Characteristics of alpha-hemolytic strains of Escherichia coli isolated from dogs with gastroenteritis.

Twenty-four hemolysin producing (Hly+) strains of Escherichia coli isolated from dogs with gastroenteritis were investigated for their virulence markers and their phenotypic properties. The strains were distributed over eleven known E. coli O-serogroups and most of them were heterogeneous for their phenotypes. All strains were found to produce alpha-hemolysin which was detected by Southern hybridization and colony immunoblotting using a specific gene probe and a monoclonal antibody. Eight strains were carrying plasmids encoding alpha-hemolysin sequences (hly-plasmids) and 16 strains carried chromosomal hly-determinants. Twelve of the strains showed enterotoxic activities which were tested for in different assays. Among these, three O42:H37 and two O70:H-strains carrying hly-plasmids were found to harbour other plasmids encoding the heat-stable enterotoxin STA1. The other seven strains showing enterotoxicity in the ileal loop or the suckling mouse assay were negative for STA1, STA2, or LT. None of the 24 strains were positive for invasiveness or for production of Vero (Shiga-like) toxins. The production of alpha-hemolysin was closely associated with the production of cytotoxic necrotizing factor (CNF), which was detected in 17 of 24 strains. Of these, 16 elaborated CNF1 and one strain produced an unknown CNF type. Surprisingly, all strains carrying ST-plasmids and six of eight strains carrying hly-plasmids were negative for CNF. Thus, in canine E. coli strains CNF production seems to be closely associated with production of chromosomally encoded alpha-hemolysin whereas hly-plasmids are more often associated with ST-producing, CNF negative isolates.