Occurrence and antibiotic resistance profiles of mastitis pathogens in North West South African dairy herds

Bovine mastitis remains one of the most expensive infectious diseases in the dairy industry, threatening both milk quality and food security worldwide. Although diverse pathogens can cause mastitis, bacteria are the predominant culprits responsible for these infections. Here, we investigated the occurrence and antibiotic resistance profiles of bacteria associated with mastitis at the quarter level from dairy farms in the Dr. Kenneth Kaunda District of South Africa’s North West Province, an underexplored region in global mastitis surveillance. A total of 1,730 quarter milk samples were collected from seven dairy farms. Bacteria were isolated and identified using culture-based methods, the automated identification Sensititre™ Aris 2X ID/AST system, and RapID panels. Antimicrobial susceptibility was determined via Kirby-Bauer Disk-diffusion assays. Bacterial growth was detected in 89.7% of milk samples, revealing the occurrence of non-aureus Staphylococcus as the predominant isolate (48.2%), followed by Bacillus cereus (20.5%), Corynebacterium species (15.8%), Staphylococcus aureus (6.6%), and Escherichia coli (5.2%). The antimicrobial resistance patterns show non-aureus Staphylococcus is resistant to penicillin G (24.1%) and amoxicillin-clavulanic acid (18.8%). Classical contagious pathogens like S. aureus no longer dominate bovine mastitis on these farms; instead, there is a noticeable ecological shift toward environmental and opportunistic bacteria, especially non-aureus Staphylococcus species, P. aeruginosa , E. cloacae , and Bacillus species. This suggests that environmental microbiomes specific to individual farms are increasingly influencing mastitis rather than uniform contagious transmission. These findings demonstrate that dairy herds in this province harbor a high burden of mastitis-associated bacteria, including multidrug-resistant strains, underscoring the urgent need for improved milk hygiene and antibiotic stewardship within a “One Health” framework.