PATHOGENIC E.coli

Escherichia coli, the normal colonists of the human gastrointestinal tract may occasionally be associated with diseases of humans. These apart, there are some strains of E.coli that have the potential to cause various enteric diseases. 

Five classes (virotypes) of E.coli (enterovirulent E.coli)that cause diarrhoeal diseases are now recognized:

• enterotoxigenic E.coli (ETEC),

• enteroinvasive E.coli (EIEC),

• enterohemorrhagic E.coli (EHEC),

• enteropathogenic E.coli (EPEC), and

• enteroaggregative E.coli (EAggEC).

Each class falls within a serological subgroup and manifests distinct features in pathogenesis. 

Enterotoxigenic E.coli (ETEC): 
Infection is acquired by ingestion of food or water contaminated with ETEC. Contamination of water with human sewage may lead to contamination of foods. Infected food handlers may also contaminate foods. The infective dose is 10⁶-10¹⁰ bacilli. With high infective dose, diarrhea can be induced within 24 hours. Infants may require fewer organisms for infection to be established. It is responsible for traveler's diarrhoea.

The bacteria colonize the GI tract by means of a fimbrial adhesin (CFA I and CFA II). These fimbrial adhesins adhere to specific receptors on enterocytes of the proximal small intestine. The symptoms of diarrhoea are due to ETEC strains produce enterotoxins. Enterotoxins produced by ETEC include the LT (heat-labile) toxin and or the ST (heat-stable) toxin, the genes for which may occur on the same or separate plasmids. LTs are similar to cholera toxin in structure and mode of action. Like cholera toxin, LTs are holotoxin consisting of A subunit and B subunit. The B subunit of LTs binds to specific ganglioside receptors (GM1) on the epithelial cells of small intestine and facilitates the entry of A subunit where it activates adenylate cyclase. Stimulation of adenylate cyclase causes an increased production of cAMP, which leads to hypersecretion of water and electrolytes into the lumen and inhibition of sodium reasborption. In several strains, the plasmids carry genes for both enterotoxin and colonization factor production. The enterotoxin production is limited to following O serotypes: O6, O8, O15, O25, O63, O78, O148 and O159.
The ETEC stains are indistinguishable from the resident E.coli by biochemical tests. These strains are differentiated from nontoxigenic E.coli present in the bowel by a variety of in vitro immunochemical, tissue culture, or DNA hybridization tests designed to detect either the toxins or genes that encode for these toxins. Detection of LTs is achieved by ligated rabbit ileal loop test, morphological changes in Chinese hamster overy cells and Y1 adrenal cells, ELISA, immunodiffusion, coagglutination etc.

Enteroinvasive E.coli (EIEC):
 EIEC closely resemble Shigella in their pathogenic mechanisms and the kind of clinical illness they produce. EIEC penetrate and multiply within epithelial cells of the colon causing widespread cell destruction. The ability to invade the epithelial cells is associated with the presence of a large plasmid containing structural genes for several outer membrane proteins that are similar to those found in Shigella. Though they lack specific fimbriae to adhere to the cells, they appear to produce an adhesin. EIEC stains are associated with following O groups: O28, O112, O115, O124, 0136, O143, O147 and O152. These strains are antigenically and biochemically similar to Shigella (non-lactose fermenter and non-motile).
EIEC are believed to produce both an enterotoxin and a cytotoxin. The clinical syndrome is identical to Shigella dysentery and includes diarrhoea with blood and mucus, fever, severe abdominal cramps, malaise and toxemia.

The sample of faeces is inoculated on McConkey's agar. The non-lactose fermenting colonies can by typed suing suitable antisera. The invasive property of EIEC can be demonstrated by penetration of HeLa cells monolayers and Sereny test (development of keratoconjunctivitis in guinea pig).

Enterohemorrhagic E.coli (EHEC): 
EHEC are represented by an important strain (serotype O157:H7), which causes a diarrhoeal syndrome distinct from EIEC (and Shigella) in that there is copious bloody discharge but no fever. The initial watery diarrhoea with abdominal cramps turns into grossly bloody diarrhoea in a few days. Hemolytic Uremic Syndrome (HUS) follows certain outbreaks. This disease is often associated with ingestion of inadequately cooked hamburger meat. EHEC strains may produce one or more types of cytotoxins, which are collectively referred as Shiga-like toxins (SLT) since they are antigenically and functionally similar to Shiga toxin produced by Shigella dysenteriae. SLTs were previously known as verotoxin, so named because its cytopathic effects on Vero cells. They may produce two antigenically distinct SLTs, SLT-1 and SLT-II. The toxins provoke cell secretion and kill colonic epithelial cells. EHEC strains have been associated with many serogroups including O4, O26, O45, O91, O111, O145 and O157. 

Laboratory diagnoses involve culturing the faeces on McConkey's agar or on sorbitol McConkey's agar, where they don't ferment sorbitol. Strains can then be identified by serotyping using specific antisera. SLTs can be detected by ELISA and genes coding for them can be detected by DNA hybridization techniques.

Enteropathogenic E.coli (EPEC): 
Infection is acquired through ingestion of contaminated water or food. They produce a non-fimbrial adhesin called intimin, which is an outer membrane protein that mediates the final stages of adherence. EPEC do not produce any demonstrable extracellular toxin. There is a characteristic morphological lesion with destruction of microvilli and without invasion of the organism. These strains cause a specific type of cell damage called 'attaching and effacing' lesions. Microvillous destruction appears to be caused by breakdown of cytoskeletal components, since actin filaments derived from altered cytoskeleton are found at the site of adherence. The diarrhoea and other symptoms of EPEC infections probably are caused due to interference with normal cellular signal transduction, rather than by production of toxins. These strains belong to following serogroups: O26, O55, O111, O127, O86, O119, O124, O125, O126 and O128. Infantile diarrhoea is characterised by diarrhoea without blood or mucus, infrequent vomiting, may progress to dehydration shock and death. Some types of EPEC are referred to as Enteroadherent E.coli (EAEC), based on specific patterns of adherence. They are an important cause of traveler's diarrhea in Mexico and in North Africa.

Detection of these strains is by culture of faeces and typing with specific antisera. DNA probes are available against eae gene, which are responsible for attaching and effacing lesions.

Enteroaggregative E.coli (EAEC): 
The distinguishing feature of EAggEC strains is their ability to attach to tissue culture cells in an aggregative manner, which is due to certain thin pili (GVVPQ fimbriae). These strains are associated with persistent diarrhoea with vomiting and dehydration in young children. They resemble ETEC strains in that the bacteria adhere to the intestinal mucosa and cause non-bloody diarrhoea without invading or causing inflammation. A distinctive heat-labile plasmid-encoded toxin called the EAST (EnteroAggregative ST) toxin has been demonstrated. They also produce a hemolysin. The role of the toxin and the hemolysin in virulence has not been proven.