Bacteria are single-celled organisms that lack a nucleus. They are nearly ubiquitous in nature; according to the webpage for the NIH Human Microbiome Project, the bacteria in our bodies outnumber our cells by nearly 10 to 1.
When studying bacteria, microbiologists often culture or grow bacterial colonies on small plates containing a growth medium like agar. Microbiologists typically categorise these colonies according to shape, size, texture, colour and other features. This practice is known as colony morphology.
A bacterial colony may be circular, or it may be irregular in shape, with an uneven outline like that of a cloud or an inkblot.
In filamentous colonies, bacteria form many hairlike filaments as they grow outward from the centre of the colony. Rhizoid colonies are similar, but they grow outward in fewer and thicker filaments, like the roots of a miniature tree.
Other important features to consider when assessing the form of a bacterial colony include its colour and opacity, and its surface appearance and texture (e.g., veined, dry, moist, glistening, wrinkled, rough).
Some bacterial colonies are basically flat. Others have a raised profile, meaning that they bulge slightly from the surface of the plate. Convex colonies bulge up from the plate in a pronounced semicircle.
Umbonate colonies look like raised colonies, but they have a small projection in the middle. Crateriform colonies also resemble raised colonies, but they have a tiny depression like a dimple in the middle.
The margin describes the edge of a bacterial colony. A margin might be entire, in which case the edge is smooth and circular; or it might be undulate, in which case the edge is lumpy and irregular.
Some margins are filiform, with tiny filamentous strands of bacteria growing out from a central mass. Curled margins have a wavelike edge, while lobate margins have fingerlike projections growing outward from the edges of the colony.
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