Bacterial Ultrastructure

BACTERIAL ULTRASTRUCTURE

Cell Size and Shape

Bacteria are the smallest free-living organisms, their size being measured in micrometres (microns). Because of this small size a microscope affording a considerable degree of magnification (×400–1000) is necessary to observe them. Bacteria vary in size from a cell as small as 0.1–0.2 μm in diameter to those that are >5 μm in diameter. Bacteria this large, such as Thiomargarita namibiensis, are extremely rare: the majority of bacteria are 1–5 μm long and 1–2 μ m in diameter. By comparison, eukaryotic cells may be 2 μ m to >200 μm in diameter. The small size of bacteria has a number of implications with regard to their biological properties, most notably increased and more efficient transport rates. This advantage allows bacteria far more rapid growth rates than eukaryotic cells.

While the classification of bacteria is immensely complex, nowadays relying very much on 16S ribosomal DNA sequencing data, a more simplistic approach is to divide them into major groups on purely morphological grounds. The majority of bacteria are unicellular and possess simple shapes, e.g. round (cocci), cylindrical (rod, also called bacillus, spelt with a lower case initial letter to distinguish from Bacillus, the genus) or ovoid. Some rods are curved (vibrios), while longer rigid curved organisms with multiple spirals are known as spirochaetes. Rarer morphological forms include the actinomycetes which are rigid bacteria resembling fungi that may grow as lengthy branched filaments; the mycoplasmas which lack a conventional peptidoglycan (murein) cell wall and are highly pleomorphic organisms of indefinite shape; and some miscellaneous bacteria comprising stalked, sheathed, budded and slimeproducing forms often associated with aquatic and soil environments.

Often bacteria remain together in specific arrangements after cell division. These arrangements are usually characteristic of different organisms and can be used as part of a preliminary identification. Examples of such cellular arrangements include chains of rods or cocci, paired cells (diplococci), tetrads and clusters.