Contribution of fepAsm, fciABC, sbaA, sbaBCDEF, and feoB to ferri-stenobactin acquisition in Stenotrophomonas maltophilia KJ.

Abstract

Background: Stenotrophomonas maltophilia, an opportunistic pathogen, is ubiquitously distributed in the environment. In response to iron-depletion stress, S. maltophilia synthesizes the sole catecholate-type siderophore, stenobactin, for ferric iron acquisition. FepAsm, a TonB-dependent transporter (TBDT), is the sole known outer membrane receptor responsible for ferri-stenobactin uptake in S. maltophilia K279a. However, S. maltophilia KJ and its isogenic fepA mutant displayed comparable ability to utilize FeCl₃ as the sole iron source for growth in iron-depleted conditions, suggesting the involvement of additional TBDT in ferri-stenobactin uptake in the KJ strain. Here, we aimed to determine additional TBDT required for ferri-stenobactin uptake and the post-TBDT ferri-stenobactin transport system in the KJ strain.

Methods and results: Twelve TBDTs, whose expression were significantly upregulated in 2,2'-dipyridyl-treated KJ strain, were selected as candidates for ferri-stenobactin uptake. The involvement of these selected candidates in ferri-stenobactin acquisition was investigated using deletion mutant construction and FeCl₃ utilization assay. Among the 12 TBDTs tested, FepAsm, FciA, and SbaA were the TBDTs for ferri-stenobactin uptake in KJ strain. Because fciA is a member of fciTABC operon, the involvement of fciTABC operon in ferri-stenobactin uptake was also investigated. Of the fciTABC operon, fciA, fciB and fciC, but not fciT, contributed to ferri-stenobatin acquisition. SbaE is the homolog of FepD/FepG, the inner membrane transporters for ferri-enterobactin in E. coli; therefore, sbaBCDEF operon was selected as a candidate for the post-TBDT transport system of ferri-stenobactin. All proteins encoded by sbaBCDEF operon participated in ferri-stenobactin acquisition. Due to the contribution of the putative periplasmic esterase SbaB to ferri-stenobactin acquisition, FeoB, a ferrous iron inner membrane transporter, was included as a candidate and proved to be involved in ferri-stenobactin acquisition. Accordingly, contributions of feoB and sbaE to ferri-stenobactin acquisition illustrated that ferric and ferrous iron could be transported across the inner membrane via SbaE and FeoB, respectively.

Conclusions: FepAsm, fciABC, sbaA, sbaBCDEF, and feoB contribute to ferri-stenobatin acquisition in Stenotrophomonas maltophilia KJ.