Characterization of cyst cell formation in the purple photosynthetic bacterium Rhodospirillum centenum

Rhodospirillum centenum is an anoxygenic photosynthetic bacterium that is capable of differentiating into several cell types. When grown phototrophically in liquid, cells exhibit a vibrioid shape and have a single polar flagellum. When grown on a solid surface, R. centenum will differentiate into rod-shaped swarm cells that display numerous lateral flagella. Upon starvation for nutrients, R. centenum also forms desiccation-resistant cysts. In this study, it was determined that R. centenum has heat- and desiccation-resistance properties similar to other cyst-forming species. In addition, microscopic analyses of the morphological changes that occur during cyst cell development were performed. It was observed that R. centenum typically forms multi-celled clusters of cysts that contain from four to more than 10 cells per cluster. It was also determined that cell density has a minor effect on the percentage of cyst cells formed, with cell densities of 10 5 –10 7 cells per 5 μl spot yielding the highest percentage of cyst cells. The striking similarities between the life cycle of R. centenum and the life cycle exhibited by Azospirillum spp. are discussed.