What is the primary purpose of plating in a viable plate count?

What is the primary purpose of plating in a viable plate count?

• A. To allow viable cells to form colonies that can be counted.
• B. To sterilize the sample.
• C. To measure turbidity.
• D. To observe color changes.

Answer: (A)

The main idea here is to estimate the number of living, reproducing cells by turning them into discrete colonies. In a viable plate count, you plate a diluted sample on nutrient agar and incubate so that each viable cell (one that can reproduce) forms a visible colony. Counting these colonies gives you the number of colony-forming units per milliliter in the original sample, after accounting for the dilution. This method specifically measures viability, because dead cells do not form colonies. It’s not about sterilizing the sample, measuring turbidity with a spectrophotometer, or watching color changes on a differential plate; those approaches assess other properties, not direct enumeration of viable cells. For accurate counts, you aim for a dilution that yields a countable, well-separated range of colonies.

The main idea here is to estimate the number of living, reproducing cells by turning them into discrete colonies. In a viable plate count, you plate a diluted sample on nutrient agar and incubate so that each viable cell (one that can reproduce) forms a visible colony. Counting these colonies gives you the number of colony-forming units per milliliter in the original sample, after accounting for the dilution. This method specifically measures viability, because dead cells do not form colonies. It’s not about sterilizing the sample, measuring turbidity with a spectrophotometer, or watching color changes on a differential plate; those approaches assess other properties, not direct enumeration of viable cells. For accurate counts, you aim for a dilution that yields a countable, well-separated range of colonies.