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dTrimethoprim-sulfamethoxazole (SXT) is a possible alternative for the treatment of community-and hospital-acquired methicillin-resistant Staphylococcus aureus (MRSA) due to the susceptibility of most MRSA strains to the drug. However, after longterm treatment with SXT, thymidine-dependent (TD) SXT-resistant small-colony variants (SCVs) emerge. In TD-SCVs, mutations of thymidylate synthase ([TS] thyA) occur. Until now, it has never been systematically investigated that SXT is triggering the induction and/or selection of TD-SCVs. In our study, we performed induction, reversion, and competition experiments in vitro and in vivo using a chronic mouse pneumonia model to determine the impact of SXT on the emergence of TD-SCVs. SCVs were characterized by light and transmission electron microscopy (TEM) and auxotrophism testing. Short-term exposure of S. aureus to SXT induced the TD-SCV phenotype in S. aureus SH1000, while selection of TD-SCVs with thyA mutations occurred after long-term exposure. In reversion experiments with clinical and laboratory TD-SCVs, all revertants carried compensating mutations at the initially identified mutation site. Competition experiments in vitro and in vivo revealed a survival and growth advantage of the ⌬thyA mutant under low-thymidine availability and SXT exposure although this advantage was less profound in vivo. Our results show that SXT induces the TD-SCV phenotype after short-term exposure, while long-term exposure selects for thyA mutations, which provide an advantage for TD-SCVs under specified conditions. Thus, our results further an understanding of the dynamic processes occurring during SXT exposure with induction and selection of S. aureus TD-SCVs.

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Chitin present in fungal cell walls has been considered as a diagnostic polymer for the detection of fungal infections.