Swarming motility is modulated by expression of the putative xenosiderophore transporter SppR-SppABCD in Pseudomonas aeruginosa PA14.

In the present study, we characterised the putative peptide ABC transporter SppABCD, which is co-transcribed with the TonB-dependent receptor SppR in Pseudomonas aeruginosa PA14. However, our data show that this transporter complex is not involved in the uptake of peptides. The fact that the TonB-dependent receptor SppR is regulated by an iron starvation ECF sigma factor suggested that this transporter is probably involved in the uptake of xenosiderophores. Therefore, we screened culture supernatants of 23 siderophore-producing bacteria for their ability to induce the expression of the SppR-regulating ECF sigma factor. However, none of them had an effect on the expression of this ECF sigma factor. Since the spp operon is not expressed under standard laboratory conditions, we overexpressed it from plasmids in PA14, which led to an impairment of its swarming motility on semisolid agar. Since we excluded the possibility that the uptake of a culture medium component was responsible for the observed phenotype, we hypothesize that the Spp transport system is involved in the uptake of a compound from the periplasmic space or a compound secreted by P. aeruginosa. Furthermore, we found that rhamnolipid synthesis was decreased while biofilm and exopolysaccharide synthesis was slightly increased upon overexpression of the spp operon. Moreover, we observed an impact of spp overexpression on regulation of genes involved in siderophore and phenazine biosynthesis.