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CLASSIFICATION

There are curretly five accepted species, E. coli, E. blattae, E. fergusonii, E. hermanii and E. vuneris . They can be differentiated particularly on the basis of the following reactions: Indole, citrate, lysine decarboxylase, growth in KCN and malonate utilization.

BIOCHEMISTRY

They frequently produce indole (except E. blattae and E.vulneris ), ferment glucose by the mixed acid fermentation, do not produce H2S, phenylalaninedeaminase or urease, do not utilize citrate as sole carbon source (except some strains of E. blattae , and E. fergusonii. ) Most are motile, ferment a variety of carbohydrates and decarboxylae arginine, lysine and/or ornithine.

Most genetic studies have been done on E. coli . It consists of a single circular DNA molecule of about 4 x 10⁶ base pairs with a molecular weight of 4 x 10⁹ and a total length of about 1.4mm. Many of the genes have been mapped.

E. coli can be subdivided according to their somatic (cell-wall) or O antigens and their flagellar or H antigens. Currently there are over 160 recognized O types and 55 recognized H types making over 8000 possible OH serotypes. There are also capsular and fimbrial antigens.

Enterotoxins:
There are at least two types of enterotoxins; Heat Stable (ST) and Heat Labile (LT).

Verotoxins or Shiga-like toxins:
The two terms are interchangeable. The term Verotoxin is based on the reactions of these toxins on Vero cells. There are at least two families of these toxins VT1 (SLT I) which is closely related to Siga-toxin (produced by some strains of Shigella dysenteriae) and VT2 (SLT II) which is only about 50% realted Shiga toxin.

Other Toxins:
Among the various other toxins described are the Cytolethal distending toxin (CLDT), Vir Cytotoxin, Cytotoxic necrotising factors (CNF), a possible Enteropathogenic E. coli EPEC) enterotoxin and a possible E. coli Sudden Infant Death Syndrome (SIDS)-toxin.

Haemolysins:
Many strains produce an extracellular haemolysin known as alpha-haemolysin, some strains also produce a cell associated haemolysin known as beta-haemolysin. A feature of many Enterohaemorrhagic E. coli (EHEC) strains is the production of another extracellular haemolysin known as enterohaemolysin.

Adhesins & Fimbriae:
Of the most important fimbriae are the K88, K99 and CFA fimbriae associated with enterotoxigenic E. coli (ETEC). They have differing species specificities. The p-fimbriae are associated with urinary tract pathogens. E. coli also produce common fimbriae not specifically associated with virulence.
Intimin is the most important adhesin. It causes the intimate association found among the enteropathogenic and enterohaemorrhagic E. coli . This is associated with the 'attachment and effacement' phenomenon, which causes destruction of the intestinal surface cells. Other outer membrane proteins can act as adhesins.

NORMAL FLORA

E. coli colonise the intestine of virtually all warm blooded animals from shortly after birth, where they are part of the commensal flora.

Enteric Infections:
There are a number of types causing human and animal intestinal diseases. They include the Enteroaggregative E. coli (EAggEC), Enterohaemorrhagic E. coli (EHEC), Enteroinvasive E.coli (EIEC), Enteropathogenic E. coli (EPEC) and Enterotoxigenic E. coli (ETEC).

Extraintestinal Infections:
Uropathogenic E. coli (UPEC) cause urinary tract infections and there are Neonatal Menigitis E. coli (NMEC).

Animal Infections:
Apart from causing similar infections in animals as some of the human ones, there are specific animal diseases including: calf septicaemia, bovine mastitis, porcine oedema disease, and respiratory tract infections (air sac disease) in poultry.

E. coli can be grown very easily on most microbiological media. Generally the production of lactose-fermenting colonies on media containing bile-salts is a strong indication of the presence of E. coli . To isolate dammaged cells (e.g. after antibiotic therapy) may require special media. With E. coli infections it is generally the problem to characterise the pathogenic types among the commensal types. For this very special techniques tailor-made for each type must be applied. For the isolation of E. coli from sites which are normally sterile, but where their presence indicates infection such as blood or urine, there are many standard techniques available. With the emergence of EHEC as important pathogens special media are being developed to identify these from faeces and food. Most commonly used is Sorbitol MacConkey (SMAC) Agar on which some but not all EHEC will appear as non-fermenting colonies while most other E. coli ferment sorbitol.

E. coli can not only survive but acitively grow in many environmental waters. As E. coli are present in the faeces of humans and most animals their presence has been used as an indicator of faecal pollution for many years. However, caution has to be excersised, because E. coli have been isolated from pristine waters and birds particularly have been associated with causing E. coli to be present in some waters. Currently methods involving some form of counting bacteria which give characteristic E.coli biochemical reactions are the main means by which pollution can be monitored.

As E. coli can be grown very easily on simple media and its genetic characteristics have been essentially determined, they have found extensive use as vehicles for the preparation of biological polymers, including polypeptide hormones, proteins, carbohydrates etc. By incorporating into the E. coli genome the genetic information required to produce such substances, it is a simple process to produce these in large amounts.

Currently most of the specific E. coli vaccines are for use agains animal diseases. E. coli toxins have been used toxoided (made harmless but still antigenic) and used as vaccines. There are also some studies on the development of compound toxoids containing immunologically competent components from a number of bacterial toxins including some of the E. coli toxins.
There are also some studies suggesting that E. coli cell-wall preparations could be used as vaccines to prevent the effects of endotoxic shock in cases of Gram-negative septicaemia.

Enterobacteriaceae

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