Strain (biology)

In biology, a strain is a low-level taxonomic rank below the rank of species. This is seen in microbiology where strains are got from a single cell colony, and are quarantined by being in a Petri dish. Strains are also commonly referred to in virology, in horticulture, and with rodents used in experimental studies.
Microbiology/Virology.
A strain is a genetic variant or subtype of a microorganism, that is to say a virus or bacterium or fungus. For example, a "flu strain" is a certain biological form of the influenza or "flu" virus. Properly speaking, bacteria, archaea and viruses do not have species, because they do not have eukaryote-type sexual reproduction. Bacteria do exchange DNA, but they may exchange it between different kinds of bacteria, which shows their difference from eukaryotes. Strains are therefore an essential part of bacterial identification. 
Plants.
In botany, the term is not used to rank plants. It is sometimes used to refer to all the descendants produced from a common ancestor that share a uniform morphological or physiological character. A strain is a designated group of offspring that have descended from a modified plant, produced ether by conventional breeding or by biotechnological means or result from genetic mutation. 
For example, some rice strains are made by inserting new genetic material into a rice plant, all the descendants of the genetically modified rice plant are a strain with a unique genetic code that is passed on to later generations. The strain designation, a number or a formal name, covers all the plants that descend from the originally modified plant. The rice plants in the strain can be breed to other rice strains or cultivars, and if desirable plants are produced, these are further breed to stabilize the desirable traits; the stabilized plants that can be propagated and come true, are given a cultivar name and released into production to be used by farmers.
Rodents.
A mouse or a rat strain is a group of animals that is genetically uniform. Strains are used in laboratory experiments. Mouse strains can be inbred or genetically engineered, while rat strains are usually produced by inbreeding.