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Background  Previous studies have established a correlation between electrophoretic polymorphism of esterase B, and virulence and phylogeny of  Escherichia coli . Strains belonging to the phylogenetic group B2 are more frequently implicated in extraintestinal infections and include esterase B 2  variants, whereas phylogenetic groups A, B1 and D contain less virulent strains and include esterase B 1  variants. We investigated esterase B as a marker of phylogeny and/or virulence, in a thorough analysis of the esterase B-encoding gene.    Results  We identified the gene encoding esterase B as the acetyl-esterase gene ( aes ) using gene disruption. The analysis of  aes  nucleotide sequences in a panel of 78 reference strains, including the  E. coli  reference (ECOR) strains, demonstrated that the gene is under purifying selection. The phylogenetic tree reconstructed from  aes  sequences showed a strong correlation with the species phylogenetic history, based on multi-locus sequence typing using six housekeeping genes. The unambiguous distinction between variants B 1  and B 2  by electrophoresis was consistent with Aes amino-acid sequence analysis and protein modelling, which showed that substituted amino acids in the two esterase B variants occurred mostly at different sites on the protein surface. Studies in an experimental mouse model of septicaemia using mutant strains did not reveal a direct link between  aes  and extraintestinal virulence. Moreover, we did not find any genes in the chromosomal region of  aes  to be associated with virulence.    Conclusion  Our findings suggest that  aes  does not play a direct role in the virulence of  E. coli  extraintestinal infection. However, this gene acts as a powerful marker of phylogeny, illustrating the extensive divergence of B2 phylogenetic group strains from the rest of the species.

aes, the gene encoding the esterase B in Escherichia coli, is a powerful phylogenetic marker of the species