Objective: Antibiotic exposure is a risk factor for isolation of methicillin-resistant Staphylococcus aureus (MRSA). We reasoned that this relationship might be especially strong if the comparison was targeted to antibiotics with activity against methicillin-sensitive Staphylococcus aureus (MSSA), but not MRSA.
Subjects: Forty-eight cases with initial MRSA clinical isolates (15 urinary, 19 wound, 11 sputum, 3 blood) were matched to 48 with MSSA clinical isolates from the same location. In addition, 10 whose MRSA was noted screening around an index case were matched to 10 with negative screening cultures sampled around the same index case.
Design: MSSA was always/usually sensitive to dicloxacillin, amoxicillin/clavulanate, cephalosporins, imipenem, quinolones, and erythromycin, whereas MRSA was always/usually resistant. These antibiotics were defined as "targeted" and would be expected to facilitate the proliferation of MRSA by suppressing competing bacteria. We compared the use of all antibiotics over 60 days in cases and controls.
Results: The comparison of antibiotic exposure in the 48 clinical isolates revealed that 29 (60%) with MRSA isolates versus 16 (33%) with MSSA isolates had received a targeted antibiotic (RR 1.81, CI 1.33-3.34, P = 0.006). Twelve (25%) with MRSA clinical isolates versus 3 with MSSA isolates (6%) had received a nontargeted antibiotic (RR 4.00, CI 2.10-14.18, P = 0.009). The comparison of antibiotic exposure in MRSA screening isolates versus negative controls revealed that 7 (70%) had received a targeted antibiotic versus 2 (20%) of controls (RR 3.50, CI 1.57-16.85, P = 0.009)). One in the screening group versus 2 controls had received a nontargeted antibiotic. Twenty-one (62%) of 34 with MRSA (urine or wound) isolates versus 4 MSSA controls (12%) had received a quinolone (RR 5.25, CI 1.80-15.30, P < 0.000).
Conclusion: There was a significant association between antibiotic exposure and MRSA isolates. The association was especially strong for quinolones with urinary or wound isolation of MRSA. Our data do not support the hypothesis that targeted antibiotic use was more likely to be associated with MRSA isolation than nontargeted antibiotic use. The use of nontargeted antibiotics was low, with greater use in the MRSA clinical group.