Antimicrobial Resistance in Bacteria Isolated from Dairy Farms in Agrarian Settlements of Pontal do Paranapanema, São Paulo, Brazil

Family farming is central to Brazilian milk production, yet smallholder dairy systems in agrarian settlements often have limited access to veterinary assistance and microbiological diagnosis. This study describes the bacteria recovered from 2432 quarter milk samples with California Mastitis Test (CMT) scores of 2+ or 3+, collected in 2020 from dairy herds in eight agrarian settlements of the Pontal do Paranapanema region, São Paulo State, Brazil, and the antimicrobial susceptibility of the resulting isolates. Bacterial growth was obtained from 1296 samples (53.3%), whereas 1121 (46.1%) yielded no growth. Staphylococcus spp. (26.3% of samples, pure culture), Bacillus spp. (12.6%) and Corynebacterium spp. (3.8%) were the most frequently recovered genera. Susceptibility was determined by disk diffusion. Among 648 Staphylococcus isolates tested, resistance was most frequent to ampicillin (13.7%), penicillin (12.8%) and cephalothin (11.6%); among 363 Bacillus isolates, to cephalothin (22.0%), amoxicillin (19.8%) and penicillin (19.3%); and among 115 Corynebacterium isolates, to cephalothin (46.1%). Resistance was unevenly distributed and concentrated in a few settlements. β-Lactams accounted for the most frequent resistance phenotypes among the main genera recovered from CMT-positive quarters. These descriptive data support the use of bacterial culture and susceptibility testing to guide mastitis treatment in smallholder herds of the region.