{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,7,15]],"date-time":"2026-07-15T23:56:31Z","timestamp":1784159791947,"version":"3.55.0"},"reference-count":42,"publisher":"Frontiers Media SA","license":[{"start":{"date-parts":[[2021,5,7]],"date-time":"2021-05-07T00:00:00Z","timestamp":1620345600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":["frontiersin.org"],"crossmark-restriction":true},"short-container-title":["Front. Vet. Sci."],"abstract":"<jats:p>CTX-M-2-producing<jats:italic>Klebsiella oxytoca<\/jats:italic>(<jats:italic>K. oxytoca<\/jats:italic>) has not received much attention in animal husbandry compared with<jats:italic>Klebsiella pneumoniae<\/jats:italic>(<jats:italic>K. pneumoniae<\/jats:italic>), a major reservoir of extended-spectrum \u03b2-lactamase (ESBL) genes. Bacteriological examinations of 1,466 mastitic milk samples between October 2012 and December 2014 were conducted. Ninety-five<jats:italic>K. pneumoniae<\/jats:italic>isolates (total prevalence: 6.5%) and 81<jats:italic>K. oxytoca<\/jats:italic>isolates (total prevalence: 5.5%) were obtained. Seventeen<jats:italic>K. pneumoniae<\/jats:italic>isolates obtained from 15 animals reared on 11 farms and 9<jats:italic>K. oxytoca<\/jats:italic>isolates obtained from 9 animals reared on the same farm were phenotypically confirmed to be ESBL producers. All nine ESBL-producing<jats:italic>K. oxytoca<\/jats:italic>isolates were obtained from one farm between June and November 2013 and related to a significantly (<jats:italic>p<\/jats:italic>&amp;lt; 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<jats:italic>K. pneumoniae<\/jats:italic>isolates were distinguished from each other by more than 6-band differences except for two isolates from two animals, whereas all nine<jats:italic>K. oxytoca<\/jats:italic>isolates showed an identical PFGE pattern. Transferability of the<jats:italic>bla<\/jats:italic><jats:sub>CTX\u2212M\u22122<\/jats:sub>gene was found in 14<jats:italic>K. pneumoniae<\/jats:italic>and 9<jats:italic>K. oxytoca<\/jats:italic>isolates by conjugation analysis. Of these isolates, the<jats:italic>bla<\/jats:italic><jats:sub>CTX\u2212M\u22122<\/jats:sub>gene was detected on plasmids belonging to the incompatibility (Inc) groups P and N derived from five<jats:italic>K. pneumoniae<\/jats:italic>and nine<jats:italic>K. oxytoca<\/jats:italic>isolates, respectively, although the plasmids from the remaining nine<jats:italic>K. pneumoniae<\/jats:italic>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 \u03b2-lactams from<jats:italic>K. oxytoca<\/jats:italic>, showed an identical<jats:italic>Dra<\/jats:italic>I digestion pattern. These results suggest that the CTX-M-2-producing<jats:italic>K. oxytoca<\/jats:italic>strain with the above-mentioned characteristics may have clonally spread within a farm, whereas the<jats:italic>bla<\/jats:italic><jats:sub>CTX\u2212M\u22122<\/jats:sub>gene in<jats:italic>K. pneumoniae<\/jats:italic>possibly disseminated among the farms through different plasmids. Thus, monitoring of ESBL genes, including the<jats:italic>bla<\/jats:italic><jats:sub>CTX\u2212M\u22122<\/jats:sub>gene, among causative agents of bacterial mastitis in cows can help to develop relevant treatments and control practices.<\/jats:p>","DOI":"10.3389\/fvets.2021.659222","type":"journal-article","created":{"date-parts":[[2021,5,7]],"date-time":"2021-05-07T06:27:48Z","timestamp":1620368868000},"update-policy":"https:\/\/doi.org\/10.3389\/crossmark-policy","source":"Crossref","is-referenced-by-count":15,"title":["Genetic Characterization of CTX-M-2-Producing Klebsiella pneumoniae and Klebsiella oxytoca Associated With Bovine Mastitis in 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