Antimicrobial susceptibility of coagulase-negative Staphylococcus species isolated from bovine milk

摘要

Coagulase-negative Staphylococcus (CNS) isolates (n =168) obtained from milk from heifers and dairy cows were screened for minimum inhibitory concentration (MIC) to antimicrobials used commonly for mastitis therapy. Of the 10 CNS species included in the study, the predominant species were Staphylococcus chromogenes (n =61), Staphylococcus epidermidis (n =37), Staphylococcus hyicus (n =37), and Staphylococcus simulans (n =16). The majority of CNS was susceptible to ampicillin, oxacillin, cephalothin, and ceftiofur. Erythromycin and pirlimycin were also very effective in vitro inhibitors of CNS. The only exception was observed with S. epidermidis. Of 37 S. epidermidis evaluated, 13 (35%) exhibited efflux-based resistance to erythromycin (>=16og/ml) encoded by msrA and one isolate carried ermC encoding ribosomal methylase-based resistance to both erythromycin (>=64og/ml) and pirlimycin (>=64og/ml). A total of 17 S. epidermidis, 11 S. chromogenes, and one S. hyicus exhibited phenotypic resistance to ampicillin (>=0.5og/ml). Constitutive o-lactamase production was observed in all ampicillin resistant isolates except 4 S. epidermidis that exhibited inducible o-lactamase production. Induced o-lactamase production was also observed in 13 S. epidermidis that were phenotypically susceptible to the entire MIC panel. All isolates that produced o-lactamase either constitutively or by induction carried blaZ. S. epidermidis (n =12, 32%) that were resistant to methicillin (oxacillin >=0.5og/ml) carried low affinity penicillin-binding protein encoded by mecA. Most multi-drug resistant (MDR) S. epidermidis (>=2 resistance genes) were resistant to ampicillin, erythromycin and methicillin. All except one MDR S. epidermidis had icaAB, which encodes for polysaccharide intercellular adhesion. Based on pulsed field gel electrophoresis, MDR S. epidermidis were closely related genotypically, and were isolated from different cows on the same farm suggesting clonal dissemination. Bovine S. epidermidis share antimicrobial resistance patterns and virulence determinants of strains observed in human infections. Studying CNS at the species level can provide valuable information about species-specific differences that can be vital data for effective mastitis therapy and management.