Antimicrobial resistance in Staphylococcus aureus at the University of Chicago Hospitals:: A 15-year longitudinal assessment in a large university-based hospital

OBJECTIVES: To describe a longitudinal profile of resistance to beta-lactam antimicrobials among isolates of Staphylococcus aureus at a large university teaching hospital and to evaluate the impact of the methicillin resistance phenotype on resistance trends for non-beta-lactam antimicrobials. DESIGN: Retrospective evaluation of antimicrobial susceptibility data for all 17,287 S. aureus isolates obtained from January 1986 through December 2000. SETTING: The University of Chicago Hospitals, a family of tertiary-care, university-affiliated hospitals in Chicago, Illinois, consisting of 547 adult and pediatric beds. RESULTS: The annual rate of resistance to methicillin increased from 13% in 1986 to 28% in 2000 (P < .001) and has not plateaued. For each non-beta-lactam antimicrobial tested, the annual rates of resistance were far higher among methicillin resistant S. aureus (MRSA) isolates than among methicillin-susceptible S. aureus (MSSA) isolates. The annual rates of resistance to the macrolide, lincosamide, and streptogramin (MLS) antimicrobials erythromycin and clindamycin increased among MSSA isolates (P <.01), but remained lower than 20%. Resistance to the MLS antimicrobials was higher among MRSA isolates (higher than 60%), but the annual rate decreased significantly during the study (P < .01). CONCLUSION: The prevalence of methicillin resistance among S. aureus isolates has continued to increase; resistance to non-beta-lactam antimicrobials is far more common among MRSA isolates. Recent decreases in the proportion of MRSA isolates resistant to non-beta-lactam antimicrobials suggest important changes in the epidemiology of this pathogen.