Bacterial cells can be enumerated by direct microscopic count procedures.
Instruments also are available for direct counting of bacterial cells. Particle counters, such as a Coulter particle counter, permit the discrimination of particles based on size so that particles the size of bacteria are counted automatically. As long as there are no nonliving interfering particles in the same size range of bacteria, this is a rapid counting method.
Most Probable Number (MPN) Procedures
Another approach to bacterial enumeration is the determination of the most probable number (MPN). MPN is a statistical method based on probability theory. In a most probable number enumeration procedure,multiple serial dilutions are performed to reach a point of extinction. The point of extinction is the dilution level at which not even a single cell is deposited into one or more multiple tubes (FIG. 10-10). A criterion is established for indicating whether a particular dilution tube contains bacteria. Such a criteria can be development of cloudiness or turbidity in a liquid growth medium or gas production (detected by its accumulation in an inverted tube). The pattern of positive and negative test results are then used to estimate the concentration of bacteria in the original sample, that is, the most probable number of bacteria, by comparing the observed pattern of results with a table of statistical probabilities for obtaining those results (Table 10-1). Often a 95% confidence limit is given in MPN tables.
The most probable number procedure is a statistical method based on probability theory used to determine bacterial populations by diluting bacterial samples to the point of extinction.
FIG. 10-10The most probable number (MPN) procedure involves inoculation of multiple tubes with replicate samples of dilutions. The pattern of tubes that show growth (brown) and those that do not (orange) are compared with a statistical table to calculate the MPN of bacteria in the original sample. In this example all 5 tubes at the 10° show growth, 4 of 5 tubes at the 10~! dilution show growth, and only 1 of 5 tubes at the 10~2 dilution show growth. Therefore the MPN bacteria in the original sample is 170 per 100 ml (see Table 10-1).
Date: 2015-02-28; view: 1998