Swarming in Serratia marcescens is a specialized form of bacterial populational surface migration behavior. A precocious swarming mutant S. marcescens O6 whose rssC is interrupted by mini-Tn5 transposon was obtained. The 364 amino-acid RssC showed a high identity to members of acyltransferase family. Characterization of the rssC knock-out mutant S. marcescens PC105 showed aberrant and hypervirulent phenotypes including precocious swarming behavior, reduced cell attachment ability and increase in swimming motility, flagellin synthesis, capsular polysaccharide (CPS) production, hemolysin activity, and cytotoxicity activity. Besides a more intense pattern of O-antigen sugar unit ladder, S. marcescens PC105 also showed an altered cell surface topology. A potential FlhDC binding site was identified at the rssC promoter region. In accordance, rssC transcriptional activity was up-regulated by FlhDC(Sm). Briefly, rssC is hierarchically classified as class II members in FlhDC(Sm) regulon, and reciprocally defect in rssC results in aberrant swarming and hypervirulence in S. marcescens. (C) 2008 Elsevier Inc. All rights reserved.