There are several methods to quantify the count of microbes. Historically the very first of them (if microscopy is excluded) was the use of dilutions series in nutrient broths. This methods was later developed to MPN (Most Probable Number) Method, which was the standard until the rise of Colony Count methods. Colony counts give accurate numbers but suffer of the poorer recovery of the microbial cells, compared to the broth methods. It is also unavailable for certain, difficult types of samples.
"Third Generation" of quantitative mb analyses was established some 30 years ago. They are based on analytical methods which give the results much faster than the traditional methods by measuring some chemical or physical features of the samples which are in a relationship with the count (or activity) of microbes of the original samples.
The increase of the nutrient broth turbidity, caused by the growth of microorganisms, is maybe the most common "3rd Generation" method used today. It is an optical method which gives levels of either total growth or specified microbes (in latter case, selective broths with confirmation tests are applied). An inversed corretation between microbial load and detection time (= the moment of the testing period when turbidity begins to rise) can be found and correlation lines/tables between colony counts and detection times are easy to construct.
The figure above is an example of the growth curves, derived by PMEU Spectrion. X axis shows the time scale of the test, y axis indicates the values of RT. This curve shows that PMEU Spectrion actually illustrates the classic growth curves with the lag, log and stat phases (and, if the cultivation will continue long enough, also the kill phase). The moment when the turbidity begins to rise is inversely related to the count of microbes in the original sample.
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