One hundred two isolates of methicillin-resistant Staphylococcus aureus (MRSA) randomly selected from across the Canadian province of Ontario were tested for their susceptibility to ciprofloxacin, norfloxacin, and nalidixic acid by the agar dilution method. Forty-nine percent (50 of 102) had high levels of resistance to these quinolone compounds. For the 50 resistant isolates, ciprofloxacin and norfloxacin had high MICs for 90% of isolates (MIC90s) of 128 ,ug/ml and >128 ,ug/ml, respectively; for these isolates, the nalidixic acid MIC90 was >640 ,ug/ml. The majority (98%) of the 50 isolates were also resistant to tobramycin (MIC90, >128 ,ug/ml), while 42% of the isolates were resistant to gentamicin (MIC90, 64 ,ig/ml). Quinolone-resistant MRSA isolates were susceptible to bacteriophages from several groups, indicating independent selection of resistant strains. These results suggest that a reappraisal of the use of fluoroquinolones against MRSA in Canada is necessary.

Staphylococcus aureus is the predominant cause of bacteremia worldwide. We assessed the molecular epidemiology and antibiotic resistance of MRSA isolates causing bacteremia in Southern Mississippi. Diverse genetic backgrounds in terms of SCCmec, PFGE, and MLST types of MRSA were identified as causing bacteremia in Mississippi. A strong association of Panton – Valentine Leukocidin (PVL) genes with elevated vancomycin MIC is one of the important findings of our study.