The pel genes of the Pseudomonas aeruginosa PAK strain are involved at early and late stages of biofilm formation

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Abstract
Pseudomonas aeruginosais a Gram-negative bacterium associated with nosocomial infections and cystic fibrosis. Chronic bacterial infections are increasingly associated with the biofilm lifestyle in which microcolonies are embedded in an extracellular matrix. Screening procedures for identifying biofilm-deficient strains have allowed the characterization of several key determinants involved in this process. Biofilm-deficientP. aeruginosaPAK strains affected in a seven-gene cluster calledpelwere characterized. Thepelgenes encode proteins with similarity to components involved in polysaccharide biogenesis, of which PelF is a putative glycosyltransferase. PelG was also identified as a putative component of the polysaccharide transporter (PST) family. Thepelgenes were previously identified in theP. aeruginosaPA14 strain as required for the production of a glucose-rich matrix material involved in the formation of a thick pellicle and resistant biofilm. However, in PA14, thepelmutants have no clear phenotype in the initiation phase of attachment. It was shown thatpelmutations in the PAK strain had little influence on biofilm initiation but, as in PA14, appeared to generate the least robust and mature biofilms. Strikingly, by constructingpelmutants in a non-piliatedP. aeruginosaPAK strain, an unexpected effect of thepelmutation in the early phase of biofilm formation was discovered, since it was observed that these mutants were severely defective in the attachment process on solid surfaces. Thepelgene cluster is conserved in other Gram-negative bacteria, and mutation in aRalstonia solanacearum pelGhomologue,ragG, led to an adherence defect.