| The Gram-negative soil bacterium Agrobacterium tumefaciens is the causative agent of crown gall disease in plants. Unique among bacterial pathogens, A. tumefaciens genetically alters its host, a capacity that has been harnessed to create transgenic plants for agricultural applications and plant research. Saprophytic populations of A. tumefaciens reside in soils around the world, living on the surfaces of soil particles and plant material. A. tumefaciens is an ideal model organism for the study of microbial persistence and the transition into the pathogenic state, because it is genetically tractable and capable of adherence to a variety of abiotic and biotic surfaces. This work focuses on the mechanisms underlying surface adherence, or biofilm formation, on living and non-living surfaces. Biofilms are generally defined as communities of microorganisms adhered to a surface and encased in an extracellular polymeric matrix. The biofilm lifestyle is thought to offer protection for the bacterial community against environmental stresses such as desiccation, antibacterial compounds, and in pathogenic situations, host defense responses. Surface adherence is important for microbial persistence in the environment, and is the critical first step to initiating pathogenesis. Genetics, molecular biology and microscopy techniques were employed to study A. tumefaciens biofilms on model surfaces and plant tissues and have identified several genes that are required for surface adherence and normal maturation of biofilm populations. Our screen identified mutants in motility and cell-surface functions, each exhibiting a biofilm defect. ExoR and SinR are two transcriptional regulators identified through our genetic screen, and they are described in detail herein. ExoR regulates the synthesis of exopolysaccharides, a component of the biofilm matrix. Disruption of the exoR gene abolishes bacterial surface adherence and inhibits motility. SinR belongs to a family of oxygen-responsive regulators and modulates biofilm maturation and structure. Disruption of the sinR gene inhibits A. tumefaciens biofilm maturation, yielding sparsely populated communities in comparison to wild type biofilms. In contrast, elevated SinR levels accelerate the rate of biofilm maturation and yields a more homogeneous biofilm. The results we describe reveal that A. tumefaciens biofilm formation is a tightly regulated and complex process that is highly relevant to plant interactions. |
