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

Abstract

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.

Infections in Patients with Cystic Fibrosis

“…First, the use of oral TMP-SMX has been associated with the emergence of small colony variants (SCV) of S aureus. [16][17][18] The resistance mechanism in these organisms is their ability to use thymidine obtained from the environment, bypassing the folic acid synthesis pathway, the target of TMP-SMX. Such organisms are considered to be thymidine dependent.…”

“…Such organisms are not as fit as wild-type organism and grow slowly, producing small, streptococcallike colonies on most enrichment media. 16 Another SCV that is seen in S aureus are strains with defect in electron transport. These strains are resistant to aminoglycosides.…”

“…4) and chromogenic S aureus agar, are effective in the recovery of SCV and wild-type strains. 16,26 SCV S aureus have typical morphology on mannitol salts agar, whereas on chromogenic media, these organisms may grow more slowly and not give typical phenotypic reactions. 16,26 Organism identification is straightforward, with catalase and latex agglutination or coagulase testing being highly accurate.…”

“…16,26 SCV S aureus have typical morphology on mannitol salts agar, whereas on chromogenic media, these organisms may grow more slowly and not give typical phenotypic reactions. 16,26 Organism identification is straightforward, with catalase and latex agglutination or coagulase testing being highly accurate. 27 MALDI-TOF MS works well for identification of S aureus from patients with CF, although its ability to speciate SCV has not been reported.…”

Are lower airway or throat cultures predictive of sinus bacteriology in cystic fibrosis?

“…Burkholderia cepacia-selective agar was used for recovery of B. cepacia and similar organisms. 10,11 Cultures and stains for viruses, fungi, and mycobacteria were carried out on all BALF samples.…”