Which of the following is NOT a direct method to measure microbial growth?

• A. Turbidity measurement using a spectrophotometer
• B. Plate count using serial dilution
• C. Counting with a Petroff-Hausser chamber
• D. Hemocytometer counting

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the difference between direct and indirect methods of measuring microbial growth. Direct methods involve counting the actual number of cells, while indirect methods estimate growth based on related properties.
2. Step 2: Review the methods listed: Plate count using serial dilution, Petroff-Hausser chamber counting, and Hemocytometer counting are all direct methods because they involve counting cells either by colony formation or microscopic enumeration.
3. Step 3: Analyze turbidity measurement using a spectrophotometer. This method measures the cloudiness or optical density of a culture, which correlates with cell concentration but does not count cells directly.
4. Step 4: Conclude that turbidity measurement is an indirect method because it estimates microbial growth based on light scattering rather than direct cell counting.
5. Step 5: Therefore, identify turbidity measurement using a spectrophotometer as the method that is NOT a direct method to measure microbial growth.