Some of the ways these selective forces may affect bacterial morphology are summarized in Table 1. The last five are Secondary in that they represent a suite of morphologically associated mechanisms that bacteria use to deal with the Primary forces. The first three are Primary in that they represent fundamental conditions that determine whether cells live or die, because cells must grow and multiply and keep from being killed. Consistent with these expectations, shape contributes a measure of survival value in the face of three “Primary” selective pressures: 1) nutrient acquisition, 2) cell division, and 3) predators and in optimizing five “Secondary” mechanisms: 4) attachment to surfaces, 5) passive dispersal, 6) active motility, and 7) internal or 8) external differentiation ( Table 1). coli can impart a fitness advantage of ~10% compared to its unaltered competitors, so improvements need not be dramatic to be useful. Even a 0.01% increase in the growth rate of E. How, then, might morphology contribute to natural selection? Simply put, bacteria with different shapes present different physical features to the outside world, and these features help cells cope with and adapt to external conditions. The simplest conclusion is that morphological adaptation serves an important biological function. So, for example, although they have a non-peptidoglycan-based cell wall, the Archaea exhibit a range of morphological forms similar to that of the Bacteria. Secondly, prokaryotes with different genealogies may converge morphologically, indicating that a similar shape may confer advantages in certain environments. Progressive development of a trait implies that selective forces are operating. First, shape has a vector through evolutionary time – rod-like organisms having arisen first and coccoid forms being derivatives at the ends of evolutionary lines. Two evolutionary arguments also support the utility of bacterial shape. Another clue is that some bacteria can modify their morphology in response to environmental cues or during the course of pathogenesis, suggesting that shape is important enough to merit regulation. One argument favoring this assertion is that even though bacteria have a wide variety of shapes, any one genus typically exhibits a limited subset of morphologies, hinting that, with a universe of shapes to choose from, individual bacteria adopt only those that are adaptive. The first issue to get settled is that the shape of a bacterium has biological relevance. Portions of this topic have also been discussed by Beveridge, Dusenbery, Koch, and Mitchell. More depth, more examples, and a bit more quantitative treatment can be found in a recent review and the references therein. I will highlight a few research areas that bear on why bacteria have certain morphologies, but only in a brief and qualitative way. What has not been as well explored is why bacteria find it advantageous to exhibit such a prodigious number of different shapes and so the purpose of this article is to examine some of the reasons that lie behind this variety. And, indeed, this approach has produced exciting new information, highlighted by other articles in this issue. This emphasis is understandable because we are both more familiar with and more comfortable with answering how-type questions. The expectation is that by answering this (deceptively) simple question we may acquire knowledge that will point us to a universal mechanism of shape control. The discussion of bacterial morphology has been dominated by questions about how a cell manages to create a rod shape, which, of course, is but one example of the more general question of how a cell constructs any shape.
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