Which statement about CFU counts as a measure of viability is true?

• A. CFU counts overestimate viability because they count cells that can't grow.
• B. CFU counts may underestimate viability because some viable cells are VBNC or require specific conditions to form colonies.
• C. CFU counts measure only dead cells.
• D. CFU counts measure only viruses.

Answer: (B)

The main idea is that CFU counts reflect viability only for cells that can form colonies under the specific culture conditions used. Some viable cells stay alive but don’t form colonies on standard plates—these are the viable but non-culturable (VBNC) cells. Others may require particular nutrients, signals, or environmental triggers to resume growth. Because these cells can be alive but not produce colonies under the tested conditions, CFU counts can underestimate the true number of viable cells. In contrast, CFU counts do not measure dead cells, and they are not a measure of viruses.