Genomic analysis of bovine dairy ecosystem-derived methicillin-resistant Macrococcus caseolyticus reveals diversity of genetic elements carrying mecB and

Macrococcus spp. are widespread animal commensals, but are becoming increasingly recognized as veterinary and human pathogens. Methicillin-resistant strains may spread mec genes to the closely related but more pathogenic staphylococci, including Staphylococcus aureus . To gain insight into the genetic features of methicillin-resistant M. caseolyticus within the bovine ecosystem, 11 mec -containing strains isolated from raw cow’s milk were characterized using whole-genome sequencing. The methodology included mec typing, multilocus sequence typing, antibiotic resistance gene identification, and phylogenetic analysis to assess relatedness and compare the isolates with publicly available whole-genome sequences. The McRI _mecD -3 element was identified in seven strains belonging to sequence types (ST) 26, ST40, ST47, and ST66. McRI _mecD -1 was found in ST22 ( n = 1), while McRI _mecD -2 was found in ST6 ( n = 1). The mecB gene was detected in ST69 ( n = 1) on a staphylococcal cassette chromosome mec (SCC mec ) element and in ST11 on a large mecB -carrying pMCCL2-like plasmid found previously in S. aureus . In both mecB -containing strains, the gene was part of transposon Tn 6045 . Six strains carrying McRI _mecD -3 were genetically related to M. caseolyticus isolated earlier from bulk tank milk from the UK, from bovine mastitis milk, or from nasal cavities of calves in Switzerland. Phylogenetic comparison to publicly available M. caseolyticus suggested the presence and persistence of dairy-specific lineages over time and across different geographical locations. Our data contribute to a better understanding of the genomic characteristics of mecB and mecD and illustrate the importance of bovine-derived M. caseolyticus as a reservoir for transferable antimicrobial resistance.

IMPORTANCE

Macrococcus caseolyticus is an animal commensal but is also increasingly recognized for carrying the methicillin resistance genes mecB and mecD . These genes may spread to closely related pathogens, such as Staphylococcus aureus , with implications for animal and public health. Methicillin-resistant M. caseolyticus strains may be present in dairy cattle and dairy products, such as raw milk, representing a potential animal and public health concern. Our study analyzes mecB - and mecD -carrying M. caseolyticus isolated from raw milk derived from dairy cows. Our data provide molecular characterization of mecB and mecD determinants and highlight the importance of M. caseolyticus as a reservoir for transferable antimicrobial resistance. The data from this study will help to surveil the dissemination of bacterial strains carrying mecB and mecD within the bovine ecosystem.