Question

Which dilution range is considered statistically reliable for plate count methods?

• A. 1–10 colonies
• B. 10–50 colonies
• C. 30–300 colonies
• D. 300–1000 colonies

Hint:

Plates with fewer than $30$ colonies are reported as "Too Few To Count" (TFTC), and those with more than $300$ colonies are reported as "Too Numerous To Count" (TNTC) due to overcrowding.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks for the standard, statistically validated range of bacterial colonies on an agar plate that is considered reliable for calculating microbial populations in food analysis.

Step 2: Detailed Explanation:

• Principle of Colony Counting: Standard plate count methods (pour-plate or spread-plate) require counting visible colonies to estimate Colony Forming Units (CFUs) per milliliter or gram of original sample.

• Statistical Constraints:

• If a plate has too few colonies, random sampling errors become highly significant, making the count statistically unreliable. This rules out options (A) and (B).

• If a plate has too many colonies, the plates become overcrowded. Overcrowding leads to overlapping colonies, which causes undercounting. It also depletes nutrients, inhibiting the growth of some cells. This is termed "Too Numerous To Count" (TNTC), ruling out option (D).

• The Ideal Range: Standard regulatory and analytical bodies (such as FDA-BAM and ISO) have established that plates containing between 30 and 300 colonies provide the optimal balance, minimizing both statistical deviation and crowded overlapping. For certain specialized mold/yeast counts, a range of $15 - 150$ may be used, but $30 - 300$ remains the standard for bacteria.

Step 3: Final Answer:
The dilution range of $30 - 300$ colonies per plate is the standard for statistical reliability in microbiology, making option (C) the correct choice.