Temporal trends of antimicrobial resistance and resistance genes in Mannheimia haemolytica from cattle in Japan.

Mannheimia haemolytica is a major bacterial pathogen associated with bovine respiratory disease (BRD), and the emergence of antimicrobial resistance (AMR) in this bacterium is a growing concern for cattle health management. Therefore, this study investigated temporal trends in AMR, resistance genes, and integrative and conjugative element (ICE)-associated genes of M. haemolytica isolated from cattle in Japan between 2004 and 2018. In total, 337 isolates, including 26 from cattle imported from Australia, were examined for susceptibility to 17 antimicrobials, serotyped by PCR, and screened for 14 AMR genes and 4 ICE-associated genes. Resistance was observed to 10 of 17 antimicrobials tested, with high rates for ampicillin, kanamycin, streptomycin, nalidixic acid, thiamphenicol, chloramphenicol, and tetracycline, which except for nalidixic acid and chloramphenicol, are used either alone or in combination as first-line treatments for BRD in Japan. All isolates were classified as serotypes A1, A2, or A6, with serotypes A2 and A6 showing pronounced increases in resistance over time. Eight AMR genes (blaROB-1, aphA, strA, strB, floR, catA, tetH, and tetR) were identified in association with their corresponding AMR phenotypes, and most resistant isolates carried one to four ICE-associated genes (parB, ICErel1, int1, and int2). Notably, isolates harboring all four ICE-associated genes predominantly belonged to serotype A6. These findings imply the continuing expansion of AMR in M. haemolytica in Japan via resistance gene acquisition facilitated by ICEs, particularly within serotype A6. This study provides essential insights for developing effective strategies to mitigate AMR in cattle.