Читать книгу Analytical Food Microbiology - Ahmed E. Yousef - Страница 59

“Counting” in food microbiology refers to the determination of the size of a microbial population within a specific quantity of food (i.e., population concentration). Enumerating the number of colonies on agar plates may also be referred to as “counting,” therefore careful distinction between these two usages is urged. Throughout this manual, the former will be referred to as “population count” and the later as “colony count.”

Some enumeration techniques, such as the direct microscopic counting method, allow determination of the number of cells per unit volume or weight of the sample. The plate count technique, however, determines the number of cells or cell clumps capable of forming colonies on agar plates. Since it is impossible to distinguish colonies arising from individual cells and those from cell clumps, the final population count determined by this method is expressed as colony forming units per unit volume or weight, i.e., CFU/ml or CFU/g.

To begin the counting process, the analyst should lay out the incubated plates in order of dilution to evaluate the executed dilution scheme and technique. The lowest dilution plated should have yielded the plate with the most colonies, and the number of colonies are expected to decrease by approximately a factor of ten as dilutions increase, provided that a decimal dilution scheme is used. If this is not the case, analysts should be cautious when interpreting results. Counting colonies on plates can be done visually, preferably with the help of a colony counter. Colony counters, such as the darkfield Quebec colony counter (Figure 3.2), provide background lighting and magnification so that small colonies are not overlooked. To carry out this process accurately and rapidly, the analyst should mark the counted colonies with a marker pen on the bottom of the plate, to make sure that the same colonies are not counted repeatedly.