Getting bacteria into shape
You probably know that bacteria come in a range of different shapes. Rod-shaped cells are called bacilli (as in Bacillus anthracis), spherical cells are called cocci (as in Staphylococcus aureus), and helical bacteria come in three forms, vibrio – curved or comma-shaped rods (such as Vibrio cholerae), spirilla – thick, rigid helices (such as Rhodospirillum rubrum) and spirochetes – thin, flexible helices (such as Treponema pallidum). A few rare bacteria (such as Haloquadratum walsbyi) are even cubic. So we know bacteria have different shapes. This article is about why and how.
The shapes of bacterial cells do not occur randomly, but have arisen over millions of years of evolution because each shape confers some selective advantage on the species in the environment in which it lives. Bacteria are very small, so they have a large surface-to-volume ratio. This allows rapid uptake of nutrients and gasses and supports a highly active internal biochemistry. Some bacteria expand their surface area even further by surface features such as filaments and stalks.
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For optimum motility, there is a fairly narrow range of optimum sizes and shapes. The fastest swimmers are medium-length rods with a particular length-to-width ratio, since this is the most efficient shape for swimming. A typical bacterium can move at about 100 times its body length in a second (e.g. about 50 µm/sec), whereas the fastest swimming fish such as tuna can move only about 10 body lengths per second, and the fastest land animal, the cheetah, can only manage 25 body lengths per second. Bacteria which live in highly viscous environments use their helical shape to great advantage, literally corkscrewing their way though the medium.
Cell size and shape may also be a defence against predation in some cases, with certain bacteria making themselves too large, too small or too awkwardly shaped (e.g. with surface projections) to be consumed by planktonic feeders, which often have a relatively narrow range of acceptable prey size.
So if there is an optimum size and shape for bacterial cells in a particular environment, how is cell morphology produced? With no control over shape, all cells would be spherical, a shape produced by the turgor pressure of the cytoplasm on the outer membrane, rather like blowing up a balloon. In most (but not all) bacteria, shape is maintained by the cell wall, specifically the peptidoglycan layer, which has the approximate strength of strong, stiff fabric. Digest the peptidoglycan with lysozyme or inhibit its deposition using antibiotics such as penicillin and the cells become spherical protoplasts or spheroplasts. The shape of the wall is determined by the way it is deposited, and this is controlled by a cytoskeleton. In bacteria, the cytoskeleton is made up of two types of proteins. Tubulin-like proteins are responsible for the construction of the septum and the poles of the cell. Actin-like proteins localize peptidoglycan synthesis in the lateral walls of rod-shaped cells. As in eukaryotes, the cytoskeleton is produced by self-organized assembly, although the details of the processes involved are only just becoming clear.
Links:
- It’s hip to be square! Nature Reviews Microbiology 2007 5: 400-401
- Bacterial morphology: why have different shapes? Current Opinion in Microbiology 2007 10: 596-600
- Bacterial morphogenesis: learning how cells make cells. Current Opinion in Microbiology 2007 10: 591-595
Tags: Antibiotics, Bacteria, Biology, Microbiology, Podcast, Science
