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.
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.
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