Which method is commonly used to determine the original cell density in an Erlenmeyer flask during a microbiology experiment?

• A. Observation under a light microscope
• B. Gram staining
• C. Use of a spectrophotometer to measure absorbance
• D. Serial dilution followed by plate counting

Answer: (D)

Verified step by step guidance

1. Understand that determining the original cell density means finding out how many viable cells are present in a given volume of culture before any treatment or measurement.
2. Recognize that direct observation under a light microscope can count cells but does not distinguish between live and dead cells, and is often not practical for precise quantification in dense cultures.
3. Know that Gram staining is a differential staining technique used to classify bacteria based on cell wall properties, not for quantifying cell numbers.
4. Learn that using a spectrophotometer to measure absorbance (optical density) provides an indirect estimate of cell density but does not give an exact count of viable cells.
5. Identify that the most accurate and commonly used method to determine original viable cell density is serial dilution followed by plate counting, where the culture is diluted stepwise, plated on agar, incubated, and colonies counted to calculate the number of colony-forming units (CFU) per milliliter in the original sample.