Prevalence and antimicrobial resistance of methicillin-resistant and methicillin-susceptible <i>Staphylococcus</i> in small- to medium-scale and large-scale dairy farms in Thailand

In Thailand, small- to medium-scale (SM) dairy farms typically have fewer than 100 cows. They are often family-owned or independently operated, and vary in infrastructure and mechanization depending on their size. In contrast, large-scale (L) farms, with more than 100 cows, are more industrialized, utilizing advanced technology, higher production systems, and usually employ multiple workers. To date, few studies have reported the prevalence of methicillin-resistant staphylococci (MRS) and methicillin-susceptible staphylococci (MSS) and their antimicrobial resistance (AMR) at different farm scales. This study aimed to investigate the prevalence of <i>Staphylococcus spp.</i>, MRS, MSS and their AMR as well as their genetic backgrounds on SM and L dairy farms in Thailand. A total of 157 mastitis milk samples were collected from 106 cows on 42 SM farms, and 65 samples from 37 cows on one L farm, all located in Kanchanaburi Province. Antimicrobial susceptibility testing was conducted by determining the minimum inhibitory concentration. Whole-genome sequencing and analysis were performed for genetic characterization. There was a significant difference in the prevalence of <i>Staphylococcus</i> spp. on L farm (26.2%) and SM farms (14%) (<i>P</i> = 0.031, χ² test). The phenotypic resistance of trimethoprim/sulfamethoxazole in L farm (58.8%) was significantly greater than that in SM farms (27.3%) (<i>P</i> = 0.049, χ² test). Six methicillin-resistant staphylococci (27.3%), including <i>Staph. haemolyticus</i> sequence type (ST) 3 (N = 1) and ST42 (N = 3) and <i>Staph. epidermidis</i> ST59 (N = 2) were discovered on SM farms, whereas a single <i>Staph. aureus</i> ST398 (5.9%, N = 1) was found on an L farm. These strains were multidrug-resistant and carried multiple, diverse antimicrobial resistance genes, including β-lactam resistance genes (<i>mecA</i>, <i>blaZ</i>), tetracycline resistance genes [<i>tet</i>(K), <i>tet</i>(M)], and macrolide resistance genes [<i>msr</i>(A), <i>mph</i>(C)]. Compared with MRS, MSS carried fewer diverse antimicrobial resistance genes and had distinct STs at both farm scales. At each farm scale, a particular type of resistance may originate from a certain species or specific ST. In conclusion, the prevalence of <i>Staphylococcus</i> spp. and their resistance traits and genetic background on SM and L farms differ according to different production farm scales. The specific management and monitoring of the information on <i>Staphylococcus</i> spp. circulated on each farm type could help to limit the spread of antimicrobial-resistant staphylococci.