Characterization of the dapA-nlpB genetic locus involved in regulation of swarming motility, cell envelope architecture, hemolysin production, and cell attachment ability in Serratia marcescens

Swarming migration of Serratia marcescens requires both flagellar motility and cellular differentiation and is a population-density-dependent behavior. While the flhDC and quorum-sensing systems have been characterized as important factors regulating S. marcescens swarming, the underlying molecular mechanisms are currently far from being understood. Serratia swarming is thermoregulated and is characterized by continuous surface migration on rich swarming agar surfaces at 30°C but not at 37°C. To further elucidate the mechanisms, identification of specific and conserved regulators that govern the initiation of swarming is essential. We performed transposon mutagenesis to screen for S. marcescens strain CH-1 mutants that swarmed at 37°C. Analysis of a "precocious-swarming" mutant revealed that the defect in a conserved dapA_Sm-nlpB_Sm genetic locus which is closely related to the synthesis of bacterial cell wall peptidoglycan is responsible for the aberrant swarming phenotype. Further complementation and gene knockout studies showed that nlpB_Sm, which encodes a membrane lipoprotein, NlpB _Sm, but not dapA_Sm, is specifically involved in swarming regulation. On the other hand, dapA_Sm but not nlpB_Sm is responsible for the determination of cell envelope architecture, regulation of hemolysin production, and cellular attachment capability. While the nlpB _Sm mutant showed similar cytotoxicity to its parent strain, the dapA_Sm mutant significantly increased in cytotoxicity. We present evidence that DapA_Sm is involved in the determination of cell-envelope-associated phenotypes and that NlpB_Sm is involved in the regulation of swarming motility.