Abundance in sewage of bacteriophages infecting Escherichia coli O157:H7.

Bacterial virulence factors such as toxins are often encoded by bacteriophages. Among other examples, factors encoded by phages have been described in some of the emerging or re-emerging pathogens, including the pyrogenic exotoxin A production in group A streptococci, the cholera toxin in Vibrio cholerae, or enterotoxin production in enterohemorrhagic (EHEC) strains of E. coli. Most described virulence factors in Shiga toxin (Stx)-producing E. coli strains are located in mobile genetic elements such as plasmids and bacteriophages. Stx, which are one of the most important virulence elements in Shiga toxin-producing E. coli (STEC), are encoded in the genome of temperate bacteriophages infecting E. coli and other Enterobacteriaceae. Studies on Stx phages indicate that they are transmitted between different bacteria in vivo and in vitro. Phages could also be transmitted extraintestinally, hence the observed presence of infectious Shiga toxin phages in sewage and in fecally contaminated rivers. Stx phages also show a higher persistence under natural inactivation and disinfectant treatments in aquatic environments.This background shows that phages or lysogenic strains carrying Stx2 phages might be the natural reservoir of Stx2 genes and that lysogenization could be the main cause of the emergence of STEC strains, as suggested by several authors. It has also been suggested that lysogenization/conversion processes could take place in food and water and probably inside the human and animal gut. Ingestion of Stx2 phages could produce conversion of non-Stx2-E. coli strains, present inside the gut and producing new pathogenic strains. To control these phenomena, it is first necessary to gain more information about the distribution of Stx phages in the environment. For this purpose, a method of detecting Stx2 phages present in environmental water samples has been developed. The particularity of this method is that it allows detection of all (infectious and noninfectious) Stx2 phages in a water sample; in a second stage, the method allows detection of those phages able to infect and replicate on E. coli O157:H7. Although this method has been applied to Stx2 phages able to infect E. coli O157:H7, it is also applicable to detection in the natural environment of other genes carried by other bacteriophages and other bacteria.