Loss of the WaaL O-Antigen Ligase Prevents Surface Activation of the Flagellar Gene Cascade in Proteus mirabilis

Abstract

Proteus mirabilis is a Gram-negative bacterium that undergoes a physical and biochemical change from a vegetative swimmer cell (a typical Gram-negative rod) to an elongated swarmer cell when grown on a solid surface. In this study, we report that a transposon insertion in the waaL gene, encoding O-antigen ligase, blocked swarming motility on solid surfaces but had little effect on swimming motility in soft agar. The waaL mutant was unable to differentiate into a swarmer cell. Differentiation was also prevented by a mutation in wzz, encoding a chain length determinant for O antigen, but not by a mutation in wzyE, encoding an enzyme that polymerizes enterobacterial common antigen, a surface polysaccharide different from the lipid A::core. In wild-type P. mirabilis, increased expression of the flhDC operon occurs after growth on solid surfaces and is required for the high-level expression of flagellin that is characteristic of swarmer cells. However, in both the waaL and the wzz mutants, the flhDC operon was not activated during growth on agar. A loss-of-function mutation in the rcsB response regulator or overexpression of flhDC restored swarming to the waaL mutant, despite the absence of O antigen. Therefore, although O antigen may serve a role in swarming by promoting wettability, the loss of O antigen blocks a regulatory pathway that links surface contact with the upregulation of flhDC expression.

Role of the Umo Proteins and the Rcs Phosphorelay in the Swarming Motility of the Wild Type and an O-Antigen ( waaL ) Mutant of Proteus mirabilis

“…The mechanism by which FliL transmits a signal to regulate gene expression has not been defined (3). More recently, we proposed a role for the O antigen in sensing surfaces (35). We hypothesized that O antigen was needed to sense solid surfaces and to relay this signal to the cell through the Rcs phosphorelay.…”

“…Our previous study indicated a role for O antigen in surface sensing and the Rcs phosphorelay in transmitting a surface contact signal to transcriptional regulation (35). Mutations in the waaL gene (PMI3163, formerly rfaL), encoding O-antigen ligase, and wzz (PMI2183, formerly cld), encoding a chain-length determinant for O antigen, result in the failure to derepress the class I flagellar activator FlhDC on solid surfaces.…”

Proteus mirabilis interkingdom swarming signals attract blow flies

“…The swarming behavior involves a complex repeating cycle of differentiation between two cell types, the vegetative (swimmer) and swarmer cells (Janda and Abbott, 2005). The swimmer cells dominate in liquid and change into swarmer cells with longer cell length and more flagella after 3-4 h when they are placed on solid surfaces (Morgenstein et al, 2010). The flagellar rotation is inhibited during this conversion, and an extracellular signal is required to control this multicellular behavior.…”

“…RfaL (WaaL) is the lipopolysaccharide O-antigen ligase. In P. mirabilis, the deletion of rfaL causes reduced differentiation into swarmer cells primarily due to its repression of the flhDC operon (Morgenstein et al, 2010). In P. aeruginosa, the RfaL protein is a membrane protein with 11 potential transmembrane segments (Abeyrathne and Lam, 2007).…”

“…mirabilis is a Gram-negative urinary tract pathogen for humans (Morgenstein et al, 2010). The prominent feature of this bacterium is its ability to swarm.…”