Chapter 9 - Statistical considerations of other methods in quantitative microbiology

Abstract

Alternative methods of quantitative analysis of cell numbers include direct microscopic counts using the ratio method and relatively crude techniques such as the Howard mould count. Other procedures include electronic cell counts and indirect methods such as impedance measurement and oxygen uptake. Quantitative polymerase chain reaction (qPCR) procedures, developed originally for identification, are used increasingly for estimation of numbers of specific organisms in foods and other matrices. Such indirect methods are often predicated on the Normal distribution of the analyte since assays of pure chemicals usually conform to a Normal distribution. But only rarely is there a good agreement with Normality between actual counts of microbial cells and the results of indirect methods of analysis. This is even more evident when dilutions of food and environmental samples are examined. It is realistic to recognise that if organisms show over-dispersion in extracts, then indirect methods used to estimate their density will also reflect over-dispersion. Hence, it is usually necessary to relate the output of the test system to, for example, the log10 of cell numbers.