6.2—Measuring microorganism growth

Microbial growth can be estimated by counting colonies, measuring turbidity or tracking another validated proxy. The method must be consistent so that differences reflect population change rather than sampling or instrument variation.

Use a known dilution or counting area, repeat measurements, keep incubation conditions constant and plot the proxy against time. A calibration is needed before converting absorbance or turbidity into cell number.

Two cultures with the same turbidity reading may contain different cell sizes or clumps, so direct colony counts or a calibration curve may be needed to compare viable cells.

Optical density measures cloudiness, not automatically living cells. Do not infer growth rate from one endpoint or compare values made with different dilution and timing conditions.