Microbiology - Current Issue

Salmonella Typhimurium is a major Salmonella serovar that is found globally. It is responsible for outbreaks of self-limiting gastroenteritis that are broadly linked to the industrialization of food production. S. Typhimurium is a pathogen with a broad host range and remarkable metabolic versatility. The ∼5 Mb genome includes the pSLT virulence plasmid and has a characteristic prophage repertoire. The major virulence determinants are encoded by a variety of pathogenicity islands. Emerging multidrug-resistant lineages of epidemics of S. Typhimurium are currently causing bloodstream infections in sub-Saharan Africa. The versatility and adaptability of S. Typhimurium pose an important public health challenge.

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. Pseudomonas aeruginosa is a common opportunistic pathogen that is less susceptible to sphingosine than many Gram-negative bacteria. Here, we determined that the deletion of the sphBCD operon reduced growth in the presence of sphingosine. Using deletion mutants, complementation and growth assays in P. aeruginosa PAO1, we determined that the sphC and sphB genes, encoding a periplasmic oxidase and periplasmic cytochrome c, respectively, were important for growth on sphingosine, while sphD was dispensable under these conditions. Deletion of sphBCD in P. aeruginosa PA14, Pseudomonas protegens Pf-5 and Pseudomonas fluorescens Pf01 also showed reduced growth in the presence of sphingosine. The P. aeruginosa sphBC genes were also important for growth in the presence of two other sphingoid bases, phytosphingosine and sphinganine. In WT P. aeruginosa, sphingosine is metabolized to an unknown non-inhibitory product, as sphingosine concentrations drop in the culture. However, in the absence of sphBC, sphingosine accumulates, pointing to SphC and SphB as having a role in sphingosine metabolism. Finally, the metabolism of sphingosine by WT P. aeruginosa 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 P. aeruginosa 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.