Genomic analysis of population structure, antimicrobial resistance, and virulent factors of methicillin-resistant Macrococcus caseolyticus in global lineages

Macrococcus caseolyticus is an opportunistic pathogen frequently detected in dairy products as well as veterinary infections. The present study examined the population structure, antimicrobial resistance, and virulent factors of methicillin-resistant M. caseolyticus isolates in retail meat from Shanghai (n=10) and global isolates from GenBank (n=87). All strains were divided into five lineages that distributed in Europe (82.4%, n = 80), Asia (11.3%, n = 11), North America (4.1%, n = 4), Oceania (1%, n = 1) and Africa (1%, n = 1). MLST typing revealed novel alleles in Chinese M. caseolyticus strains. Furthermore, a total of 24 AMR genes associated with 10 classes of antimicrobial agents were identified in the isolates from global lineages, carried by dominant plasmids such as rep7a, rep22 and repUS56. Comparing to other lineages, genomes from the Chinese lineage carried significantly more AMR genes (pM. caseolyticus in China. Finally, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were enrolled for morphology comparison between M. caseolyticus and S. aureus, showing that M. caseolyticus has a larger diameter and thicker cell wall. The present study showed geographical variation with regards to MLST profiles, antimicrobial resistance, and virulent factors in global M. caseolyticus lineages. This study suggests that such local evolution of foodborne or livestock origin M. caseolyticus may serve as vehicles for domestic transmission of methicillin resistance in retail meat in China.HighlightsGlobal M. caseolyticus strains were divided into five lineages from A to E.MLST typing revealed novel alleles in Chinese M. caseolyticus strains.Chinese lineage carried significantly more AMR genes and less virulent factors.Macrococcus caseolyticus has a larger diameter and thicker cell wall compared with S.aureus.Macrococcus caseolyticus may enhance the domestic transmission of methicillin resistance in China.