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

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.

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.