The following points highlight the three main types of bacterial food poisoning. The types are: 1. Ingestion of Pre-Formed Toxin 2. Non-Invasive Infection 3. Invasive Infection.
Type # 1. Ingestion of Pre-Formed Toxin:
Toxins may be produced in and ingested with the food as in Staphylococcus aureus food poisoning and the Bacillus cereus emetic syndrome. Botulism is similar in this respect though in this case gastrointestinal symptoms are of minor importance.
The absence of person to person spread and a relatively short incubation period between ingestion of food and the onset of symptoms are usual characteristics of this type of food poisoning.
Type # 2. Non-Invasive Infection:
In a non-invasive infection, viable bacteria ingested with food, colonize the intestinal lumen. This is principally associated with the small intestine where competition from the endogenous microflora is less intense. To prevent their removal by the flushing action of the high flow rates in this section of the gut, the pathogen generally attaches to and colonizes the epithelial surface.
It does this by producing adhesins, molecules often associated with fimbriae on the bacterial cell surface, which recognize and attach to specific receptor sites on the microvilli. Loss of the ability to adhere to the gut wall will dramatically reduce a pathogen’s virulence – its ability to cause illness.
Once attached, the pathogen produces a protein enterotoxin which acts locally in the gut changing the flow of electrolytes and water across the mucosa from one of absorption to secretion. Several enterotoxins act by stimulating enterocytes (the cells lining the intestinal epithelium) to over-produce cyclic nucleotides.
Most extensively studied in this respect is the cholera toxin produced by Vibrio cholerae. The toxin (MW 84 000) comprises five B subunits and a single A subunit. The B subunits bind to specific ganglioside (an acidic glycolipid) receptors on the enterocyte surface. This creates a hydrophilic channel in the cell membrane through which the A unit can pass.
Once inside the cell, a portion of the A unit acts enzymically to transfer an ADP-ribosyl group derived from cellular NAD to a protein regulating the activity of the enzyme adenylate cyclase. As a result, the enzyme is locked into its active state leading to an accumulation of cyclic adenosine monophosphate (cAMP) which inhibits absorption of Na+ and Cl– ions while stimulating the secretion of Cl–, HCO3– and Na+ ions.
To maintain an osmotic balance, the transfer of electrolytes is accompanied by a massive outflow of water into the intestinal lumen. This far exceeds the absorptive capacity of the large intestine and results in a profuse watery diarrhoea (Figure 6.7).
A number of other enterotoxins have been shown to act in the same way as the cholera toxin including the heat labile toxin (LT) produced by some types of entero-toxigenic E. coli.
Other toxins such as the heat stable toxin of E. coli are similar in the respect that they stimulate the production of a cyclic nucleotide in enterocytes. In this case it is cyclic guanosine monophosphate (cGMP) which differs slightly from cAMP in its activity but also produces diarrhoea as a result of electrolyte imbalances.
A different enterotoxin is produced by Clostridium perfringens as it sporulates in the gut. The toxin binds to receptors on the surface of cells of the intestinal epithelium, producing morphological changes in the membrane which affect absorption/ secretion processes thus precipitating diarrhoea.
A traditional method of analysing for the presence of enterotoxins is based upon their in vivo action. The ileum of a rabbit under anaesthesia is tied off to produce a number of segments or loops which serve as test chambers. These are injected with cultures, culture filtrates or samples under test.
If an enterotoxin is present it produces, after about 24 h, an accumulation of fluid in the loop which becomes distended. A number of alternative assays, based on the effects of enterotoxins on cells in tissue culture are also used. These have the advantages of being more economical, more humane and easier to quantify than the ligated ileal loop assay but are less directly related to the clinical action of the toxin.
Type # 3. Invasive Infection:
Other-diarrhoea causing pathogens invade the cells of the intestinal epithelium but do not normally spread much beyond the immediate vicinity of the gut. Some, such as Salmonella preferentially invade the ileum to produce a profuse watery diarrhoea. Bacterial cells invade and pass through the epithelial cells to multiply in the lamina propria, a layer of connective tissue underlying the enterocytes.
The precise mechanism of fluid secretion into the intestinal lumen is not known and is probably multifactorial. A heat-labile enterotoxin which stimulates adenylate cyclase activity has been identified in some salmonellas as well as a cytotoxin.
It has also been suggested that the local acute inflammation caused by the infection and responsible for the fever and chills that are often a feature of salmonellosis, causes an increase in levels of prostaglandins, known activators of adenylate cyclase.
Other entero-invasive pathogens like Shigella and entero-invasive E. coli invade the colonic mucosa and produce a dysenteric syndrome characterized by inflammation, abscesses and ulceration of the colon and the passage of bloody, mucus- and pus-containing stools. Bacterial cells adhere to the enterocytes via outer membrane protein adhesins.
They are then engulfed by the enterocytes in response to a phagocytic signal produced by the bacterium and multiply within the cytoplasm invading adjacent cells and the underlying connective tissue. The strong inflammatory response to this process causes abscesses and ulcerations of the colon.
Invasiveness can be diagnosed by examination of the fluid accumulated and the mucosal surface in rabbit ileal loops. A less definitive test for invasiveness is the Sereny test which measures the ability of an organism to cause keratoconjunctivitis in the eye of guinea pigs or rabbits.
Some shigellas also produce a protein exotoxin, known as Shiga toxin, which has a range of biological activities. It inhibits protein synthesis by inactivating the 60S ribosomal subunit and is a powerful cytotoxin. It has neurotoxic activity causing paralysis and death in experimental animals and is an enterotoxin capable of causing fluid accumulation in ligated rabbit ileal loops.
As an enterotoxin, it appears unrelated to cholera toxin since it does not stimulate adenylate cyclase or cross-react with antibodies to cholera toxin. Its role in the pathogenesis of shigellosis is unclear since strains incapable of producing Shiga toxin remain pathogenic. Enteroinvasive E. coli causes a similar syndrome but does not produce Shiga toxin.
Some authors have linked the enterotoxin activity of Shiga toxin with the watery diarrhoea which often precedes dysentery. Interestingly, a similar sequence of watery diarrhoea with supervening bloody diarrhoea is seen with entero-haemorrhagic E. coli.
This organism, which both colonizes the epithelial surface in the colon and multiplies in the lamina propria, produces a number of Shiga-like toxins, sometimes known as verotoxins because of their activity against Vero cells in culture.
Recently, common features have been identified among the various diarrhoea- causing toxins and a number of bacterial exotoxins important in other diseases such as diphtheria. Each consists of five, linked B units which are able to bind to the target cell and facilitate transport of the active A unit into the cell.