Chapter 26 Infections of the gastrointestinal tract
In healthy, fasting individuals, the stomach is either sterile or may contain only a few organisms, because of its low pH and enzymes. The diet has a major effect on the gut flora. The small intestine may be colonized with streptococci, lactobacilli and yeasts (especially Candida albicans); the proportions of these and other organisms vary, depending on dietary habits. In the ileum, a typical Gram-negative flora (e.g. Bacteroides spp. and Enterobacteriaceae) is seen, and the large intestine has a dense population of varied flora. These include members of the Enterobacteriaceae, Enterococcus faecalis, Bacteroides spp., Clostridium spp., bifidobacteria and anaerobic streptococci. The anaerobes outweigh the aerobes by far and comprise the vast majority of the bacteria in the large intestine. Roughly 20% of the faeces contains bacteria, approximately 1011 organisms per gram. As in the oral cavity, the gastrointestinal tract harbours a vast multitude of unculturable, and yet to be discovered organisms.
Gastrointestinal infections are a major cause of morbidity and mortality worldwide. For example, recent studies have revealed that, globally, severe diarrhoea and dehydration are responsible each year for the death of 1 575 000 children under the age of 5. Most of these diseases are preventable and are caused by poor food, and personal hygiene, poor sanitation and lack of quality pipeborne water systems.
A diverse array of infections of the gastrointestinal tract is caused by an equally varied population of microbial agents (Fig. 26.1). The agents of diarrhoeal diseases, including those that are considered common agents of food poisoning, are listed in Table 26.1. The common bacterial diarrhoeal diseases in the developed world include those caused by:
Campylobacter coli and Campylobacter jejuni are among the most common diarrhoea-inducing agents in the western world. They are curved, slender, Gram-negative bacilli present in the gut as well as in the oral cavity.
Symptoms vary from mild to severe, with any part of the small or large intestine affected. Dogs and cats are probable sources of infection, but mass-produced poultry is the most common source. Eating contaminated food is a common cause of infection; note that campylobacters do not multiply in food. Patients may become symptomless carriers after recovery.
Shigella causes bacillary dysentery, as opposed to amoebic dysentery caused by intestinal amoebae. It is an important cause of morbidity and death in young children, particularly in the developing world.
Infection is by ingestion of organisms. Once ingested, the bacteria attach to the mucosal villus epithelium, enter and multiply in these cells. The resultant death of the infected cells initiates an inflammatory reaction in the submucosa and lamina propria. Finally, necrosis and ulceration of the villus epithelium ensue, making the stools bloody and mucous. This type of severe reaction is usually due to Shigella dysenteriae, which is known to produce a potent enterotoxin and a cytotoxin. This infection may be life-threatening.
Dysentery due to other shigellae is generally milder and varies from asymptomatic excretion to a severe attack of diarrhoea with abdominal pain. Shigella sonnei is the usual agent of dysentery in the UK, while Shigella boydii is common in the Middle East and South-East Asia.
Spread of the disease is from hand to mouth. It usually occurs in nursery schools where the index case (i.e. the person with the disease) contaminates hands at the toilet, and further contaminates lavatory handles and hand towels if personal hygiene is deficient. Subsequent handling of these bacteria-laden fomites (inanimate surfaces acting as vehicles of disease transfer) by healthy individuals results in hand-to-mouth transmission of the agents, leading to the disease. Thus, ‘food, flies and fomites’ are classical means of spread.
The diagnosis is made by examination of stool sample and culture on MacConkey’s agar and selective media such as desoxycholate-citrate agar (DCA). Pale, non-lactose-fermenting (NLF) colonies are then isolated and identified by biochemical tests; serological identification is performed subsequently.
A large number of different Salmonella species exist, together with an even more bewildering number (about 1500) of serotypes. Of these, about 14 are important pathogens. The common diarrhoea-causing organism is Salmonella typhimurium. The other major pathogens of this group are Salmonella typhi and Salmonella paratyphi-A, -B and -C, which cause enteric fever, a septicaemic illness in which diarrhoea is a late feature of the disease.
The genesis of salmonella food poisoning is ill understood. Patients have mild gastrointestinal disturbances with an incubation period of about 1–2 days. Abdominal pain, diarrhoea (with or without fever) and vomiting are commonly present. Septicaemia is rare.
The organism is found in domestic animals and poultry and is spread via the faecal–oral route. On entering the gastrointestinal tract, the salmonellae may either produce an enterotoxin (similar to toxigenic E. coli) or invade the mucosa of small intestine (like shigellae).
Examination of stool sample and culture on MacConkey (indicator) medium and selective media such as DCA or Wilson–Blair medium; pale, NLF colonies on MacConkey medium and black, shiny colonies on Wilson–Blair medium. Subsequent identification is by biochemical tests and determination of serological status. The major antigens that are useful for the serotyping of salmonellae are the ‘O’ (somatic or body antigen) and the ‘H’ (flagellar) antigens.
Prevention includes control of animal food quality, good farming and abattoir practices, rigorous kitchen hygiene and good personal hygiene among food handlers, and exclusion of known human carriers (‘excretors’) from food handling. However, the best form of prevention is thorough cooking of food and avoidance of consumption of raw or partly cooked eggs and other animal-derived food.
E. coli is a normal commensal of the gastrointestinal tract, but certain strains, for some unknown reason, can behave as pathogens. As described in Chapter 15, they produce enterotoxins, and the enteroinvasive strains have the ability to invade the gut mucosa.
Accompanied by acute and profuse diarrhoea, this infection has an incubation period of 1–3 days. The disease is mainly caused by enteropathogenic E. coli (EPEC), but in a minority of cases, enterotoxigenic E. coli (ETEC) strains contribute (Chapter 15). It is common in the developing world because of poor sanitation and poverty; infection spreads directly from case to case and via fomites (see above for shigellae), and in some cases, the mother may be the source of infection.