The Ctp Type IVb Pilus Locus of Agrobacterium tumefaciens Directs Formation of the Common Pili and Contributes to Reversible Surface Attachment

Agrobacterium tumefaciens can adhere to plant tissues and abiotic surfaces and forms biofilms. Cell surface appendages called pili play an important role in adhesion and biofilm formation in diverse bacterial systems. TheA. tumefaciens C58 genome sequence revealed the presence of thectpABCDEFGHI genes (c luster oft ype IVp ili; Atu0216 to Atu0224), homologous totad -type pilus systems from several bacteria, includingAggregatibacter actinomycetemcomitans andCaulobacter crescentus . These systems fall into the type IVb pilus group, which can function in bacterial adhesion. Transmission electron microscopy ofA. tumefaciens revealed the presence of filaments, significantly thinner than flagella and often bundled, associated with cell surfaces and shed into the external milieu. In-frame deletion mutations of all of thectp genes, with the exception ofctpF , resulted in nonpiliated derivatives. Mutations inctpA (a pilin homologue),ctpB , andctpG decreased early attachment and biofilm formation. The adherence of thectpA mutant could be restored by ectopic expression of the paralogouspilA gene. The ΔctpA ΔpilA double pilin mutant displayed a diminished biovolume and lower biofilm height than the wild type under flowing conditions. Surprisingly, however, thectpCD ,ctpE ,ctpF ,ctpH , andctpI mutants formed normal biofilms and showed enhanced reversible attachment. In-frame deletion of thectpA pilin gene in thectpCD ,ctpE ,ctpF ,ctpH , andctpI mutants caused the same attachment-deficient phenotype as thectpA single mutant. Collectively, these findings indicate that thectp locus is involved in pilus assembly and that nonpiliated mutants, which retain the CtpA pilin, are proficient in attachment and adherence.