Bacterial density Can be estimated by the formulae given or from the table using the number of positive tubes in the multiple dilutions. The number of sample portions selected will be governed by the desired precision of the result. MPN tables are based on the assumption of a Poisson distribution (random dispersion). However, if the sample is not adequately shaken before the portions are removed or if clumping of bacterial cells occurs, the MPN value will be understimate of the actual bacterial density.