Susceptibility profiles and genetic typing of oral MRSA isolates

Strains of methicillin resistant Staphylococcus aureus (MRSA) are a significant cause of nosocomial infection in hospitals. There is increasing concern about the involvement of MRSA outwith health care facilities, so-called community acquired MRSA. The oral flora represents a potential source of this organism in both hospital and community settings. Objective: To analyse the genotype and antibiotic susceptibility profile of a collection of MRSA isolates derived from the mouths of 11 hospital in-patients and 24 out-patients. Methods: Swabs from relevant specimens submitted to a diagnostic oral microbiology laboratory were inoculated onto Columbia blood agar and mannitol salt agar for incubation at 37^oC for 48 hours. Colonies resembling staphylococci underwent coagulase and DNAse tests. Antibiotic susceptibility tests were performed using the Stokes method. Methicillin resistant isolates were genotyped by pulsed field gel electrophoresis. Results: Of the MRSA isolates from hospital in-patients, 55% were EMRSA15 and 45% were EMRSA16. Of the isolates from out-patients, 83% were EMRSA15, 13% were EMRSA16 and one isolate was SMRSA 106. All isolates were resistant to penicillin, amoxicillin and flucloxacillin, but sensitive to fucidin and tetracycline. All in-patient isolates were resistant to erythromycin and cephradine and 73% were resistant to neomycin. Of the out-patient isolates, 79% of isolates were resistant to erythromycin and cephradine and 29% were resistant to neomycin. Conclusion: Strains of MRSA may be isolated from the oral cavity of both hospital in-patients and those in the community. The mouth should be considered as a source of infection with MRSA in both of these settings.