If you allowed your dilution tubes to incubate for 24 hours before plating them, do you think the results of the experiment would be impacted? Assume that unlimited resources are present in the tubes.

If you allowed your dilution tubes to incubate for 24 hours before plating them, do you think the results of the experiment would be impacted? Assume that unlimited resources are present in the tubes.

• A. Yes, the results would be impacted because bacteria would grow in the tubes and affect accurate colony counts.
• B. No, the results would not be impacted.
• C. Yes, but only for yeast cultures.
• D. It would depend on the type of agar used.

Answer: (A)

Growth during the dilution step changes what you actually plate. In a serial dilution used to estimate CFU, you assume the number of viable cells in each tube is fixed at the moment of dilution. If you incubate for 24 hours with abundant nutrients, those cells will multiply, so when you plate later, the colonies reflect a higher starting cell count than the intended dilution factor. That inflates colony numbers and skews the CFU per milliliter calculation. Because resources are unlimited in the tubes, growth will occur, so the results would be impacted. To avoid this, plate promptly after diluting or halt growth before plating.

Growth during the dilution step changes what you actually plate. In a serial dilution used to estimate CFU, you assume the number of viable cells in each tube is fixed at the moment of dilution. If you incubate for 24 hours with abundant nutrients, those cells will multiply, so when you plate later, the colonies reflect a higher starting cell count than the intended dilution factor. That inflates colony numbers and skews the CFU per milliliter calculation. Because resources are unlimited in the tubes, growth will occur, so the results would be impacted. To avoid this, plate promptly after diluting or halt growth before plating.