Bacteria are much smaller than eucaryotic cells; most fall into a size range of about 1-5^m, although some may be larger than this. Some of the smallest bacteria, such as the mycoplasma measure less than 1^m, and are too small to be resolved clearly by an ordinary light microscope.
Because of their extremely small size, it was only with the advent of the electron microscope that we were able to learn about the detailed structure of bacterial cells. Using the light microscope however, it is possible to recognise differences in the shape and arrangement of bacteria. Although a good deal of variation is possible, most have one of three basic shapes (Figure 3.2):
• curved: these range from comma-shaped (vibrio) to corkscrew-shaped (spirochaete)
In recent years, square, triangular and star-shaped bacteria have all been discovered!
All these shapes confer certain advantages to their owners; rods, with a large surface area are better able to take up nutrients from the environment, while the cocci are less prone to drying out. The spiral forms are usually motile; their shape aids their movement through an aqueous medium.
As well as these characteristic cell shapes, bacteria may also be found grouped together in particular formations. When they divide, they may remain attached to one another, and the shape the groups of cells assume reflects the way the cell divides. Cocci, for example, are frequently found as chains of cells, a reflection of repeated division in one plane (Figure 3.2(f)). Other cocci may form regular sheets or packets of cells, as a result of division in two or three planes. Yet others, such as the staphylococci, divide in several planes, producing the irregular and characteristic 'bunch of grapes' appearance. Rod-shaped bacteria only divide in a single plane and may therefore be found in chains, while spiral forms also divide in one plane, but tend not to stick together. Blue-greens form filaments; these are regarded as truly multicellular rather than as a loose association of individuals.
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