When colonies are too numerous to count, how is the colony count determined?

• A. The percentage of total plate area covered with a confluent lawn of bacteria times 10,000
• B. The number of colonies per square centimeter
• C. The dilution factor used
• D. The time to visible growth

Answer: (A)

When there are too many colonies to count, you switch to an estimation approach based on how much of the plate is covered by growth. The idea is that a confluent lawn reflects a high density, so you quantify the area of the plate that shows colonies and convert that coverage into a rough number of colony-forming units on the plate. A standard conversion multiplies the percent plate coverage by 10,000 to yield the estimated CFU on that plate. For example, about 30% coverage would correspond to roughly 3,000 CFU on the plate. If you started from a dilution, you would then use that plate-count estimate to back-calculate CFU per mL in the original sample. This method is used specifically when counting individual colonies isn’t possible; other options either rely on counting discrete colonies (not feasible with a dense lawn), or rely on growth rate (time to visible growth) or dilution factor alone without converting the dense growth into a plate-wide estimate.