Detection of Staphylococcus and Streptococcus Resistant to Antibiotics in Subclinical Bovine Mastitis in Ecuador

Subclinical bovine mastitis (SBM) is an inflammatory condition of the udder that remains a major concern for the dairy industry due to its high incidence and the direct and indirect associated costs. Antibiotics are widely used for prophylaxis and therapy in livestock, especially for SBM. However, overuse and misuse have contributed to the emergence of antimicrobial resistance (AMR), enabling resistant bacteria to enter the food chain and potentially spread to humans. This study aimed to detect antibiotic-resistant Staphylococcus and Streptococcus associated with SBM in dairy cows from Pioter, north-central Ecuador. For this, a commercial screening test, morphological and biochemical assays, standard culture techniques, mass spectrometry, and PCR (polymerase chain reaction) were applied. Among 99 isolates, 77 were Staphylococcus and 22 were Streptococcus. Among the identified Staphylococcus isolates, S. aureus was the predominant species (36.4%). Resistance in Staphylococcus exceeded 70% for fosfomycin and was under 30% for the other antibiotics tested. In Streptococcus, S. uberis predominated (54.5%), with resistance primarily to penicillin and tetracycline (>50%). PCR identified mecA, nuc, and lukSF-PV genes in 7.8%, 29.9%, and 6.5% of Staphylococcus isolates, respectively. In Streptococcus, the ermB and blaZ genes were found in 18.2% and 50% of isolates, respectively. These data provide a baseline on SBM-associated AMR in the study area and highlight the need for ongoing surveillance and improved milking practices to mitigate risks to the dairy sector and public health.