Shiga toxin 2-converting bacteriophages associated with clonal variability in Escherichia coli O157:H7 strains of human origin isolated from a single outbreak

Shiga toxin 2 (Stx2)-converting bacteriophages induced from 49 strains of Escherichia coli O157:117 isolated during a recent outbreak of enterocolitis in Spain were examined in an attempt to identify the variability due to the stx(2)-converting phages. The bacterial isolates were divided into low-, medium-, and high-phage-production groups on the basis of the number of phages released after mitomycin C induction. Low- and medium-phage-production isolates harbored two kinds of phages but released only one of them, whereas high-phage-production isolates harbored only one of the two phages. One of the phages, phiSC370, which was detected only in the isolates with two phages, showed similarities with phage 933W. The second phage, phiLC159, differed from phiSC370 in morphology and DNA structure. When both phages were present in the same bacterial chromosome, as occurred in most of the isolates, only phiSC370 was detected in the supernatants of the induced cultures. If phiLC159 was released, its presence was masked by phiSC370. When phiSC370 was absent, large amounts of phiLC159 were released, suggesting that there was some regulation of phage expression between the two phages. To our knowledge, this is the first description of clonal variability due to phage loss. The higher level of phage production was reflected in the larger amounts of stx(2) toxin produced by the cultures. Some relationship between phage production and the severity of symptoms was observed, and consequently these observations suggest that the virulence of the isolates studied could be related to the variability of the induced stx(2)-converting phages.