The role ofCaulobactercell surface structures in colonization of the air-liquid interface
AbstractIn aquatic environments,Caulobacterspp. are often present at the boundary between liquid and air known as the neuston. I report an approach to study temporal features ofCaulobacter crescentuscolonization and pellicle biofilm development at the air-liquid interface, and have defined the role of cell surface structures in this process. At this interface,C. crescentusinitially forms a monolayer of cells bearing a surface adhesin known as the holdfast. When excised from the liquid surface, this monolayer strongly adheres to glass. The monolayer subsequently develops into a three-dimensional structure that is highly enriched in clusters of stalked cells known as rosettes. As this pellicle film matures, it becomes more cohesive and less adherent to a glass surface. A mutant strain lacking a flagellum does not efficiently reach the surface, and strains lacking type IV pili exhibit defects in organization of the three-dimensional pellicle. Strains unable to synthesize holdfast fail to accumulate at the boundary between air and liquid and do not form a pellicle. Phase contrast images support a model whereby the holdfast functions to trapC. crescentuscells at the air-liquid boundary. Unlike the holdfast, neither the flagellum nor type IV pili are required forC. crescentusto partition to the air-liquid interface. While it is well established that the holdfast enables adherence to solid surfaces, this study provides evidence that the holdfast has physicochemical properties required for partitioning of non-motile mother cells to the air-liquid interface, which facilitates colonization of this microenvironment.ImportanceIn aquatic environments the boundary at the air interface is often highly enriched with nutrients and oxygen. Colonization of this niche likely confers a significant fitness advantage in many cases. This study provides evidence that the cell surface adhesin known as a holdfast enablesCaulobacter crescentusto partition to and colonize the air-liquid interface. Additional surface structures including the flagellum and type IV pili are important determinants of colonization and biofilm formation at this boundary. Considering that holdfast-like adhesins are broadly conserved inCaulobacterspp. and other members of the diverse classAlphaproteobacteria, these surface structures may function broadly to facilitate colonization of air-liquid boundaries in a range of ecological contexts including freshwater, marine, and soil ecosystems.