Overexpression of the Efflux Pumps SmeVWX and SmeDEF Is a Major Cause of Resistance to Co-trimoxazole in Stenotrophomonas maltophilia

Co-trimoxazole is one of the antimicrobials of choice for treating infections. Most works on the molecular epidemiology of the resistance to this drug combination are based on the analysis of genes. Nevertheless, the existence of clinical co-trimoxazole-resistant isolates that do not harbor genes has been reported. To investigate potential mutations that can reduce the susceptibility of to co-trimoxazole, spontaneous co-trimoxazole-resistant mutants isolated under different co-trimoxazole concentrations were studied. All mutants presented phenotypes compatible with the overexpression of either SmeVWX (94.6%) or SmeDEF (5.4%). Indeed, the analysis of a selected set of strains showed that the overexpression of either of these efflux pumps, which was due to mutations in their regulators and , respectively, was the cause of co-trimoxazole resistance. No other efflux pump was overexpressed in any of the studied mutants, indicating that they do not participate in the observed resistance phenotype. The analysis of mutants overexpressing or lacking SmeDEF or SmeVWX shows that SmeDEF contributes to the intrinsic and acquired resistance to co-trimoxazole in , whereas SmeVWX only contributes to acquired resistance. It is important to highlight that all mutants were less susceptible to other antibiotics, including chloramphenicol and quinolones. Since both SmeVWX and SmeDEF are major determinants of quinolone resistance, the potential cross-selection of resistance to co-trimoxazole and quinolones, when either of the antimicrobials is used, is of particular concern for the treatment of infections.