viable cell count
vi·a·ble cell count
viable cell countthe number of living CELLS in a given volume or area for a given sample. Viable cell counts are generally applicable to single-celled ORGANISMS such as BACTERIA, SPORES, YEASTS and certain PROTOZOA. There are various procedures for determining viable cell counts:
- (a) by direct counting of a sample stained with a vital stain, for example methylene blue (see VITAL STAINING), using a counting chamber (see HAEMOCYTOMETRY). The living cells can be distinguished from dead cells by their different reaction with the stain.
- (b) by COLONY counts from the number of colonies that develop on a solid MEDIUM that has been inoculated with the sample. This may involve the technique of DILUTION PLATING. It is assumed that each colony that develops on the medium arises from a single cell in the original sample.
- (c) by the multiple tube method (also called most probable number method) in which sets of tubes containing liquid growth medium are inoculated with progressively smaller volumes of the sample. After incubation, the tubes are examined for the presence or absence of growth. It is assumed that growth will occur in all tubes that receive at least one viable organism. The number of viable cells can then be determined from the pattern of growth and no growth in the tubes by using probability tables. In some cases cells tend to form clumps or to grow in chains or filaments so that an accurate viable count cannot be obtained by using the above methods. Counts may then be expressed as colony forming units (cfu) for a given volume or area of the sample, where a cfu is any entity that can give rise to a single colony.
Viruses may be counted by using an ELECTRON MICROSCOPE; however, it is not possible to distinguish infectious from noninfectious virus particles. The PLAQUE assay is therefore used, in which dilutions of a viral suspension are either mixed with an excess of sensitive host cells and poured as an overlay on to solid medium or used to inoculate suitable TISSUE CULTURE. After incubation the plaques that form are counted.