Antimicrobial Effectiveness of Methicillin, Amoxicillin and Ampicillin against Methicillin-Resistant Staphylococcus Aureus (MRSA) Strains Isolated from Bovine Mastitis

Staphylococcus aureus is a Gram-positive pathogen that is commonly associated with bovine mastitis, which is an infectious condition affecting the udder of cows. The objective of the present study is to evaluate the antimicrobial efficacy of methicillin, amoxicillin, and ampicillin for detecting methicillin-resistant Staphylococcus aureus (MRSA) strains isolated from cases of bovine mastitis. Methodologically, it was established to carry out the antimicrobial activity using the plate disc diffusion method (Kirby-Bauer) with the use of amoxicillin and ampicillin at 2 µg and 10 µg, also following the recommendations of the Clinical and Laboratory Standards Institute. To detect resistant methicillin strains, oxacillin was used at 1 µg, and cefoxitin at 30 µg. The study was conducted against 50 strains of the bacteria under investigation, which were isolated and identified. The applied treatments demonstrated a highly significant effect (p< 0.05). In addition, a resistance of 50% and 60% to concentrations of 2 µg of amoxicillin and ampicillin, respectively, was observed. Furthermore, a resistance of 60% and 68% to 10µg of amoxicillin and ampicillin, respectively, was observed, indicating that Staphylococcus aureus is developing mechanisms that confer antimicrobial resistance. To further investigate this, the antimicrobial activity was conducted using 1 µg oxacillin discs and 30 µg cefoxitin, revealing that 36% and 32% of the isolates were resistant to these drugs, respectively. Phenotypically, 32% (n=16) of the isolates were identified as methicillin-resistant Staphylococcus aureus (MRSA), demonstrating resistance to all beta-lactams tested.