POST-GENOMICS IDENTIFICATION OF A GENETIC DETERMINANT OF MAMMARY PATHOGENIC ESCHERICHIA COLI

Escherichia coli is a major pathogen of bovine mastitis, a disease of great economical and animal wellfare importance in dairy production, and the main reason for antibiotic use in dairy farms. Good farm management practices have proven insufficient to tackle the incidence of E. coli mastitis. The Mammary Pathogenic E. coli (MPEC) are sero- and genotypically diverse, and the virulence mechanisms allowing E. coli to cause intra-mammary infections in cows are unknown. This work aimed to study the virulence determinants of MPEC. Whole-genome sequence analysis of 40 MPEC and 22 “dairy-farm” E. coli (DFEC) strains revealed that the only genes present in the core-genome of MPEC and that are significantly less prevalent in DFEC (p < 0.05) comprise the fecIRABCDE locus for ferric-dicitrate assimilation. We found that protein expression of the FecA receptor in the outer-membrane is citrate-dependent, and that FecA is over-expressed when bacteria grow in milk in constrast to laboratory medium. In addition, transcription of fecA and of the inner-membrane transporter fecB is up-regulated in-vivo during intra-mammary infection in cows. The presence of fec correlated with the ability of wild-type strains to grow in milk. This was corroborated by knockdown of fec in MPEC strain P4, which showed impaired growth in milk, and insertion of fec in the non-pathogenic K71 DFEC isolate, which became able to grow in milk similarly to MPEC. By intra-mammary challenge in cows, the fec^- P4 MPEC mutant did not induce mastitis whereas the fec⁺ K71 DFEC mutant became pathogenic in the mammary gland. For the first time, a single molecular locus was shown to be ubiquitous in, and necessary for E. coli that cause mammary infections in cattle. These results have far-reaching implications, raising the possibility of targeting this system for mastitis control.