The Relative Errors of Bacteriological Plate Counting Methods

Abstract

One of the principal measures of error of dilution plate counts, the variation among replicate plates, was separated into 2 components : a) variation due to pipetting of aliquots of inoculum into plates, and b) residual variation due to distribution of organisms in these aliquots plus other factors. The former was detd. by gravimetric measurement and the latter by the difference between the pipetting variation and the total variation observed with actual counts of Shigella dysenteriae. Three pipetting methods gave coeffs. of variation of 1, 2 and 2.7% and were shown to be significantly different. However, 5 plating procedures based on these methods gave coeffs. of variation ranging from 7.3 to 8.9% and did not differ significantly. This apparent contradiction was explained on the basis of the addition theorem. It was demonstrated that with any of the methods used, the pipetting variation is unlikely to contribute significantly to the error of the count even if the plating variation reaches the uncommonly low value of 5%. Conclusions: Attempts to improve the precision of plate counts should be directed elsewhere than toward increasing the precision of measurement of aliquots into plates; relatively imprecise methods may be used for such measurement when necessary.