Roles of FadRACB system in formaldehyde detoxification, oxidative stress alleviation and antibiotic susceptibility in Stenotrophomonas maltophilia

Background: Formaldehyde toxicity is invariably stressful for microbes. Stenotrophomonas maltophilia, a human opportunistic pathogen, is widely distributed in different environments and has evolved an array of systems to alleviate various stresses. Objectives: To characterize the role of the formaldehyde detoxification system FadRACB of S. maltophilia in formaldehyde detoxification, oxidative stress alleviation and antibiotic susceptibility. Methods: Presence of the fadRACB operon was verified by RT-PCR. Single or combined deletion mutants of the fadRACB operon were constructed for functional assays. Formaldehyde, menadione and quinolone susceptibilities were assessed by observing cell viability in formaldehyde-, menadione- and quinolone-containing media, respectively. Susceptibility to hydrogen peroxide was evaluated by disc diffusion assay. The agar dilution method was used to assess bacterial antibiotic susceptibilities. Expression of fadRACB was assessed by quantitative RT-PCR. Results: The fadR, fadA, fadC and fadB genes were arranged in an operon. Mutants of fadA and/or fadB were more susceptible to formaldehyde and oxidative stress than the WT KJ strain of S. maltophilia. No significant difference was observed in the ability of a fadC single mutant to ameliorate formaldehyde and oxidative stress; however, simultaneous inactivation of fadA, fadB and fadC further enhanced susceptibility to formaldehyde and oxidative stress. In addition, compared with WT KJ, the triple mutant KJ Delta FadACB was more susceptible to quinolones and more resistant to aminoglycosides. FadR functions as a repressor for the fadRACB operon. The FadRACB operon has moderate expression in aerobically grown WT KJ and is further derepressed by formaldehyde challenge or oxidative stress, but not by antibiotics. Conclusions: The FadACB system contributes to mitigation of formaldehyde toxicity and oxidative stress and cross-protects S. maltophilia fromquinolones.