What is the maximum (and minimum) number of colonies that can be counted accurately on one plate?
A 8. Standard protocol for most samples dictates that Coliscan®Easygel® dishes containing more than 300 colonies (CFU) should be identified as TNTC (too numerous to count). However, it is possible to achieve quite accurate results or estimates with up to 1000 colonies/plate by careful observation. Standard protocol for Coliscan®MF dishes dictates a maximum of 80 CFU/dish due to the fact that the MF dish is a much smaller diameter (60 mm vs. 100 mm for standard dishes). It should also be noted that less than 20 CFUs/dish for either medium is considered to be statistically questionable for accuracy of count. (Note that Coliscan® Easygel® and other Easygel® media types used as pour plates in food and water testing have a big advantage over methods such as 3M’s Petrifilm® and Hach’s Coliblue® media which are limited to a small sized surface area for colony growth.) The best way to count colonies is to position a light source so that you can easily see the colonies while looking through the
