Genetic Characterization of CTX-M-2-Producing and Associated With Bovine Mastitis in Japan.

CTX-M-2-producing () has not received much attention in animal husbandry compared with (), a major reservoir of extended-spectrum β-lactamase (ESBL) genes. Bacteriological examinations of 1,466 mastitic milk samples between October 2012 and December 2014 were conducted. Ninety-five isolates (total prevalence: 6.5%) and 81 isolates (total prevalence: 5.5%) were obtained. Seventeen isolates obtained from 15 animals reared on 11 farms and 9 isolates obtained from 9 animals reared on the same farm were phenotypically confirmed to be ESBL producers. All nine ESBL-producing isolates were obtained from one farm between June and November 2013 and related to a significantly ( < 0.05) higher monthly prevalence of mild mastitis (in June, August, September, October, and November 2013). Pulsed-field gel electrophoresis (PFGE) patterns of ESBL-producing isolates were distinguished from each other by more than 6-band differences except for two isolates from two animals, whereas all nine isolates showed an identical PFGE pattern. Transferability of the gene was found in 14 and 9 isolates by conjugation analysis. Of these isolates, the gene was detected on plasmids belonging to the incompatibility (Inc) groups P and N derived from five and nine isolates, respectively, although the plasmids from the remaining nine were untypeable. All the transconjugants exhibited elevated minimum inhibitory concentrations of ampicillin, cefotaxime, and ceftiofur compared with those in the wild-type, recipient strain. Restriction fragment length polymorphism analysis demonstrated that the IncN plasmids extracted from eight of nine transconjugants, which received resistance against β-lactams from , showed an identical I digestion pattern. These results suggest that the CTX-M-2-producing strain with the above-mentioned characteristics may have clonally spread within a farm, whereas the gene in possibly disseminated among the farms through different plasmids. Thus, monitoring of ESBL genes, including the gene, among causative agents of bacterial mastitis in cows can help to develop relevant treatments and control practices.