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In biology, a colony refers to individual organisms of the same species living closely together, usually for mutual benefit, such as stronger defense or the ability to attack bigger prey. Some insects (ants and honey bees, for example) live only in colonies. The Portuguese man o' war is an example of a colony of four different polyp forms.
A colony of single-cell organisms is known as a colonial organism. Colonial organisms were probably the first step toward multicellular organisms via natural selection. The difference between a multicellular organism and a colonial organism is that individual organisms from a colony can, if separated, survive on their own, while cells from a multicellular life form (e.g., cells from a brain) cannot. Volvox (technically a coenobium) is an example of the border between these two states.
A bacterial colony is defined as a visible cluster of bacteria growing on the surface of or within a solid medium, presumably cultured from a single cell. Because all organisms within the colony descend from a single ancestor, they are genetically identical (except for mutations that occur at a low frequency, as well as the more likely possibility of contamination). Obtaining such genetically identical organisms (or pure strains) can be useful in many cases; this is done by spreading bacteria on a culture plate and starting a new stock of bacteria from a single colony.
A biofilm is a colony of microorganisms often comprising several species, with properties and capabilities greater than the aggregate of capabilities of the individual organisms.