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1.
Definition
The total number of colonies refers to the total number of microbial colonies formed in the 1mL (g) sample after the sample is processed and cultured under certain conditions (such as medium composition, culture temperature and time, pH, aerobic properties, etc.
)
.
The total number of colonies is not equal to the total number of bacteria.
There are two reasons: On the one hand, each type of bacteria has different requirements for environmental conditions when growing.
For example, anaerobic bacteria and psychrotrophic bacteria are difficult to grow and reproduce under the conditions of the total number of colonies.
Some bacteria with special nutritional requirements are also restricted.
Therefore, the results obtained only reflect the total number of mesophilic, aerobic and facultative anaerobic bacterial colonies developed in ordinary nutrient agar
.
On the other hand, bacterial cells exist in single, double, chain, grape-like or piled forms, so the colonies that appear on the plate may be derived from single cells or cell clumps
2.
Hygiene significance
The total number of bacterial colonies is an important item in food hygiene indicators and is mainly used as a sign to determine the degree of contamination of food
.
It is generally believed that the greater the total number of bacterial colonies in food, the greater the possibility of contamination by pathogenic bacteria
3.
Inspection method
Carried out in accordance with GB4789.
2-2016, the standard stipulates the determination method of aerobic plate count in food
.
The inspection procedure for the total number of colonies is shown in Figure 4-4
Figure 4-4 Inspection procedure for the total number of colonies
Four, matters needing attention
(1) There must be a concept of "aseptic operation"
.
The glassware used must be completely sterilized.
Scissors, tweezers and other utensils must be sterilized.
(2) Attention should be paid to the representativeness of sampling.
Liquid samples must be shaken before sampling.
When sampling solid samples, it is advisable to collect a few more locations instead of focusing on one point
.
(3) The diluent can be sterilized saline, distilled water or peptone water (1g/L), peptone water is the most suitable, because peptone water has a better protective effect on bacterial cells
.
If diluting samples with higher salt content, distilled water should be used
(4) When doing 10-fold incremental dilution, the pipette or tip inserted into the sample homogenate should not be lower than the liquid level 2.
5cm to prevent excessive liquid sticking to the outside of the pipe when the pipette or tip is taken out of the diluent
.
When serially increasing dilution, each diluent should be fully shaken to make it evenly mixed.
(5) After the sample homogenate is added to the petri dish, pour 15-20 mL of plate counting agar cooled to 46° C.
into the petri dish in time
.
In order to distribute the colonies evenly on the plate, turn the petri dish immediately to make it evenly mixed
(6) Add the sample homogenate in the petri dish (especially the dilution of 1:10), sometimes with food particles.
Related Links: Carbon Source and Enzyme Test