Different distribution of antimicrobial resistance genes and virulence profiles of Staphylococcus aureus strains isolated from clinical mastitis in six countries

摘要

Staphylococcus aureus is recognized worldwide as oneof the main contagious mastitis agents in cattle andcan express a set of antimicrobial resistance genes andvirulence-associated genes that explain the wide rangeof outcomes of intramammary infections. Staphylococcusaureus strains are heterogeneous: their differentresistance and virulence patterns, associated with hostlevelfactors and treatment factors, are related to theseverity of infection. The aim of this study was to determinephenotypic antibiotic susceptibility, occurrence ofselected antimicrobial resistance genes and other virulencegenes in 93 S. aureus strains isolated from clinicalmastitis in 6 countries: Argentina, Brazil, Germany,Italy, the United States (New York State), and SouthAfrica. These isolates were tested against a total of 16drugs (amoxicillin-clavulanate, ampicillin, cefazolin,cefoperazone, cefquinome, enrofloxacin, erythromycin,gentamicin, kanamycin, lincomycin, oxacillin, penicillin,rifampin, spiramycin, sulfamethoxazole/trimethoprim,tylosin) by minimum inhibitory concentration (MIC)assay, and examined for the presence of 6 antibiotic-resistancegenes (blaZ, mecA, mecC, ermA, ermB, ermC)and 6 virulence-associated genes (scn, chp, sak, hla, hlb,sea) via PCR analysis. The phenotypic results of thisstudy revealed the presence of 19.4% penicillin-resistantstrains, whereas 22.6% of the strains were classified ashaving resistance (5.4%) or intermediate resistance(17.2%) to erythromycin. Most (96.8%) of the isolateswere inhibited by cephalosporins, and all were susceptibleto amoxicillin-clavulanate. Two strains (1 fromGermany, 1 from Italy) were resistant to oxacillin andwere positive for mecA. Among the other antimicrobialresistance genes, the most frequently detected wasblaZ (46.2%), and 32.3% of the isolates were positivefor erm genes: ermC (21.5%) and ermB (10.8%). Themost prevalent virulence gene was hla (100%), followedby hlb (84.9%) and sea (65.6%). These results show alow prevalence of antibiotic multidrug resistance in S.aureus isolates, even if the detection of selected antimicrobialresistance genes did not always correspond withthe occurrence of phenotypic antibiotic resistance; theimmune evasion cluster gene prevalence was quite lowin the samples analyzed.