The current definition of colony forming unit (CFU) is inaccurate and misleading. The current definition implies that a given culture test protocol will detect all bacteria present in a sample. More than 25 years ago, Roszak and Colwell debunked this conceit. (Roszak, D. B., and Colwell, R. R., Survival Strategies of Bacteria in the Natural Environment, Microbiological Reviews, Vol 51, No. 3, Sept. 1987, pp. 365379.) Subsequently a substantial body of literature about the vast difference between culturability (under a given set of growth conditions) and viability has grown. Any definition of CFU must include consideration of this difference. It must also indicate that the term applies to algae and fungi as well as bacteria. None of the definitions currently provided in Standards under the jurisdiction of D19, E54 or F04 meet all of these criteria.
Keywordscalculating results; counting colonies; membrane filter methods; pour plate methods; reporting results; rounding off; significant figures; spread plate methods; verification of counts;