Presence of new mecA and mph(C) variants conferring antibiotic resistance in Staphylococcus spp. isolated from the skin of horses before and after clinic admission

Abstract

Because of the frequency of multiple antibiotic resistance, Staphylococcus species often represent a challenge in incisional infections of horses undergoing colic surgery. To investigate the evolution of antibiotic resistance patterns before and after preventative peri- and postoperative penicillin treatment, staphylococci were isolated from skin and wound samples at different times during hospitalization. Most staphylococci were normal skin commensals and belonged to the common coagulase-negative group. In some cases they turned out to be opportunistic pathogens present in wound infections. MICs were determined for 12 antibiotics, and antibiotic resistance genes were detected by microarray. At hospital admission, horses harbored staphylococci that were susceptible to antibiotics or resistant to one group of drugs, mainly due to the presence of new variants of the methicillin and macrolide resistance genes mecA and mph(C), respectively. After 3 days, the percentage of Staphylococcus isolates displaying antibiotic resistance, as well as the number of resistance genes per isolate, increased moderately in hospitalized horses without surgery or penicillin treatment but dramatically in hospitalized horses after colic surgery as well as penicillin treatment. Staphylococcus species displaying multiple resistance were found to harbor mainly genes conferring resistance to beta-lactams (mecA and blaZ), aminoglycosides [str and aac(6')-Ie-aph(2')-Ia], and trimethoprim [dfr(A) and dfr(D)]. Additional genes conferring resistance to macrolides [mph(C), erm(C), and erm(B)], tetracycline [tet(K) and tet(M)], chloramphenicol [cat(pC221) and cat(pC223)], and streptothricin (sat4) appeared in several strains. Hospitalization and preventive penicillin use were shown to act as selection agents for multidrug-resistant commensal staphylococcal flora.

FIG. 1.

Distribution of antibiotic resistance (based on phenotypic and/or genotypic testing) in 72 CoNS strains and three S. aureus strains isolated, before and after admission to the clinic and before and after penicillin treatment, from 19 horses seen at the equine clinic of the Vetsuisse Faculty of the University of Berne between August 2004 and May 2005. Antibiotics: AMC, amoxicillin-clavulanic acid; CHL, chloramphenicol; CLI, clindamycin; ENR, enrofloxacin; ERY, erythromycin; GEN, gentamicin; OXA, oxacillin; PEN, penicillin; STH, streptothricin; STR, streptomycin; SYN, quinupristin-dalfopristin; TET, tetracycline; TMP, trimethoprim; VAN, vancomycin; XNL, ceftiofur.

FIG. 2.

Occurrence of resistance genes in 72 CoNS strains and three S. aureus strains isolated, before and after admission to the clinic and before and after penicillin treatment, from 19 horses seen at the equine clinic of the Vetsuisse Faculty of the University of Berne between August 2004 and May 2005. The detected antibiotic resistance genes confer resistance to β-lactams (mecA and blaZ), macrolides [mph(C), erm(B), and erm(C)], tetracycline [tet(K) and tet(M)], chloramphenicol [cat(pC221) and cat(pC223)], trimethoprim [dfr(A) and dfr(D)], streptothricin (sat4), and aminoglycosides [aac(6′)-Ie-aph(2′)-Ia, ant(4′)-Ia, ant(6)-Ia, aph(3′)-III, aph(2′)-Ic, and str].