In biology, a subculture is either a new cell culture or a microbiological culture made by transferring some or all cells from a previous culture to fresh growth medium. This action is called subculturing or passaging the cells. Subculturing is used to prolong the lifespan and/or increase the number of cells or microorganisms in the culture. Role Cell lines and microorganisms cannot be held in culture indefinitely due to the gradual rise in toxic metabolites, use of nutrients and increase in cell number due to growth. Once nutrients are depleted and levels of toxic byproducts increase, the bacteria in the overnight culture enter the stationary phase, where proliferation is greatly reduced or ceased (the cell density value plateaus). When microorganisms from this overnight culture are transferred into the fresh media, nutrients trigger the growth of the microorganism and it goes through the lag phase, a period of slow growth and adaptation to the new environment, and then the log phase, a period where the cells grow exponentially. Subculture is therefore used to produce a new culture with a lower density of cells than the originating culture, fresh nutrients and no toxic metabolites allowing continued growth of the cells without risk of cell death. Subculture is important for both proliferating (e.g. a microorganism like E. coli) and non-proliferating (e.g. terminally differentiated white blood cells) cells. Subculturing can also be used for growth curve calculations (ex. generation time) and obtaining log-phase microorganisms for experiments (ex. Bacterial transformation). Typically, subculture is from a culture of a certain volume into fresh growth medium of equal volume, this allows long-term maintenance of the cell line. Subculture into a larger volume of growth medium is used when wanting to increase the number of cells for, for example, use in an industrial process or scientific experiment. Passage number It is often important to record the approximate number of divisions cells have had in culture by recording the number of passages or subcultures. In the case of plant tissue cells somaclonal variation may arise over long periods in culture. Similarly in mammalian cell lines chromosomal aberrations have a tendency to increase over time. For microorganisms there is a tendency to adapt to culture conditions, which is rarely precisely like the microorganism's natural environment, which can alter their biology. Protocols for passaging The protocol for subculturing cells depends heavily on the properties of the cells involved. Non-adherent cells Many