Читать книгу Analytical Food Microbiology - Ahmed E. Yousef - Страница 58

Inoculated plates are incubated at a time‐temperature combination appropriate for the growth and colony formation by the microorganism being counted. Microorganisms vary in their ability to grow at different temperatures. While psychrophiles prefer refrigeration temperatures (1 to 10°C), mesophiles grow optimally at temperatures close to that of the human body (37°C), and thermophiles grow best at higher temperatures (e.g., 55°C). Psychrotrophic bacteria grow optimally in the mesophilic range, but they are also capable of growing under refrigeration. Choice of incubation temperature, therefore, depends on the microorganisms of interest and their natural habitat, as well as presence of competing microbes.

In food microbiology, several incubation temperatures are typically used. For potentially pathogenic organisms, such as Salmonella enterica, incubation occurs at 32–37°C, a temperature range suitable for mesophiles. A somewhat cooler temperature (e.g., 25°C) is more preferred by spoilage organisms such as yeasts, molds, and psychrotrophic bacteria. “Room temperature” is typically taken to mean 22°C, but this temperature may vary depending on the room used for incubation and even the season and area of the world. Refrigeration at 4°C is typically used to maintain cultures without allowing further growth. A refrigerated incubation can also be used for cold enrichments of psychrotrophic microorganisms such as Listeria monocytogenes.

Plates containing inoculated agar media are typically inverted before incubation. If plates are incubated with lids upward, water condensate falls on the agar surface causing the spreading of colonies. When plates are inverted, condensed water (from moist agar) accumulates on the plate lid. Excessive water condensation on the lid, however, is undesirable and should always be minimized. Pouring hot agar (> 50°C) aggravates this problem. In the case of spread plates, it is preferable to pour the agar in these plates 24–48 hours before use. Some microbiologists choose to “dry” the spread plates soon after preparation for several hours in a warm clean incubator. For the safety of the analyst, plate lids with excessive water condensation should be replaced with dry sterile lids.