Which growth-phase parameter can serve as a proxy for culture viability in certain tests?

• A. Optical density at 600 nm.
• B. Colony-forming unit counts (CFU) or viability staining.
• C. pH measurement.
• D. Temperature of incubation.

Answer: (B)

Measuring viability focuses on whether cells are alive and able to reproduce. Optical density at 600 nm tracks how much cell mass is present, but it doesn’t distinguish living from dead cells, so it’s a rough growth indicator rather than a true viability measure. Temperature and incubation conditions influence growth, but they don’t directly report how many cells are viable. Counting colony-forming units or using viability staining provides a direct readout of viability: CFU counts reflect how many cells can actually reproduce to form colonies, and viability staining distinguishes live cells from dead ones quickly. Because these methods directly indicate whether cells are alive and capable of growth, they serve as the best proxy for culture viability in tests. (Note: CFU can underestimate viability in cells that are alive but non-culturable, and viability staining reflects membrane integrity rather than reproductive ability, but these approaches still offer the most direct viability information among the options.)