Why can optical density overestimate live cell counts in a culture?

• A. OD measures only viable cells
• B. OD measures turbidity from dead cells only
• C. OD measures turbidity from all cells, including dead ones, not just viable cells
• D. OD measures the pH of the medium

Answer: (C)

Optical density tracks how turbid a culture is, which means it measures how much the suspension scatters light. That scattering comes from all particles in the culture, not just living cells. Dead cells, cell debris, and even clumped cells still scatter light, so they contribute to the turbidity that OD detects. As a result, the OD can stay high even when the number of viable (live) cells has decreased, leading to an overestimation of live cell counts. The other options misstate what OD measures: it is not limited to viable cells, not exclusive to dead cells, and it does not measure pH. For viability, methods like CFU counts or viability dyes are needed.