The study was intended for identification and characterization of Staphylococcus aureus isolated from bovine subclinical mastitis cases. A total of 143 milk samples were collected from apparently normal cows from Basrah province. California mastitis test was used to detect 81 (56.6%) samples as subclinical mastitis. However, by using bacteriological and biochemical tests 36(44.44%) isolates were confirmed as S. aureus. Antimicrobials susceptibility assays of isolates revealed that, all of them were completely susceptible to chloramphenicol, gentamycin and vancomycin. Oxacillin and cefoxitin susceptibility illustrated that 22(61.1%) isolates were resistant to methicillin (MRSA) and 14 (38.9%) isolates were methicillin susceptible (MSSA). Phenotypic production of slime and biofilm were evaluated by using Congo red agar and microtiter plate techniques, 31 (86.1%) isolates were slime producer and 29 (80.6%) were biofilm producers from all tested isolates. The production of slime and biofilm of MRSA isolates were 95.5% and 90.9%, whereas, for MSSA were 71.4% and 64.3% respectively. The differences in slime and biofilm production among MRSA and MSSA isolates were statistically significant.
All MRSA isolates were resistant to oxacillin, penicillin, cefoxitin, ampicillin and cefotaxime, and these isolates showed low resistance to erythromycin and lincomycin each (9.1%) and doxycycline (18.2%). However, these isolates were susceptible to vancomycin, gentamycin, nitrofurantoin and chloramphenicol. All MSSA isolates exhibited resistance to ampicillin and 85.7% of isolates to cefotaxime, however, all isolates were sensitive to azithromycin, cefoxitin, chloramphenicol, gentamycin, oxacillin and vancomycin. The differences in antimicrobial susceptibility between MRSA and MSSA isolates were highly statistically significant.
Results of this study indicate that Staphylococcus aureus is the most important agent of bovine subclinical mastitis, isolates which have resistance to methicillin and produce biofilm have propensity to be multiple antibiotic resistant