In order that the sample to be plated out contains an appropriate number of cells, the original sample is subjected to serial dilution. In the example below, it is diluted by a factor of ten at each stage to give a final dilution of 10-5 (one in a hundred thousand).
Samples of 0.1 ml of each dilution are plated out on a suitable solid medium and colonies allowed to develop.
TNTC = too numerous to count
In the earlier tubes, the suspension of cells is too concentrated, resulting in too many colonies to count. In the final tube, the suspension is so dilute that there are no cells in the sample taken. The 10-3 dilution is used to calculate the concentration of cells in the original culture, as it falls within the range of 30-300 colonies regarded as being statistically reliable.
Colony count (10-3 dilution) = 65 A Number of cfu per ml diluted suspension = 65 x 10 = 650 A Number of cfu per ml original susp'n = 650 x 103 = 6.5 x 105
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