumstances, and the pathogenesis is summarized in Figure 22.16:

Figure 22.16 Clostridium perfringens is linked with two forms of food-associated infection. The common, enterotoxin-mediated infection (left) is usually acquired by eating meat or poultry that has been cooked enough to kill vegetative cells, but not spores. As the food cools, the spores germinate. If reheating before consumption is inadequate (as it often is in mass catering outlets), large numbers of organisms are ingested. The rare form associated with β-toxin-producing strains (right) causes a severe necrotizing disease.

The clinical features of the common type of infection are shown in Table 22.3. The laboratory investigation of suspected Cl. perfringens infection is outlined in the Appendix. The organism is an anaerobe and grows readily on routine laboratory media. Enterotoxin production can be demonstrated by a latex agglutination method.

Antibacterial treatment of Cl. perfringens diarrhea is rarely required. Prevention depends on thorough reheating of food before serving, or preferably avoiding cooking food too long before consumption.

Cl. perfringens is also an important cause of wound and soft tissue infections, as described in Chapter 26.

Bacillus cereus spores and vegetative cells contaminate many foods, and food-associated infection takes one of two forms:

Two different toxins are involved, as illustrated in Figure 22.17. The clinical features of the infections are summarized in Table 22.3. Laboratory confirmation of the diagnosis requires specific media as described in the Appendix. The emetic type of disease may be difficult to assign to B. cereus unless the incriminated food is cultured.

Figure 22.17 Bacillus cereus can cause two different forms of food-associated infection. Both involve toxins.

As with Cl. perfringens, prevention of B. cereus food-associated infection depends upon proper cooking and rapid consumption of food. Specific antibacterial treatment is not indicated.

Treatment with broad-spectrum antibiotics can be complicated by antibiotic associated Cl. difficile diarrhea

All the infections described so far arise from the ingestion of organisms or their toxins. However, diarrhea can also arise from disruption of the normal gut flora. Even in the early days of antibiotic use, it was recognized that these agents affected the normal flora of the body as well as attacking the pathogens. For example, orally administered tetracycline disrupts the normal gut flora, and patients sometimes become recolonized not with the usual facultative Gram-negative anaerobes but with Staphylococcus aureus, causing enterocolitis, or with yeasts such as Candida. Soon after clindamycin was introduced for therapeutic use, it was found to be associated with severe diarrhea in which the colonic mucosa became covered with a characteristic fibrinous pseudomembrane (pseudomembranous colitis; Fig. 22.18). However, clindamycin is not the cause of the condition; it merely inhibits the normal gut flora and allows Cl. difficile to multiply. This organism is commonly found in the gut of children and to a lesser extent in adults, but can also be acquired from other patients in hospital by cross-infection. Cl. difficile is a spore former and survives in the environment as it is resistant to heat and acid, for example. The spores contaminate the environment and become vegetative bacteria that can be transmitted between patients on the wards. In common with other clostridia, Cl. difficile produces exotoxins, two of which have been characterized: one is a cytotoxin and the other an enterotoxin that cause haemostasis and tissue necrosis in the colon, resulting in diarrhea.

Figure 22.18 Antibiotic-associated colitis due to Clostridium difficile. Sigmoidoscopic view showing multiple pseudomembranous lesions.

(Courtesy of J. Cunningham.)

Toxin A and toxin B are encoded within a short chromosomal segment carried by pathogenic strains of Cl. difficile, referred to as the pathogenicity locus, as is a regulatory gene tcdC. There is also a binary toxin encoded by two chromosomal genes separate from the chromosomal pathogenicity locus. One gene mediates cell surface binding and intracellular translocation and the other causes cell death.

An emergent epidemic Cl. difficile variant strain called Cl. difficile 027 has been shown to produce more toxin A and toxin B than most hospital strains. A study reported that the binary toxin genes were associated with partial deletions in the tcdC gene that down-regulates the toxin A and B genes, and that severe Cl. difficile-associated diarrhea was significantly associated with them. Finally, Cl. difficile 027 was associated with much higher levels of toxins A and B than in other strains. This strain detected in the USA, Canada, the UK and other parts of Europe is not only highly transmissible but causes more severe disease in individuals in both hospitals and the community. It has been associated with higher case fatality rates, with some infected individuals requiring a colectomy and intensive care unit support, and has also been shown to be more resistant to the fluoroquinolone antibiotics than other strains.

Although initially associated with clindamycin, Cl. difficile diarrhea has since been shown to follow therapy with many other broad-spectrum antibiotics; hence the term antibiotic-associated diarrhea or colitis. The infection is often severe and may require treatment with the anti-anaerobic agent metronidazole, or with oral vancomycin. However, the emergence of vancomycin-resistant enterococci, probably originating in the gut flora, has led to the recommendation that oral vancomycin be avoided wherever possible (see Ch. 33).