Question

Which method is most appropriate for quantifying bacteria in a liquid food samples?

• A. Direct microscopic count on slide
• B. Serial dilution and standard plate count
• C. Gram staining alone
• D. Simple pH measurement

Hint:

Standard Plate Count (SPC) only measures viable (living, actively dividing) cells, which is the most critical parameter in food safety, whereas direct microscopic counts do not differentiate between living and dead bacteria.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks for the most appropriate and widely accepted analytical method used to quantify (enumerate) viable bacterial populations in liquid food matrices (e.g., milk, fruit juices, or beverages).

Step 2: Detailed Explanation:

• Quantification in Food Analysis: Accurately determining bacterial concentrations is critical for verifying compliance with food safety regulations and assessing microbial quality.

• Serial Dilution and Standard Plate Count (SPC): This is the gold standard method for viable microbial enumeration.

• Serial Dilution: High bacterial concentrations are systematically diluted (typically $10$-fold steps: $10^{-1}$, $10^{-2}$, etc.) in sterile diluent to ensure that when plated, the colonies will be sparse enough to be resolved individually.

• Standard Plate Count: Aliquots of the dilutions are transferred onto or into nutrient agar (using spread-plate or pour-plate methods). After incubation, plates containing $30 - 300$ colonies are counted. Each colony represents a single Colony Forming Unit (CFU) present in the original sample.

• Why Other Methods are Less Suitable:

• Direct Microscopic Count: Uses a hemocytometer slide. It is rapid but counts both live and dead cells, is physically tiring, and has low sensitivity (unsuitable for low bacterial counts).

• Gram Staining: This is a qualitative, differential staining technique to determine cell-wall morphology (Gram-positive vs. Gram-negative), not a quantitative enumeration tool.

• Simple pH Measurement: Measures chemical acidity/alkalinity. While acid production indicates bacterial metabolism, it cannot be used to calculate physical cell numbers.

Step 3: Final Answer:
Serial dilution followed by the standard plate count is the standard quantitative, viable-cell enumeration technique for liquid foods, making option (B) the correct choice.