Identification and antimicrobial susceptibility of milk pathogen isolated from dairy production systems

Livestock has been recognized as a reservoir of antibiotic-resistant bacteria. Prevalence of resistance has been associated with herd size and intensification of animal production systems. Brazil is one of the emergent hotspots of bacterial resistance, which is also associated with animal husbandry. This study aimed to evaluate the resistance profile of pathogens that cause subclinical mastitis and the relationship between resistance status at farm level and different production systems. Milk samples from cows diagnosed with subclinical mastitis were collected from farms that adopt different husbandry systems with different production intensities, i.e., agroecological, low input, high input, Free-Stall and Compost-bedded pack barn. Etiological agents were isolated and microbiologically identified, and antibiotic susceptibility testing was conducted, using the disk diffusion method. The main isolated agents were Streptococcus spp. (n = 54, 30.5 %) and coagulase-positive Staphylococcus (CPS) (n = 54; 30.5 %). The recovered isolates displayed high antibiotic resistance against Sulfamethazine (80.2 %), Gentamicin (29.37 %), Penicillin (29.37 %), Oxacillin (28.82 %) and Ampicillin (26 %). Multidrug resistance was found for all agents and in all farming systems (39.54 %). Neither production systems (p = 0.26) nor farming systems (p = 0.24) significantly affected the resistance rates of samples. Therefore, intensive production systems may not be a root cause of increased rates of antimicrobial resistance in the milk production chain, suggesting that other environmental factors should be investigated. It is noteworthy that high levels of multidrug resistance were even found in bacteria earlier considered as minor pathogens. This development can be taken as a warning that environmental bacteria are potential transmitters of resistance genes to the environment.