Which plating method is commonly used to count the number of viable bacteria in a sample?

• A. Spread plate method
• B. Pour plate method
• C. Streak plate method
• D. Replica plating method

Answer: (A)

Verified step by step guidance

1. Understand the goal: The question asks which plating method is commonly used to count the number of viable bacteria in a sample. Viable bacteria are those capable of growing and forming colonies.
2. Review each plating method's purpose: The streak plate method is mainly used to isolate pure colonies, not for counting. Replica plating is used to transfer colonies to multiple plates for comparison, not for counting viable cells.
3. Focus on the spread plate and pour plate methods: Both are quantitative techniques used to estimate the number of viable bacteria by spreading or mixing a known volume of sample on or in agar.
4. Compare the two quantitative methods: The spread plate method involves spreading a diluted sample evenly on the surface of solid agar, allowing colonies to grow on top, which makes counting easier and more accurate.
5. Conclude that the spread plate method is commonly preferred for counting viable bacteria because it provides clear, countable colonies on the agar surface without the complications of colonies growing inside the agar, as seen in the pour plate method.