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Multiple species of bacteria inhabit the world’s oceans and estuarine areas and their population size and spreading in the environment depend on many different variables and is highly influenced by fluctuations in the external milieu – and thus in the case of human pathogens, changing the likelihood of human infections. Consequently, it is essential to understand how bacteria adapt to changes in their environment and what strategies they employ to ensure their dissemination.
One mechanism, employed by many different species of bacteria to accommodate changes in the environment involves the differentiation into specialized cell types suitable for the particular conditions they encounter. A distinct type of differentiation utilized by many bacteria, including species of Serratia, Aeromonas, Salmonella, Proteus and Vibrio, is the differentiation between a planktonic swimmer cell and a swarmer cell that is specialized for movement over solid surfaces. Here we have anlyzed one such example, V. parahaemolyticus, which is a marine bacterium and a worldwide human pathogen, being the leading agent of seafood borne gastroenteritis in the world. As outlined here, it has an intricate life-cycle that depends on its environmental conditions. Particularly, in liquid environments it exits as a short motile cell that is propelled by a single polar flagellum. However, when it attaches to solid surfaces it induces a distinct differentiation program, which allows it to adapt to changes in its environment and colonize solid surfaces by means of swarm motility. Reports based on V. parahaemolyticus levels in estuarine environments have suggested that the level of bacteria in the water is tide dependent. Thus, suggesting that V. parahaemolyticus cells could be released from surfaces into the liquid surroundings. Nonetheless, the release of surface attached cells into liquid environments have remained unexplored for V.
parahaemolyticus as well as for swarm colonies in general.
In Chapter III of this study we revealed a new distinct cell type, which is released from swarm colonies into the liquid environment upon swarm colony flooding.
Furthermore, our results show how the swarm colony architecture fluctuates with changing environmental conditions with responses of differentiation and dedifferentiation within zonal regions of the colony. Importantly, our data shows that cells are continuously released from flooded swarm colonies, thus indicating that swarm colonies function as a continuous source of cells that can be released into the environment upon colony flooding. Surprisingly, our results indicate that long swarmer cells are not released into the liquid environment. Instead, released cells comprise of a distinct cell type that is morphologically optimized for swimming behavior and capable of spreading and exploring their new liquid environment and eventually attach to new solid surfaces where
101 they can initiate new swarm colonies. Importantly, our data indicates that release of this distinct cell type facilitates the dissemination of V. parahaemolyticus in the environment.
At this point, more experiments are required to understand where exactly the released bacterial cells are located within the swarm colony before they disperse. One possible experiment would be to label the cells with a fluorescence protein fused to the promoter of a gene specifically up-regulated in released cells. This way one could see where the released cells are located during development of the colony. More importantly, it would be possible to analyse their location within the swarm colony, before being released into the liquid environment. Further work is needed to test the capacity of the released cells (once re-attached) and of cells within a swarm colony to utilize the type VISS1 machinery to successfully compete with and neutralize other surface colonizers.
Moreover, further studies are needed to understand the exact environmental circumstances within the bacterium´s natural habitat that induce swarming. We argue that swarm colonies from other pathogenic bacteria living in estuarine areas, such as V.
alginolyticus, might also present a similar spreading mechanism like V.
parahaemolyticus based on the release of distinct swimming proficient cells from swarm colonies. Thereby, more research is required to determine if other swarming proficient bacteria species also allow for the release of swimming proficient cells from swarm colonies, whilst permitting rapid swarming surface colonization.
In Chapter IV of this thesis, we demonstrated how elastic the proteome profile of V. parahaemolyticus can be in response and in adaptation to different conditions. We also revealed which proteins are specifically regulated in cells exposed to distinct environmental scenarios, namely, center and periphery of a swarm colony and liquid medium.
Our results show that deletion of genes that encode for proteins specifically regulated in cells within the swarm flares (periphery) have a significant impact in swarm motility. However, more work is still required to understand if these targets participate directly or indirectly in swarming motility and to unravel their specific role for this biological differentiation process. As topic for further research in the swarming motility field, it is still to be discovered which players are involved in the signal transduction mechanism and in the elongation of cell morphology. It would be interesting to see if the cell division and cell wall membrane proteins detected in our proteomics studies are also up-regulated in swarmer cells of other organisms and if they play a role in the elongation
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phenotype. Moreover, further work is required to analyse if the cell wall composition of lipopolysaccharides is different between cells from the swarm flares and from the center of a swarm colony. Additional research is still needed to verify if V. parahaemolyticus, as in the case of other swarming species, also requires the production of amphipathic molecules that enable surface wettability and consequently, aid in motility upon solid surfaces.
Overall, this work shows how flexible the proteomic expression profiles are in order to greatly adapt to different environmental stresses and habitats. This would permit V. parahaemolyticus to colonize many different hosts and surfaces, and might be an explanation for the high worldwide prevalence of this bacterium.
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