Quantitative bacterial enumeration: which plating techniques are suitable? Which technique(s) can be performed quantitatively to determine the number of viable bacteria of a particular type in a sample?

Introduction / Context:
Quantitative bacteriology is fundamental in food safety, water quality, and clinical microbiology. Only specific plating approaches yield countable colonies suitable for CFU calculations. This question discriminates between quantitative and qualitative isolation techniques.

Given Data / Assumptions:

• Objective is to determine CFU/mL using calibrated dilutions and plated volumes.
• Countable plates typically contain 30–300 colonies.
• The streak plate aims for isolation, not enumeration.

Concept / Approach:

Both pour and spread plates accept a known volume of a known dilution, enabling back-calculation to CFU/mL. The streak plate is not standardized for volume, so it does not support accurate enumeration. “Drop plate” can be quantitative if standardized, but the undiluted drop method in the option is not framed for proper quantification here.

Step-by-Step Solution:

Verification / Alternative check:

Standard Methods for the Examination of Water and Wastewater, and food microbiology texts, specify pour/spread protocols for quantitative CFU counts.

Why Other Options Are Wrong:

• Streak plate: Designed to obtain isolated colonies, not an accurate count.
• Undiluted drop plate: Without serial dilution and calibrated drops, quantification is unreliable.

Common Pitfalls:

• Counting colonies from streak plates and reporting CFU/mL; this is incorrect because inoculum volume is unknown.

Final Answer:

Both (a) and (b)