oneidensis mutant by electroporation (Myers & Myers, 1997) and selected on LB medium containing the appropriate antibiotic. Microscopic visualization of biofilms, biofilm parameter analysis, and image processing were performed as described previously (Thormann et al., 2005, 2006). Transmission electron microscopy (TEM) was performed at the Cell Science Imaging Facility at Stanford
University on LM-grown cells stained with 2% uranyl acetate-negative stain on 200-mesh formvar-coated TEM copper grids. Images were obtained using a JEOL TEM1230 transmission electron microscope (Jeol Ltd, Tokyo, Japan). In a previous genetic screen, we had identified genes involved in pilus biogenesis and function, including mshA (coding for the main structural subunit of the MSHA pilus) and a homolog to
pilT, as well as genes of the mxd operon, to be critical for biofilm formation under hydrodynamic conditions (Thormann Thiazovivin et al., 2004). The biofilm phenotypes of ΔmshA and Δmxd mutants are opposite of each other in that ΔmshA biofilms do not form a contiguous surface coverage and remain loosely structured, whereas Δmxd mutant biofilms completely cover the substratum surface, but lack a three-dimensional structure (Fig. 1). Here, we examined biofilms of a constructed ΔmshAΔmxdB double mutant and found that they were entirely deficient in the initial attachment and biofilm formation (Fig. 1). The expression of mshA in trans rescued this phenotype in a static biofilm system click here such that the complemented double mutant exhibited biofilm formation to the same extent as the ΔmxdB mutant (data not shown). The initial adhesion phenotypes associated with the single and double mutants O-methylated flavonoid observed in LM were also observed in MM (data not shown). These data suggest that the mshA and mxd genes encode a complementary set of molecular machineries that constitute the dominant mechanisms enabling biofilm formation under the conditions tested. In the same
genetic screen, we also identified SO3351, a pilT homolog required for type IV pili-mediated twitching motility in other microorganisms (Mattick, 2002; Thormann et al., 2004). To test whether pilT behaves, in a genetic sense, as an msh class gene, we constructed ΔpilTΔmshA and ΔpilTΔmxdB double mutants. Biofilms of a ΔpilTΔmshA double mutant were very similar in architecture to those of a ΔpilT mutant (Fig. 1), and ΔpilTΔmxdB mutant biofilms exhibited a phenotype similar to the ΔmshAΔmxdB mutant (Fig. 1). The expression of pilT in trans rescued this phenotype in a static biofilm system such that the complemented double mutant exhibited the same extent of biofilm formation as the ΔmxdB mutant (data not shown). The initial adhesion phenotypes associated with the single and double mutants observed in LM were also observed in MM (data not shown). Various attempts to observe twitching motility in S.