Characterization of swarm-colony development reveals the release of a distinct cell type facilitating dissemination of Vibrio parahaemolyticus

Bacteria often experience changes in their external environment and have developed various strategies to respond accordingly. One mechanism to accommodate such changes involves the differentiation into specialized cell types suitable for the particular conditions. Vibrio parahaemolyticus exists as a swimmer cell, adapted for liquid conditions, and as a swarmer cell, specialized to grow on solid surfaces. Swarmer cells undergo a differentiation process that leads to elongation and production of multiple lateral flagella along the cell body, which are essential for swarming behavior. According to the position within a swarm colony, V. parahaemolyticus cells display different sizes. Particularly, long swarmer cells are only found in the periphery of the colony while the center consists of much shorter cells. Nonetheless, how the architecture develops over time or in response to environmental fluctuations is unknown. As V. parahaemolyticus is a marine bacterium and the leading agent of seafood borne gastroenteritis, the worldwide distribution of V. parahaemolyticus accentuates the need for understanding the factors contributing to its dissemination. In this study, by characterizing the swarm-colony architecture and development we revealed that a new distinct cell type is released from the swarm colony into the environment. Through mass spectrometry and confocal microscopy analysis we show that released cells comprise of a cell type that is morphologically short and distinct from cells belonging to the center and periphery of the swarm colony. Surprisingly, the cell length distribution of released cells was very homogenous and almost no long cells were detected. Thus, suggesting that long swarmer cells are not released into the liquid environment but stay surface-attached during flooding. We also revealed that released cells are capable of spreading in the liquid environment and attach to new submerged surfaces. Moreover, our data shows that released cells are optimized for swimming behavior and can chemotax towards the chitin component, N-acetylglucosamine. By using fluorescence microscopy and stereomicroscopy, we determine the temporal development of swarm colonies and show how the swarm colony architecture fluctuates with changing environmental conditions. Importantly, we show that swarm colonies act as a continuous source of cells that are released from the swarm colony into the environment. Overall, these results indicate that release of a distinct cell type from swarm colonies facilitates the dissemination of V. parahaemolyticus in the environment, likely influencing the ecology of this marine bacterium. Additionally, our research revealed the degree to which the V. parahaemolyticus proteome changes according to its distinct environmental circumstances. Particularly, we define which proteins are present specifically in the swarm flares, in the center of the swarm colony and in a planktonic condition. By performing single deletions we identified potential regulators of swarming differentiation. At last, we define which proteins are constitutively expressed in this bacterium. Altogether, this work reveals how flexible the proteome of V. parahaemolyticus is according to different ecological niches and reports on the development of swarm colony populations and how the formation and release of a distinct cell type from swarm colonies facilitates the dissemination of an important human pathogen in the environment – thus, influencing the ecology of this marine bacterium