Question

Which of the following pre-heat treatments is used in the enumeration of mesophilic aerobic spore counts before conducting serial dilution and plating?

• A. 80°C for 10 minutes
• B. 70°C for 10 minutes
• C. 110°C for 10 minutes
• D. No pre-treatment required

Hint:

Always associate the standard laboratory spore-count pre-treatment with \(80^{\circ}\text{C}\) for 10 minutes (or 12 minutes) to selectively isolate bacterial endospores from vegetative cells.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
To enumerate bacterial spores in milk, it is necessary to selectively eliminate all vegetative cells while leaving the heat-resistant endospores intact and viable for subsequent germination and colony formation.

Step 2: Detailed Explanation:
- Heating a milk sample at \(80^{\circ}\text{C}\) for 10 minutes is the standard pre-treatment used to pasteurize/heat-shock the sample.
- This specific temperature-time combination is sufficient to kill all vegetative bacterial cells (including non-spore-formers and vegetative cells of spore-forming species) but is not harsh enough to destroy bacterial endospores (such as those of Bacillus or Clostridium species).
- It also serves to "heat-shock" the spores, which triggers and synchronizes their subsequent germination on agar plates.
- Heating at \(70^{\circ}\text{C}\) may not guarantee the complete destruction of some heat-resistant vegetative cells, while \(110^{\circ}\text{C}\) would destroy many of the target mesophilic spores.

Step 3: Final Answer
The standard pre-heat treatment used for enumerating mesophilic aerobic spores is 80°C for 10 minutes.