Question

1. Observe the two sets of plates. Determine the CFU/mL of each solution (Normal and Heated), remembering to use a plate that has 30 – 300 colonies for statistical reliability. Is there a difference in the CFU/mL counts for the two treatment conditions? If so, what do you think accounts for this difference?
2. How are the colonies from the two treatment conditions different (e.g., size, color, shape)?
3. What effect does the heating and cooling procedure have on the microorganisms present in the soil samples?
4. What is the source of the colonies in the heated samples?

Answer 1

Answer:

1):

• Please note that for this answer, you need to count the number of colonies of each plate and mention those.
• CFU/ml = number of colonies * dilution factor / volume (in ml)
• Dilution factor for stock solution is 10^0, for 10% is 10^-1; for 1% is 10^-2; for 0.01% is 10^-3
• Volume plated is 0.1ml
• CFU/ml for normal and heated plates will be average of the CFU/ml of four plates.

2):

• In the first case, colonies of all bacteria present in the soil sample are obtained.
• These may vary in color from white, yellow, orange, green etc.
• Different shapes and sizes of colonies must be there. This is because soil contains a consortium of different taxa of bacteria.
• In the second case, heated sample of soil is added. Heat kills all the bacteria from the sample.
• It is important to note that some bacteria must be already present in the saline solution.
• These bacteria are halophiles. These will form colonies.
• Colony morphology of all these bacteria will be almost same. This means that all colonies will be almost same color, shape and size.

3):

• Heating kills almost all the bacteria of the soil solution. After heating, only highly thermostable bacteria (stable at a temperature > 120°C) will be left.
• Then, cooling is done. This will kill the remaining bacteria that are thermostable, partly by shock treatment, and the rest by low temperature.

4):

• The saline solution is the main source of bacterial colonies in the heated samples.

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