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Infection and Immunity, September 2005, p. 6075-6084, Vol. 73, No. 9
0019-9567/05/$08.00+0 doi:10.1128/IAI.73.9.6075-6084.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Department of Clinical Laboratory Sciences and Medical Biotechnology, National Taiwan University College of Medicine, Taipei, Taiwan, Republic of China,1 Department of Laboratory Medicine, National Taiwan University Hospital and National Taiwan University College of Medicine, Taipei, Taiwan, Republic of China2
Received 15 December 2004/ Returned for modification 16 March 2005/ Accepted 4 May 2005
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 dapASm-nlpBSm 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 nlpBSm, which encodes a membrane lipoprotein, NlpBSm, but not dapASm, is specifically involved in swarming regulation. On the other hand, dapASm but not nlpBSm is responsible for the determination of cell envelope architecture, regulation of hemolysin production, and cellular attachment capability. While the nlpBSm mutant showed similar cytotoxicity to its parent strain, the dapASm mutant significantly increased in cytotoxicity. We present evidence that DapASm is involved in the determination of cell-envelope-associated phenotypes and that NlpBSm is involved in the regulation of swarming motility.
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