Why is it necessary to dilute a sample when determining bacterial numbers using plate counts?

• A. To ensure that only Gram-positive bacteria are detected
• B. To increase the growth rate of bacteria on the agar plate
• C. To obtain countable colonies that accurately reflect the number of bacteria present
• D. To prevent contamination from environmental microbes

Answer: (C)

Verified step by step guidance

1. Understand that when performing plate counts, the goal is to estimate the number of viable bacteria in a sample by counting colonies formed on an agar plate.
2. Recognize that if the sample is too concentrated, too many bacteria will grow, resulting in a lawn of growth or overlapping colonies, making it impossible to count individual colonies accurately.
3. Dilution reduces the concentration of bacteria in the sample, allowing colonies to grow separately and be countable.
4. By counting colonies on a properly diluted plate, you can calculate the original number of bacteria in the sample using the dilution factor.
5. Therefore, dilution is necessary to obtain countable colonies that accurately reflect the number of bacteria present, rather than to select for Gram-positive bacteria, increase growth rate, or prevent contamination.