#SERIAL DILUTION LAB REPORT MICROBIOLOGY SERIES#

#SERIAL DILUTION LAB REPORT MICROBIOLOGY SERIES#

“Because of these and other difficulties certain plates in any series made from a given sample are more satisfactory for use in computing a total than are others.

They set out to determine the “limit in the number of colonies that may be allowed to grow on a plate without introducing serious errors…in connection with the proposed revisions of standard methods of milk analysis.” They note that “the kind of bacteria in the material under examination will have an influence on the size of the colonies, and consequently, on the number that can develop on a plate.” They also note that food supply can be an issue, that colonies close to each other on the plate may merge, and that neighbor colonies may inhibit growth or conversely stimulate growth. Breed and Dotterrer published a seminal paper on this topic in 1916 (2). The origin of those ranges is worth examination. The general ranges in common acceptance for countable numbers of colonies on a plate are 30 – 300 and 25 – 250. While you might get lucky and hit this with dilutions whose plate counts are in the 150-250 CFU/plate range, – at lower plate counts this target value imposed by regulation will virtually guarantee a long, difficult and quite possibly unsuccessful, validation exercise. A good example of this is the Pharm Eur “Precision” requirement for an alternate method (quantification) to have a Relative Standard Deviation (RSD) in the range of 10-15% (1). Being obsessive by training, we are trying to exceed measures of accuracy and precision in this exercise that the traditional methods cannot come close to matching. We have allowed specifications for environmental monitoring, raw material bioburden, in-process bioburden and finished product bioburden to be imposed by regulation without regard for the ability of the method to support those specifications.Ī second reason for concern is that now we are trying to introduce alternate microbiological methods into the lab. Our lax attention to precision and accuracy in our measurements helps further this perception. One reason for concern is that microbiology has a well-deserved reputation for being highly variable. A colony could arise from one cell, or several thousand.

Even among that group of microorganisms a colony does not represent a single cell, but rather cells that happened to be well separated on the plate and so can be distinguished after growth. It is a skewed estimate at best as the only cells able to form colonies are those that can grow under the conditions of the test (incubation media, temperature, time, oxygen conditions, etc). What is the fuss about in determining the number of colony forming units? After all, the CFU is only an estimate of the number of cells present. This article first appeared in the PMF Newsletter of January, 2006 and is protected by copyright to PMF.

Serial dilution lab report microbiology