Introduction to Basic plate count techniques for the enumeration of microorganisms

Microbiological examination of foods is predominantly based on culturing techniques, to detect and enumerate living microorganisms. In view of the huge variety and multiplicity of groups, genera and species that may be present, a great number of tests are used, and which can be of one of two types: qualitative tests, which are aimed at detecting the  presence or absence of the target microorganism(s), without quantifying, and quantitative tests which determine the quantity of the target microorganism(s) in the sample, generally per unit of weight or volume. Each of these tests follow differentiated procedures, which in turn depend on the target microorganism(s), but most of them utilize the same basic microbiological culturing techniques. These techniques are the detection of presence/absence, the Most-Probable-Number counts (MPN( and the standard plate counts, described in this chapter.

Standard  plate counts are used both for quantification of large microbial groups, such as aerobic mesophilic microorganisms, aerobic psychrotrophiles, yeasts and molds, sulphate-reducing clostridia, enterococci and lactic bacteria, but also particular genera and species, such as Staphylococcus aureus, Bacillus cereus and Clostridium perfringens. The basic procedure consists in inoculating the homogenized sample (and its dilutions) on a solid culture medium (with agar), contained in Petri dish, and followed by incubation of the plates until visible growth occurs. The versatility of the technique derives from the principle underlying the concept of microbiological counts, and which is based on the premise that, when placed on an appropriate solid medium, each microbial cell present in the sample will form an isolated colony. By varying the type of culture medium (enrichment medium, selective medium, differential selective medium) and the incubation conditions (temperature and atmosphere), it is possible to select the group, genus or species to count. Since microbial cells often tend to aggregate into groupings of different shapes and sizes (pairs, tetrads, chains, grape-like clusters), it is not possible to establish a direct relation between the number of colonies and the number of cells. This correlation is between the number of colonies and the number of “ colony forming units” (CFU), which may be either individual cells or clusters of a specific size-shape-number configuration that is characteristic of certain microorganisms. With regard to inoculation in a solid culture medium ( called plating) four basic procedures may be utilized: a)  pour plate (deep plating), b)  spread plate (surface plating), c)  drop plate and d)  membrane filtration.

References

Silva, N.D .; Taniwaki, M.H. ; Junqueira, V.C.A.;  Silveira, N.F.A. , Nasdcimento , M.D.D. and Gomes ,R.A.R .(2013) . Microbiological examination methods of food and water a laboratory Manual. Institute of Food Technology – ITAL, Campinas, SP, Brazil .