Genetic diversity and antimicrobial susceptibility profiles among mastitis-causing Staphylococcus aureus isolated from bovine milk samples

OBJECTIVE: To determine whether particular antimicrobial susceptibility profiles of bovine mastitis-causing Staphylococcus aureus isolates were associated with specific S aureus genotypes. SAMPLE POPULATION:357 S aureus isolates recovered from milk samples submitted for diagnostic bacteriologic testing from 24 dairy herds. PROCEDURES: Antimicrobial susceptibility of S aureus isolates was assessed by determining minimum inhibitory concentrations (MICs) to 14 antimicrobial agents. After digestion of S aureus genomic DNA by SmaI, electrophoretic patterns were obtained via pulsed-field gel electrophoresis (PFGE) and used to classify isolates into types. Gels were analyzed, and data were used to prepare dendrograms. RESULTS: 308 of 357 (86%) S aureus isolates were susceptible to all antimicrobials evaluated. Forty-nine S aureus isolates were resistant to 1 or more antimicrobials; of these isolates, 37 were resistant only to penicillin, 9 were resistant to penicillin and erythromycin, 2 were resistant to tetracycline, and 1 was resistant to erythromycin. Isolates were assigned to 7 PFGE types. An association was found between PFGE type and antimicrobial susceptibility profile. Organisms with resistance to at least one of the tested antimicrobial agents were identified in only 4 of the 7 types of S aureus. CONCLUSIONS AND CLINICAL RELEVANCE: Antimicrobial resistance was uncommon among the mastitis-causing S aureus isolates identified in the milk samples. A limited number of genotypes were associated with mastitis in these herds. Antimicrobial resistance phenotypes were associated with particular S aureus PFGE types; this association may have implications for future treatment and control of S aureus-associated mastitis in cattle.