Molecular Epidemiology and Characterization of Antimicrobial-resistant Strains Isolated from Dairy Cattle Milk in Northwest, China

Introduction: Non-aureus (NAS) species are currently the most commonly identified microbial agents causing sub-clinical infections of the udder and are also deemed as opportunistic pathogens of clinical mastitis in dairy cattle. More than 10 NAS species have been identified and studied but little is known about in accordance with dairy mastitis. The present study focused on the molecular epidemiology and genotypic characterization of isolated from dairy cattle milk in Northwest, China.

Methods: In this study, a total of 356 milk samples were collected from large dairy farms in three provinces in Northwest, China. The bacterial isolation and presumptive identification were done by microbiological and biochemical methods following the molecular confirmation by 16S rRNA gene sequencing. The antimicrobial susceptibility testing (AST) was done by Kirby-Bauer disk diffusion assay and antibiotic-resistance genes (ARGs) were identified by PCR. The phylogenetic grouping and sequence typing was done by Pulsed Field Gel Electrophoresis (PFGE) and Multi-Locus Sequence Typing (MLST) respectively.

Results: In total, 39/356 (11.0%) were identified as positive for . The overall prevalence of other species was noted to be 39.6% (141/356), while the species distribution was as follows: 14.9%, 10.4%, 7.6%, 4.2%, 1.4%, and 1.1%. The antimicrobial susceptibility of 39 strains exhibited higher resistance to erythromycin (92.3%) followed by trimethoprim-sulfamethoxazole (51.3%), ciprofloxacin (43.6%), florfenicol (30.8%), cefoxitin (28.2%), and gentamicin (23.1%). All of the strains were susceptible to tetracycline, vancomycin, and linezolid. The overall percentage of multi-drug resistant (MDR) strains was noted to be 46.15% (18/39). Among ARGs, was identified as predominant (82.05%), followed by (33.33%), (30.77%), (30.77%), (28.21%), (23.08%), (12.82%), (12.82%), D (10.26%), (10.26%), A (7.69%), and G (5.13%). The PFGE categorized 39 strains into A-H phylogenetic groups while the MLST categorized strains into eight STs with ST8 being the most predominant while other STs identified were ST3, ST11, ST22, ST32, ST19, ST16, and ST7.

Conclusion: These findings provided new insights into our understanding of the epidemiology and genetic characteristics of in dairy farms to inform interventions limiting the spread of AMR in dairy production.