Relationship among plasmids recovered from Staphylococcus aureus, milk leukocytes, and antimicrobial resistance

The efficacy and efficiency of mastitis control schemes in reducing the incidence of Staphylococcus aureus mastitis could be improved if the different strains of Staph. aureus were characterized by their pathogenicity. Plasmid finger printing of Staph. aureus strains could be a rather simple and efficient method to characterize the strains and their interaction with host cellular immune defenses. The aim of this study was to investigate the presence and distribution of plasmids in strains of Staph. aureus isolated from bovine mastitis and their relationship with mammary gland immune defenses and antimicrobial resistance. Overall, nine different plasmids were identified by their molecular weight. One or more plasmids were isolated from 293 strains (96.4%), and only 11 strains did not show the presence of any plasmid. A significant difference in polymorphonuclear neutrophil proportions was observed in association with 23-kb, 9416-bp, 2027-bp, or 1353-bp plasmids, and for macrophage proportions, significant differences were observed for 9416-bp, 6557-bp, or 1353-bp plasmids. Penicillin G minimum inhibitory concentration values showed significant differences when 6557-bp, 4361-bp, 2322-bp, or 2027-bp plasmids were present, and weak or no differences were observed for cephacetrile, rifamycin, and norfloxacin. The presence of 2027-bp plasmid was associated with a reduced neutrophil response and an increased resistance against beta-lactam antimicrobials; therefore, the strains carrying this plasmid have a higher probability to invade and colonize mammary gland, to survive to antimicrobial therapy, and to remain undetected. Results confirmed that plasmid fingerprinting is a simple and effective way to characterize Staph. aureus strains.