Differentiation

While all cells go through cycles of growth and quiescence in response to changes in their environment, some species have gone a step further to evolve a programmed set of states–a lifecycle. In some cases, as you will see in the next chapter, a lifecycle allows bacteria to prey efficiently on other cells, including ours. In other cases, such as these Caulobacter crescentus, it allows the bacterium to thrive in an environment with low nutrient levels. C. crescentus has a dimorphic (“two form”) lifecycle. Newborn cells start out life as swarmers, with a polar flagellum. They swim through the environment (either freshwater or saltwater) until they (hopefully) reach a favorable spot to put down roots, which they do by first transiently attaching to a surface with pili, then making the attachment permanent with a polymer of a sticky protein called “holdfast,” which currently holds the record for the strongest known biological adhesive (⇩). They shed their flagellum and grow a stalk in its place. Only after growing a stalk, completing the process of differentiation, will the cell begin to divide. The stalked cell will remain attached to the surface for the rest of its life, dividing asymmetrically to produce new swarmer cells, which swim away to try their luck elsewhere. Here you see a swarmer cell on the bottom, and a stalked cell on the top, in the process of dividing to produce another swarmer that would swim off to the right. You saw a similar dimorphic lifecycle in Chapter 5, in sessile (non-motile) Hyphomonas neptunium that bud to produce motile daughters. This kind of lifecycle helps prevent related cells from competing with one another for scarce local nutrients.