Prevalence and antimicrobial susceptibility profiles of Staphylococcus aureus from raw bovine milk in dairy and pastoral farms in Morogoro region, Tanzania

Increased resistance of Staphylococcus aureus isolates to existing antimicrobials constitutes a major concern in human and veterinary medicine. This study aimed at determining the prevalence, antimicrobial susceptibility profiles, and molecular characteristics of S. aureus from raw bovine milk in dairy and pastoral farms in Morogoro urban and Mvomero districts, Tanzania. In a cross-sectional study, 397 raw bovine milk samples were collected and carried to the laboratory. Conventional Gram staining, colony morphology on blood agar, and mannitol salt agar, along with biochemical tests, were used for S. aureus identification. Antimicrobial susceptibility testing (AST) was performed using the disk diffusion method, while multiplex polymerase chain reaction (PCR) was used to detect virulence and antimicrobial resistance genes. Data were analyzed using Epi Info (Version 7). Out of the 397 samples, S. aureus was confirmed in 124 (31.2%). Contamination of raw bovine milk by S. aureus in the study area was associated with poor hygienic measures. The AST revealed that all S. aureus isolates were susceptible to chloramphenicol and cefoxitin, while the highest resistance 116/124 (93.5%) was noticed for penicillin. Resistance to other antimicrobials varied between 1.6-28.2%. Of the 124 S. aureus isolates, 80 (64.5%) possessed spa gene, with 76/80(95.0%) being pathogenic. One of the S. aureus isolates, 1/124 (0.8%), harbored a mecA resistance gene. The presence of antimicrobial-resistant S. aureus isolates in raw bovine milk at the farm level is alarming and requires herd health improvement interventions to protect society.