Knowing Your E. coli

Escherichia coli on Tryptic Soy Agar

The recent outbreak of Escherichia coli poisoning in Germany is a stark reminder of the pathogenicity of certain strains of this bacterium. Most commonly found as a faculative organism in the human gastrointestinal tract, pathogenic strains of E. coli can cause a variety of diarrheal diseases in humans as well as being a major source of urinary tract infections. The strains that cause diarrheal diseases are generally divided into 6 pathotypes (1), the relationships between which are neatly illustrated below, after Donnenberg (2002) (2).

Pathotypes of Escherichia coli, after Donnenberg, 2002

The most closely associated of the six are Enteropathogenic E. coli (EPEC), Enterohaemorrhagic E. coli (EHEC) and the Verocytoxigenic E. coli (VTEC). EPEC attaches to the gut lining, altering it in the process which leads to bloody diarrhea. VTEC produces verocytotoxins (also called Shiga toxins) which disrupt protein synthesis in host cells and cause diarrhea. EHEC possess the attachment ability of EPEC and the toxin producing capacities of VTEC, and as such can be seen as a subset of the two (1). The most infamous EPEC strain (and possible the most infamous E. coli strain) is E. coli O157:H7. Most commonly associated with raw beef, this strain causes haemolytic uraemic syndrome (which results in renal failure) as well as other illnesses which can often lead to death. The “O157:H7” moniker refers to the specific O- and H- antigen proteins expressed by the bacterium.

Of the other pathotypes, Enterotoxigenic E. coli (ETEC) is a major cause of traveler's diarrhea worldwide, producing toxins upon colonisation of the gut. Enteroinvasive E. coli (EIEC) invades host cells in the epithelial layer of the gut, spreading from cell to cell and causing a mild form of dysentery. Enteroaggregative E. coli (EAggEC, or EAEC) causes persistent diarrhea, and acts by aggregating on the gut wall and producing a toxin. Diffusely Adherent E. coli is most commonly associated with urinary tract infections, however it has been suggested to have a role as a causative agent of diarrhea.

All of the above pathotypes enter the body by the faecal-oral route, most often through water contaminated with faeces. Some pathotypes, most notably the VTEC, are associated with ruminant animals, with cattle being historically their main reservoir (1).

According to the Center for Disease Control in the US, the outbreak in Germany was caused by the strain E. coli O104:H4, a VTEC type strain, but one that also shows virulence characteristics of EAggEC pathotypes (3).

References:

1. O'Sullivan et al., 2007. Methods for Detection and Molecular Characterisation of Pathogenic E. coli, ISBN 1 84170 506 3
2. Donnenberg, 2002. Escherichia coli: Virulence Mechanisms of a Versatile Pathogen
3. http://www.cdc.gov/ecoli/2011/ecoliO104/