Which of the following is an indirect method for estimating the number of microbes in a sample?

• A. Direct microscopic count
• B. Performing a plate count
• C. Measuring turbidity using a spectrophotometer
• D. Using the most probable number (MPN) method

Answer: (C)

Verified step by step guidance

1. Understand the difference between direct and indirect methods for estimating microbial numbers. Direct methods involve counting the actual cells, while indirect methods estimate numbers based on related measurements.
2. Review the direct microscopic count method: this involves directly observing and counting cells under a microscope, so it is a direct method.
3. Consider the plate count method: this involves growing colonies on agar plates and counting colony-forming units, which directly reflect viable cells, so it is also a direct method.
4. Examine the turbidity measurement using a spectrophotometer: this method estimates microbial density by measuring how much the sample scatters light, which correlates with cell concentration but does not count cells directly, making it an indirect method.
5. Look at the Most Probable Number (MPN) method: this statistical method estimates microbial numbers based on dilution and growth patterns, which is also an indirect method but different from turbidity measurement.