cell types, in particular, many microorganisms, grow in solution and not attached to a surface. These cell types can be subcultured by simply taking a small volume of the parent culture and diluting it in fresh growth medium. Cell density in these cultures is normally measured in cells per milliliter for large eukaryotic cells, or as optical density for 600nm light for smaller cells like bacteria. The cells will often have a preferred range of densities for optimal growth and subculture will normally try to keep the cells in this range. Adherent cells Adherent cells, for example many mammalian cell lines, grow attached to a surface such as the bottom of the culture flask. These cell types have to be detached from the surface before they can be subcultured. For adherent cells cell density is normally measured in terms of confluency, the percentage of the growth surface covered by cells. The cells will often have a preferred range of confluencies for optimal growth, for example a mammalian cell line like HeLa or Raw 264.7 generally prefer confluencies over 10% but under 100%, and subculture will normally try to keep the cells in this range. For subculture cells may be detached by one of several methods including trypsin treatment to break down the proteins responsible for surface adherence, chelating calcium ions with EDTA which disrupts some protein adherence mechanisms, or mechanical methods like repeated washing or use of a cell scraper. The detached cells are then resuspended in fresh growth medium and allowed to settle back onto their growth surface. See also Trypsinization References Microbiology terms