A colony counter is an instrument used to count colonies of bacteria or other microorganisms growing on an agar plate. Early counters were merely lighted surfaces on which the plate was placed, with the colonies marked off with a felt-tipped pen on the outer surface of the plate while the operator kept the count manually. More recent counters attempt to count the colonies electronically, by identifying individual areas of dark and light according to automatic or user-set thresholds, and counting the resulting contrasting spots.

Microorganism enumeration

Such counters are used to estimate the density of microorganisms within a liquid culture. An appropriate dilution, or several dilutions within the estimated appropriate range, is spread using sterile technique on the agar plate, which is then incubated under the appropriate conditions for growth until individual colonies appear. Each colony marks the spot where a single organism was originally placed, thus the number of colonies on the plate equals the number of organisms within the volume of liquid spread on the plate. That concentration is then extrapolated by the known dilution from the original culture, to estimate the concentration of organisms within that original culture.

The maximum number of colonies which may be effectively counted on a single plate is somewhere between 100 and 1,000, depending on the size of the colony and the type of organism.

See also

• Growth medium
• Petri dish
• Replica plating
• Viral plaque