Occurrence of resistance to antibiotics, metals, and plasmids in clinical strains of Staphylococcus spp.

BACKGROUND: Presence of staphylococci resistant to antibiotics is of concern, as is the possible spread of resistance determinants among Staphylococcus species. During the past several years, a series of staphylococcal isolates that demonstrated reduced susceptibility to vancomycin or other glycopeptides has been reported. However, there are no useful data on species sensitivity of coagulase-negative staphylococci to heavy metals.
METHODS: A total of 22 Staphylococcus spp. isolates recovered from clinical sources was studied for antibiotic and heavy metal resistance patterns and plasmid profiles. Isolated strains were biochemically identified by conventional tests followed by use of API Staph system. Isolates were tested for antimicrobial susceptibility by standard methods. Resistance patterns to nickel chloride (NiCl(2)), zinc sulfate (ZnSO(4)), lead acetate Pb(CH(3)COO)(2), cobalt chloride (CoCl(2)), copper sulfate (CuSO(4)), potassium chromate (K(2)Cr(2)O(7)), silver nitrate (AgNO(3)), and mercuric chloride (HgCl(2)) were surveyed by agar-dilution method. Plasmid DNA was prepared according to a previously described modification of alkaline lysis procedure.
RESULTS: In this study, isolated strains included eight (36.4%) Staphylococcus xylosus, five (22.7%) Staphylococcus aureus, four (18.2%) nonidentifiable Staphylococcus spp., one (4.5%) Staphylococcus capitis, one (4.5%) Staphylococcus lentus, one (4.5%) Staphylococcus epidermidis, one (4.5%) Staphylococcus sciuri, and one (4.5%) Staphylococcus chromogenes. All strains showed widest resistance profile to both antibiotics and metals. Methicillin resistance of Staphylococcus aureus (MRSA) was 40%. Resistance to oxacillin was 53% in coagulase-negative staphylococci (CNS) strains. No isolate was resistant to vancomycin. All isolates were resistant to K(2)Cr(2)O(7) and Pb(CH(3)COO)(2); on the other hand, all isolates were sensitive to AgNO(3). Highest incidence of metal-antibiotic multiple resistance existed between K(2)Cr(2)O(7), Pb(CH(3)COO)(2), and penicillin-G. Plasmid content and profile studies showed that isolates carried plasmids ranging from 2.224 to 20.650 kb in size.
CONCLUSIONS: Fifty percent of studied strains harbored plasmids and association between occurence of plasmids and resistance to antibiotics and heavy metals was observed.