Direct counting techniques do not rely on cell population growth. The oldest technique is microscopy where the individual cells are magnified to become visible to the naked eye. This procedure has developed from the original brightfield techniques through staining and phase-contrast approaches.
More recently epifluorescence and immunofluorescence adaptations of cell labelling are used in conjunction with laser scanning, e.g. ChemScan RDI, and flow cytometry, e.g. Advanced Analytical Technologies RBD3000 and the Chemunex D-Count systems. In both technologies the trigger for a count is derived from single cells. Digital microbiology! These counts may also be confirmed visually using a microscope.
Culture techniques rely on the growth of a microbial population to levels that are visible. This is achieved under specific conditions, e.g. time, temperature, oxygen content and pressure in liquid or on solid media containing specified nutrients. On solid media after incubation, the resultant colonies may be counted.
This technique assumes that each colony derived from an individual cell and that the incubation conditions allowed the recovery of all cells present. Despite these limitations, plate count techniques remain the 'gold standard' in microbiology. Sensitivity may be improved with the use of filtration and labour can be reduced by the use of ready-to-use media, e.g. BBL, Oxoid, BioTrace PPM and 3M PetriFilm.
Serial dilutions can be avoided by the use of spiral platers. The task of counting the colonies present can be achieved manually or with the use of automated plate readers, e.g. Synbiosis aCOLyte. Counts may also be achieved in liquid media using Most Probable Number (MPN) techniques.
Reporter assays assess the microbial population size through the metabolic activity of the cells. The population does not necessarily have to be growing. Such techniques include colorimetry, impedance and ATP (adenosine triphosphate).
Optical systems measure the colour changes in dyes added to the growth medium as they are metabolized by microorganisms. The technique can be correlated to colony forming units (cfu), e.g. BioSys 128.
Microbial metabolism produces highly charged extra-cellular end products, increasing the conductivity of the growth medium. It is this increase in conductance that is utilized in impedimetric techniques to report on the growth characteristics of microbial populations.
Intra-cellular molecules can also be utilized in reporter assays. The bioluminescent reaction produces light in direct proportion to the level of ATP present. Microbial ATP can be specifically targeted by differential extraction to report on the loading of microbes in a variety of product and environmental samples, e.g. BioTrace Bev-Trace; Cogent MLS; Millipore Milliflex; Celsis CellScan.