Motility and Chemotaxis in Agrobacterium tumefaciens Surface Attachment and Biofilm Formation

Department of Biology, Indiana University, Bloomington, IN 47405

^* To whom correspondence should be addressed. Email: cfuqua{at}indiana.edu.

	Abstract

Bacterial motility mechanisms including swimming, swarming and twitching are known to have important roles in biofilm formation, including colonization and the subsequent expansion into mature structured surface communities. Directed motility requires chemotaxis functions that are conserved among many bacterial species. The biofilm-forming plant pathogen Agrobacterium tumefaciens drives swimming motility by utilizing a small group of flagella, localized to a single pole, or the subpolar region of the cell. There is no evidence for twitching or swarming motilities in A. tumefaciens. Site-specific deletion mutations that resulted in either aflagellate, flagellated but nonmotile, or flagellated but nonchemotactic A. tumefaciens derivatives were examined for biofilm formation under static and flowing conditions. Nonmotile mutants were significantly deficient for biofilm formation under static conditions. Under flowing conditions however, the aflagellate mutant rapidly formed aberrantly dense, tall biofilms. In contrast a nonmotile mutant with unpowered flagella was clearly debilitated for biofilm formation in flow relative to wild type. A non-tumbling chemotaxis mutant was only weakly affected for biofilm formation in non-flowing conditions, but notably compromised in flow, generating less adherent biomass than wild type and a more dispersed cellular arrangement. Extragenic suppressor mutants of the chemotaxis-impaired, straight-swimming phenotype were readily isolated from motility agar plates. These mutants regained tumbling at a frequency similar to wild type. Despite this phenotype, biofilm formation by the suppressor mutants in static cultures was significantly deficient. Under flowing conditions a representative suppressor mutant manifested a phenotype similar to, yet distinct from its nonchemotactic parent.