Fundamentals of Microbiology

Colony Counter

 Colony counter is a device used to count the number of individual colonies on an agar plate. The objective is to increase the contrast between the colonies and the culture medium. It has a circular opening covered by a glass counting plate divided into squares. This plate can accommodate petri dishes of up to 100mm external diameter.

            Light source is fitted behind the plate. A 4 1/4 –inch thick lens having a standard 1.5X magnification is mounted on a sliding rod above the plate. The petriplate is placed on the counting plate. Light behind the glass plate illuminates the colonies so that they appear as brilliant spots or points on a subdued background. Magnifying glass improves the visibility so that very small colonies also can be seen clearly. In advanced versions readjustable electromagnetic counter provides instant, cumulative and accurate recording of colony counts. A switch in the electronic pen energizes the counter. Different ink colors can be used if different markings are desired. The marking pen can also be used in conjunction with the disposable membranes. Wet membranes are fixed at the back of the petri dish and after counting the sheets are indexed, dried and stored for reference.