Quantifying bacteria in a specimen Among the following routine methods, which is most likely to yield a quantitative result for viable bacterial numbers in a sample?

Introduction / Context:Clinicians and microbiologists often need quantitative data, such as colony-forming units per milliliter, to interpret urinary tract infections, food contamination, or water quality. Different methods provide either qualitative or quantitative information; knowing which method is designed for quantitation is fundamental.

Given Data / Assumptions:

• Serial dilution and plating can enumerate viable organisms.
• Gram stain provides morphology and approximate bacterial load but is not reliably quantitative.
• Wet mounts demonstrate motility or presence but not accurate counts.

Concept / Approach:The viable plate count method involves preparing tenfold dilutions of a sample, plating measured aliquots onto agar, incubating, and counting colonies. Because each colony theoretically arises from a single viable cell (or clump), the result can be expressed as CFU per mL or per gram using the formula: CFU/mL = colony count * (1 / plated volume) * dilution factor.

Step-by-Step Solution:Perform serial dilutions (for example, 10^-1 to 10^-6).Plate a known volume (for example, 0.1 mL) onto appropriate agar.After incubation, count plates with 30–300 colonies for accuracy.Compute CFU/mL using dilution factor and plated volume.

Verification / Alternative check:Results are reproducible within accepted variance and correlate with standard curves; other quantitative methods include Most Probable Number and flow cytometry, but classic dilution plating remains common.

Why Other Options Are Wrong:

• B: Gram stain is semi-quantitative at best.
• C: Wet mounts reveal motility but not standardized counts.
• D: Incorrect because option A is quantitative.
• E: DFA can identify organisms but is not a routine quantitative method.

Common Pitfalls:Counting plates outside the 30–300 range without adjusting dilutions; ignoring clumping which may underestimate cell numbers.

Final Answer:Serial dilution followed by plating (viable count)