Diluting Cultures Before Plating
Cultures need to be diluted with sterile buffer before they can be counted. If not, there will be too many microorganisms to count in the Petrie dishes.
Appendix 12 shows this. It also shows how to determine the number of microorganisms. Count only plates with 30-300 microorganisms. Less than 30 may be inaccurate. More than 300 will be impossible to count.
Turbidimetric Methods
Let’s look at Appendix 13. The leftmost sample is clear. The others are turbid. The rightmost sample is very turbid.
The amount of turbidity can be measure by optical density. Optical density is the portion of light with is blocked by bacteria. Appendix 14 shows this.
Optical density (OD) can be used to measure cell number with quite good accuracy. A standard curve like the one shown in Appendix 15. Then cell numbers can be estimated from optical density.
Cardinal Temperatures
We already talk about temperature. I would like to add just a little more.
I think that cardinal is สำคัญ
There are important reference temperatures for microbial grown. They are:
• Minimum Temperature
• Optimum Temperature
• Maximum Temperature
This is shown in Appendix 16.
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