Which statement correctly defines the concept of colony count on a plate?

• A. It is determined by the percentage of plate area covered by a confluent lawn of bacteria times 10,000
• B. It is the number of individual colonies observed
• C. It is the zone of inhibition diameter
• D. It is the time to first colony appearance

Answer: (A)

Colony count on a plate is the number of discrete colonies that appear, with each colony typically arising from a single viable organism (a colony-forming unit). This count directly reflects the number of viable microorganisms in the sample after accounting for dilution and how much was plated. To express the concentration, multiply the observed colonies by the reciprocal of the dilution factor and divide by the plated volume (in milliliters). For example, if you count 50 colonies on a plate from a 10^-3 dilution and you plated 0.1 mL, the concentration is 50 × 1000 ÷ 0.1 = 5 × 10^5 CFU/mL. If growth is confluent and no discrete colonies are visible, you cannot determine a colony count from that plate.

Other statements describe different concepts: a zone of inhibition diameter relates to antimicrobial susceptibility testing, and the time to first colony appearance pertains to growth kinetics. The idea of using a percentage of plate area covered by a confluent lawn does not represent colony counting.