Integrated regulation involving quorum sensing, a two-component system, a GGDEF/EAL domain protein and a post-transcriptional regulator controls swarming and RhlA-dependent surfactant biosynthesis in Serratia

Serratia sp. ATCC 39006 (Serratia 39006) is a Gram-negative bacterium which produces the secondary metabolite antibiotics, prodigiosin and 1-carbapen-2-em-3-carboxylic acid and secretes plant cell wall degrading enzymes. In this study we have identified mutations in the genes, pigX, rap and rsmA, which caused increased production of a previously unidentified surfactant and flagella-dependent swarming phenotype in Serratia 39006. Analysis of both the biosynthesis and regulation of surfactant production and swarming, revealed FlhC, quorum sensing, a GGDEF/EAL domain protein (PigX), a GacAS two-component system, an Rsm system and Rap as key regulators. In addition, surfactant biosynthesis required a protein similar to RhlA, involved in rhamnolipid synthesis in Pseudomonas aeruginosa. Homologues of RhlA have not previously been identified in members of the Enterobacteriaceae. Furthermore, we provide evidence that the surfactant may be responsible for dispersal of the antimicrobial pigment, prodigiosin. This study demonstrates the complex regulatory inputs into the coordinated multicellular swarming phenotype in Serratia.