The bacteriophage-associated ehly1 and ehly2 determinants from Escherichia coli O26:H- strains do not encode enterohemolysins per se but cause release of the ClyA cytolysin.

This report presents evidence that the bacteriophage-associated proteins Ehlyl and Ehly2 (enterohemolysin) of non-verotoxigenic E. coli O26:H- are not hemolysins per se. Ehly1 and Ehly2 conferred a hemolytic phenotype on wild-type but not on clyA knockout strains of E. coli K-12 when introduced in trans on plasmids. According to immunoblot analyses, and studies of the expression from a chromosomal clyA::luxAB fusion, the production of cytolysin A (ClyA) was not enhanced by the expression of Ehly1 or Ehly2. However, the analysis provided evidence that there was a low basal level expression of ClyA by E. coli K-12 strains that could be detected phenotypically when the bacteria were subject to lytic effects by bacteriophages or their gene products. In experiments when lysis of the bacteria was caused deliberately either by using lytic bacteriophages (P1 or phiW), or by triggering the SOS response with mitomycin C treatment, sufficient amounts of ClyA were released to cause detectable lysis on blood agar plates. Taken together, our findings support a model where Ehlyl and Ehly2 cause a hemolyric phenotype on blood agar as a result of their lytic and lethal effects on the bacterial hosts.