The role of Caulobacter cell surface structures in colonization of the air-liquid interface

Abstract

In aquatic environments, Caulobacter spp. are often present at the boundary between liquid and air known as the neuston. I report an approach to study temporal features of Caulobacter crescentus colonization and pellicle biofilm development at the air-liquid interface, and have defined the role of cell surface structures in this process. The flagellum enables motile swarmer cells to efficiently reach the oxygenated surface. Here, cells form a monolayer enriched in stalked cells bearing a surface adhesin known as a 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 the 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 air-liquid interface and do not form a pellicle. Phase contrast images support a model whereby the holdfast functions to trap C. crescentus cells at the air-liquid boundary. Unlike the holdfast, neither the flagellum nor pili are required for C. crescentus to 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 that enable partitioning of non-motile mother cells to the air-liquid interface, which facilitates colonization of this microenvironment.\n\nImportanceIn aquatic environments the boundary at the air interface is often highly enriched with nutrients and oxygen. The ability of microbial cells to colonize this niche likely confers a significant fitness advantage in many cases. This study provides evidence that the cell surface adhesin known as a holdfast enables Caulobacter crescentus to partition to and colonize the air-liquid interface. Additional surface structures including the flagellum and pili are important determinants of colonization and biofilm formation at this boundary. Considering that holdfast-like adhesins are broadly conserved in Caulobacter spp. and other members of the diverse class Alphaproteobacteria, 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.