| Biofilms are collections of surface bound organisms encased in and protected by an extracellular polymeric matrix that consists of DNA, proteins, or polysaccharides. Bacteria inhabiting biofilms produce one or more polysaccharides that function to stabilize and reinforce the structure of the biofilm. Exopolysaccharides that are produced by bacteria in biofilms provide protection from biocides and antimicrobial agents, consequently rendering these infections extremely difficult to clear by traditional interventions. Historically, alginate has been considered the major exopolysaccharide of the Pseudomonas aeruginosa (P. aeruginosa) biofilm matrix in the cystic fibrosis (CF) lung. However, chemical and genetic studies have demonstrated that alginate is not involved in mediating the initiation of biofilm formation in two nonmucoid P. aeruginosa strains, PAO1 and PA14.; We hypothesized that there are alternative polysaccharides involved in mediating the initiation of biofilm formation. One putative polysaccharide locus, designated psl (p&barbelow;olysaccharide s&barbelow;ynthesis l&barbelow;ocus), was derived from the annotated genome of P. aeruginosa PAO1. Our data show that a strain mutated at the psl locus has a severe biofilm initiation defective phenotype as confirmed by static and continuous flow biofilm assays on abiotic surfaces. This impaired biofilm phenotype could be complemented with the wild type sequence, and was not due to defects in motility or epopolysaccharide biosynthesis. Characterization of the pslAB mutant was extended to include studies of more clinically relevant biotic surfaces. There was a significant decrease in adherence of the pslAB mutant to lung epithelial cells as well as a cystic fibrosis cell line when compared to the wild-type. Additionally, the pslAB mutant was impaired in its ability to attach to mucin-coated coverslips. Furthermore, immunogold electron microscopy revealed the presence of a psl-specific exopolysaccharide. Taken together, these results suggest that we have found a novel exopolysaccharide produced in P. aeruginosa biofilms that is responsible for mediating cell-surface and cell-cell interactions. Further understanding of the psl-encoded exopolysaccharide's expression and protection in biofilms will provide insight into the pathogenesis of P. aeruginosa in cystic fibrosis and may open up new avenues for therapeutic interventions for CF and other infections. |
