Prevalence of virulence genes and clonality in Escherichia coli strains that cause bacteremia in cancer patients

Phenotypic analysis of Escherichia coli strains causing bacteremia in cance r patients suggests that they possess specific virulence properties. To inv estigate this hypothesis, we compared the frequency of the virulence-relate d genes cnf1, cnf2, papC, hlyC, and iut in 155 E. coli strains isolated fro m hospitalized cancer patients with epidemiologically unrelated cases of ba cteremia to their frequency in 70 E. coli strains isolated from the feces o f healthy unrelated volunteers, Of the blood isolates, 24, 37, and 26% were positive for cnf1, papC, and hlyC, respectively, versus only 6, 17, and 6% of the fecal isolates (P < 0.05 in all instances). By contrast, 47% of bot h isolates carried the hr gene. The patients' clinical characteristics did not significantly influence these frequencies. The presence on various path ogenicity islands (PAIs) of a combination of the cnf1, papC, and hlyC genes on the chromosome was strongly suggested by Southern blotting of pulsed-fi eld gel electrophoresis (PFGE) patterns with specific DNA probes. The phylo genetic relatedness among 60 strains carrying three, two, one, or no virule nce genes and 6 ECOR strains included as references was determined by neigh bor joining, the unweighted pair-group method with arithmetic mean, and Wag ner analysis of the randomly amplified polymorphic DNA (RAPD) patterns gene rated by 11 primers. Identification of a major cluster including 96.4% of t he strains carrying the cnf1, papC, and hlyC genes and ECOR subgroup B2 str ains suggested that the virulent E. coli strains causing bacteremia in canc er patients are closely related to ECOR B2 strains. The presence in the E. coli population surveyed of a strong linkage disequilibrium, and especially of a highly significant correlation between PFGE and RAPD genetic distance s, confirms that clonal propagation has a major impact on the E. coli popul ation structure. Nevertheless, low bootstrap values in the phylogenetic tre e suggested that frequent genetic exchange inhibits the individualization o f discrete genetic lineages, which are stable on an evolutionary scale.