Prevalence of thymidine-dependent Staphylococcus aureus in patients with cystic fibrosis.

During a 1-year period, the prevalence of thymidine-dependent (TD) Staphylococcus aureus in patients at two geographically distinct cystic fibrosis (CF) centers was determined. Of 200 CF patients who had their respiratory secretions cultured, 95 harbored S. aureus, and 20 (21%) had TD S. aureus as their predominant staphylococcal isolate. All 20 TD S. aureus-positive patients had received trimethoprim-sulfamethoxazole for an average of 30.9 months. It was also observed that TD S. aureus exhibited aberrant colony morphologies or did not grow on media commonly used in CF centers for S. aureus isolation, suggesting that this organism could be missed by routine culture methods. In contrast, all 20 isolates had typical staphylococcal morphology on mannitol salt agar after 48 h of incubation. Mannitol salt agar is recommended for primary isolation of TD S. aureus.