The Pseudomonas aeruginosa sphBC genes are important for growth in the presence of sphingosine by promoting sphingosine metabolism

Sphingoid bases, including sphingosine, are important components of the antimicrobial barrier at epithelial surfaces where they can cause growth inhibition and killing of susceptible bacteria. <i>Pseudomonas aeruginosa</i> is a common opportunistic pathogen that is less susceptible to sphingosine than many Gram-negative bacteria. Here, we determined that the deletion of the <i>sphBCD</i> operon reduced growth in the presence of sphingosine. Using deletion mutants, complementation and growth assays in <i>P. aeruginosa</i> PAO1, we determined that the <i>sphC</i> and <i>sphB</i> genes, encoding a periplasmic oxidase and periplasmic cytochrome c, respectively, were important for growth on sphingosine, while <i>sphD</i> was dispensable under these conditions. Deletion of <i>sphBCD</i> in <i>P. aeruginosa</i> PA14, <i>Pseudomonas protegens</i> Pf-5 and <i>Pseudomonas fluorescens</i> Pf01 also showed reduced growth in the presence of sphingosine. The <i>P. aeruginosa sphBC</i> genes were also important for growth in the presence of two other sphingoid bases, phytosphingosine and sphinganine. In WT <i>P. aeruginosa</i>, sphingosine is metabolized to an unknown non-inhibitory product, as sphingosine concentrations drop in the culture. However, in the absence of <i>sphBC</i>, sphingosine accumulates, pointing to SphC and SphB as having a role in sphingosine metabolism. Finally, the metabolism of sphingosine by WT <i>P. aeruginosa</i> protected susceptible cells from full growth inhibition by sphingosine, pointing to a role for sphingosine metabolism as a public good. This work shows that the metabolism of sphingosine by <i>P. aeruginosa</i> presents a novel pathway by which bacteria can alter host-derived sphingolipids, but it remains an open question whether SphB and SphC act directly on sphingosine.