Staphylococcus aureus urinary tract bacteriuria: single-institutional antibiotic susceptibility trends over a decade

Objectives: Methicillin resistant Staphylococcus aureus (MRSA) is a troublesome pathogen which is difficult for clinicians to treat. The purpose of this surveillance program is to assess the prevalence of MRSA urinary tract infections and determine risk factors for methicillin resistance in adults amongst urinary isolates of SA and to describe the antibiotic susceptibilities to guide empirical therapy. Methods: From 2005 through to 2014, we retrospectively reviewed urine cultures recorded in a laboratory database at a university hospital in Cambridge, UK. Susceptibility testing was performed by BSAC (British Society of Antimicrobial Chemotherapy) disc diffusion testing and reported for fluoroquinolones, gentamicin, nitrofurantoin, linezolid, trimethoprim and vancomycin. Samples were denoted “MRSA” if they were resistant to oxacillin or cefoxitin. Results: In total, 690 cultures were positive for SA, of which 293 (42.5%) were methicillin resistant. The number of SA bacteriuria decreased from around 100 per year to 40 per year. The proportion demonstrating methicillin resistance decreased from around 60% to around 20%. Both methicillin-sensitive Staphylococcus aureus (MSSA) and MRSA isolates were susceptible to vancomycin and nitrofurantoin. MRSA isolates demonstrated some increased resistance to trimethoprim and gentamicin and greatly increased resistance to fluoroquinolones. Urinary catheterization and increasing age were risk factors for methicillin resistance. Conclusion: The incidence of SA and MRSA bacteriuria decreased during the study period. A high degree of resistance to fluoroquinolones was observed in MRSA compared to MSSA. Analysis of antibiotic susceptibility profiles suggests nitrofurantoin and trimethoprim may be useful in treating uncomplicated MSSA and MRSA urinary tract infections without concurrent bacteremia.