The establishment of productive, stable surface interactions is an important process for bacteria, that can lead to formation of the adherent communities known as biofilms. These assemblages are challenges in agricultural, industrial and medical settings, and are intrinsically tolerant to many antimicrobial therapies. For a number of bacteria in the large and diverse Alphaproteobacteria (APB) group, attachment to surfaces and to other cells requires production of a structure comprised of polysaccharide localized to a single cellular pole. In the model pathogen Agrobacterium tumefaciens this structure is called the unipolar polysaccharide (UPP). Polar adhesins similar to the UPP are widespread among the APB, including other pathogens and symbionts, and the A. tumefaciens UPP is therefore a representative model for these diverse bacteria. Among these, the stalked bacterium Caulobacter crescentus produces a similar structure called the holdfast at the stalk tip, and although it has been well studied, remains poorly understood and is less broadly representative than the UPP. These polar polysaccharides can drive stable surface attachment and host interactions. In A. tumefaciens the UPP is comprised of at least two distinct polysaccharide species, and the genes required for synthesis suggest that there may be overlapping biosynthesis pathways.
We aim to determine how A. tumefaciens coordinates and regulates production of these polysaccharides during surface colonization, including dynamic localization of the biosynthetic complexes. Production of the A. tumefaciens UPP is strictly regulated by contact with the surface, and cells rarely if ever produce the UPP when free-swimming. The proposed studies will dramatically improve our current understanding of UPP properties and biosynthesis, and will elucidate its regulation via a network of intracellular signal cascades, its surface-dependent polar localization, and other environmental signals that affect its production, and thereby attachment. At the core of this control network is the ubiquitous bacterial second messenger cyclic diguanylate monophosphate, which regulates UPP production. Among the primary UPP regulatory mechanisms are a novel signaling pathway involving small metabolites called pterins, and the response to low pH. The project utilizes an extensive collection of genetic mutants and variants, quantitative microscopic imaging approaches, genomic technology, and sophisticated biochemical approaches to illuminate the cellular processes that promote attachment via the UPP. The findings generated will contribute to the understanding of the motile to sessile transition and initiation of biofilm formation. We will characterize the biosynthesis of a novel biological adhesive(s) and a potential antimicrobial target, and will reveal how bacterial cells control production of these important products to promote surface interactions that lead to biofilm formation. Our findings will provide fundamental information about these polar adhesins, identifying new targets for anti-bacterial approaches and facilitating development of new biomaterials.
Attachment to surfaces and biofilm formation are processes that are crucial to many aspects of bacterial activity, including disease interactions and chronic bacterial infections such as those observed for implanted medical devices. Members of the large and diverse Alphaproteobacteria group frequently employ adhesives that localize to a single point on the cell to drive stable surface attachment, and a unipolar polysaccharide adhesive in the model bacterial pathogen Agrobacterium tumefaciens provides a facile system for exploration of the function, regulation and deployment of this type of material. Understanding this attachment mechanism, the biogenesis of this complex material, and regulatory networks that underlie its production will provide insights into how to control initial surface interactions, bioengineer durable and biologically compatible adhesives, and inform new strategies for preventing or reversing surface-associated infections.
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