AmpR of Stenotrophomonas maltophilia is involved in stenobactin synthesis and enhanced β-lactam resistance in an iron-depleted condition

Background: Iron is an essential nutrient for almost all aerobic organisms, including Stenotrophomonas maltophilia. Fur is the only known transcriptional regulator presumptively involved in iron homeostasis in S. maltophilia. AmpR, a LysR-type transcriptional regulator, is known to regulate β-lactamase expression and β-lactam resistance in S. maltophilia. Objectives: To identify the novel regulator involved in controlling the viability of S. maltophilia in an iron-depleted condition and to elucidate the underlying regulatory mechanisms. Methods: The potential regulator involved in iron homeostasis was identified by studying the cell viabilities of different regulator mutants in 2,2′-dipyridyl (DIP)-containing medium. Iron-chelating activity was investigated using the chrome azurol S (CAS) activity assay. An iron source utilization bioassay was carried out to examine utilization of different iron sources. Gene expression was determined by quantitative real-time PCR, and the Etest method was used to evaluate antibiotic susceptibility. Results: Of the 14 tested mutants, the ampR mutant, KJΔAmpR, showed a growth compromise in DIP-containing medium. AmpR regulated stenobactin synthesis in an iron-depleted condition, but showed little involvement in the uptake and utilization of ferri-stenobactin and ferric citrate. AmpR was up-regulated by iron limitation and β-lactam challenge. S. maltophilia clinical isolates grown under conditions of iron depletion were generally more resistant to β-lactams compared with conditions of iron repletion. Conclusions: AmpR is a dual transcriptional regulator in S. maltophilia, which regulates the β-lactam-induced β-lactamase expression and iron depletion-mediated stenobactin synthesis. AmpR is, therefore, a promising target for the development of inhibitors.