The emergence of novel macrolide resistance island in Macrococcus caseolyticus and Staphylococcus aureus of food origin.

Food-derived Staphylococcaceae species with severe antimicrobial resistance, especially Staphylococcus aureus, is a major threat to public health. Macrococcus caseolyticus (M. caseolyticus) is a member of the Staphylococcaceae family which plays a vital role in fermented products and disease causation in animals. In our previous study, several Staphylococcus aureus antibiotic-resistant island msr (SaRI) were found in multidrug-resistant S. aureus. In this study, novel SaRI, SaRI-III emerged from S. aureus. Another novel SaRI-like further emerged in M. caseolyticus from food. These isolates' prevalence and genetic environment were investigated and characterized to understand the distribution and transmission of these novel SaRI strains. All SaRI-positive S. aureus isolates exhibited a multidrug resistance (MDR) phenotype, within which a series of antimicrobial resistance genes (ARGs) and virulence factor genes (VFs) were identified. In addition, three SaRI types, SaRI-I (15.1 kb), SaRI-II (16-17 kb), and SaRI-III (18 kb) carrying mef(D)-msr(F), were identified in these isolates' chromosomes. SaRI-(I-III) contains a site-specific integrase gene int and operon mef(D)-msr(F). SaRI-III has an additional orf3-orf4-IS30 arrangement downstream of mef(D) and msr(F). Moreover, the SaRI-like and macrolide-resistant transposon Tn6776 forming a novel mosaic structure coexisted in one M. caseolyticus isolate. Within this mosaic structure, the macrolide-resistant genes mef(D)-msr(F) were absent in SaRI-like, whereas an operon, mef(F)-msr(G), was identified in Tn6776. The SaRI-(I-III) and SaRI-like structure were inserted into the rpsI gene encoding the 30S ribosomal protein S9 in the chromosome. Excision and cyclisation of SaRI-III, SaRI-like, operon mef(D)-msr(F), and orf3-orf4-IS30 arrangements were confirmed using two-step PCR. This study is the first to report MDR S. aureus harbouring novel SaRI-III and M. caseolyticus containing novel mosaic structures isolated from retail foods. Similar SaRI-type resistant islands' occurrence and propagation in Staphylococcaceae species require continuous monitoring and investigation.