If more than one plate is countable, average the counts together. In this way you will have at least 1-2 plates within the countable range (25-250) to use in your calculations. When the approximate number of bacteria is unknown, plate a wide range of dilutions. Plates with less than 25 colonies do not have a statistically significant number of colonies. Counts above 250 are considered Too Numerous To Count (TNTC) because it is impossible to tell whether colonies are separated. Ideally only plates with 25-250 colonies are used. For this reason results are reported as colony forming units (CFU)/ml of bacterial culture. Not all bacterial cells produce colonies, as some bacteria tend to clump or aggregate, and some are nonviable. It is imperative that you utilize your best aseptic technique. If you perform a viable count to determine the number of bacteria in a culture, plate aliquots of the dilutions onto agar with sterile pipettes, spread with glass hockey sticks, incubate at 37☌ and count the number of colonies. Tips for handling specific types of data:
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