Cows Harbor Expanding Serotypes of Pathogenic Escherichia coli

Abstract & Commentary

Synopsis: The E. coli geneticists are working on ways to facilitate rapid diagnosis, and in time, to purge eae positive strains from bovine reservoirs.

Source: Stephen R, et al. First Isolation and Further Characterization of Enteropathogenic Escherichia coli (EPEC) O157:H45 Strains from Cattle. BMC Microbiol. 2004;4(1):10. www.biomedcentral.com/1471-2180/4/10.

Enteropathogenic Escherichia coli (EPEC) cause of infantile diarrhoea, and represent 1 of the at least 6 different categories of diarrheagenic E. coli. Most of the EPEC strains belong to a series of O antigenic groups known as EPEC O serogroups: O26, O55, O86, O111, O114, O119, O125, O126, O127, O128, O142 and O158. Readers are probably most familiar with 0111 group but these others can be pathogenic for humans.

The hallmark of EPEC pathogenesis is the presence of attaching-and-effacing (A/E) lesions on intestinal cells characterized by microvillus destruction, intimate adherence of strains to the intestinal epithelium, formation of a pathogenic entity called a pedestal, and aggregation of polarized intracellular actin. The genetic determinants for the production of A/E lesions are located on a so called pathogenicity island that contains the genes n (eae), a type III secretion system, a number of secreted proteins (ESP) and the translocated intimin receptor (Tir). Characterization of eae genes in the strains from this study revealed the existence of different eae variants. Many genetic variants of the eae gene have been identified and several intimins are probably responsible for different host- and tissue-cell tropism. In epithelial culture cells, EPEC strains produce various characteristic adherence patterns. Localized adherence is mediated by a large EPEC adherence factor (EAF) plasmid which also encodes a bundle-forming pili (BFP).

In this study from Switzerland, E. coli 0157:H45 were isolated from either "fattening" cattle or from cows positive for Shiga toxin genes. The eae gene was present in all strains so these were considered EPEC strains. Ten of 11 strains were positive for heat-stable enterotoxin and the same number showed adherence and effacement with fluorescent actin staining. These strains resemble both typical EPEC strains because they have the large virulence plasmid but have some genetic connection with atypical EPEC strains because they have astA genes.

Comment by Joseph F. John Jr, MD

The genetics of enteropathogenic E. coli have become extremely complicated and full characterization of isolates can occur only in a research microbiology laboratory. What the general clinical microbiology laboratory can determine is by use of sorbitol in lactose positive Gram-negative bacilli, so called sorbitol-positive E. coli, that strongly suggest the strains may be enteropathic.

This article works its epidemiology a bit back-to-front by noting that O157:045 strains have not been found in animals to date and realizing the novelty of the discovery in these Swiss cows. There are likely other serotypes of EPEC lurking out there in animal reservoirs.

Animal husbandry, itself, is facing new challenges of public pressure for organic farming and vegetarian palates. What the public needs is to know that any herd suspicious of harboring enteropathic strains be clearly tested so that the public can be duely warned. The E. coli geneticists, in the meantime, are working on ways to facilitate rapid diagnosis, and in time, to purge eae positive strains from bovine reservoirs.

Joseph F. John, Jr., MD, Chief, Medical Subspecialty Services, Ralph H. Johnson Veterans Administration Medical Center; Professor of Medicine, Medical University of South Carolina, Charleston, SC, and Co-Editor of Infectious Disease Alert.