Expression of cytotoxicity by potential pathogens in the standard Escherichia coli collection of reference (ECOR) strains

The standard Escherichia coli collection of reference (ECOR) strains was ex amined for ability to exert cytotoxicity towards mammalian cells. A group o f strains with functional haemolysin expression caused strong cytotoxicity and detachment in J774 macrophage cells as measured by lactate dehydrogenas e release and as observed under a microscope. The expression of haemolysin was monitored by using antisera recognizing the E, coli alpha-haemolysin, t he HlyA protein, and by quantitative haemolysis assays. The presence of the hlyA gene, which may be part of a pathogenicity island, was also confirmed . These analyses revealed that different ECOR strains express quantitativel y different levels of haemolysin. One putative enteroaggregative E, coli (E AEC) strain was also found in the ECOR collection. The EAEC strain was char acterized by the clump formation assay, PCR amplification of the EAEC DMA p robe sequence and confirmative sequence analysis of the amplified fragment. The EAEC heat-stable enterotoxin 1 gene, astA, was found in 14% (10/72) of the ECOR strains and a consensus sequence for astA was proposed by compari ng these sequences with those from pathogens. The astA gene appeared to be plasmid-located, Based on evidence from the work of other laboratories and from the present findings, it is concluded that the ECOR collection contain s strains that may represent pathogenic E, coli, It is noted that caution i s necessary when handling or disposing of those potentially pathogenic ECOR strains.