Reconsideration of the derivation of Most Probable Numbers, their standard deviations, confidence bounds and rarity values

Abstract

Aims: The purpose of this work was to derive a simple Excel spreadsheet and a set of standard tables of most probable number (MPN) values that can be applied by users of International Standard Methods to obtain the same output values for MPN, SD of the MPN, 95% confidence limits and test validity. With respect to the latter, it is considered that the Blodgett concept of ‘rarity’ is more valuable than the frequently used approach of improbability (vide de Man). Methods and Results: The paper describes the statistical procedures used in the work and the reasons for introducing a new set of conceptual and practical approaches to the determination of MPNs and their parameters. Examples of MPNs derived using these procedures are provided. The Excel spreadsheet can be downloaded from http://www.wiwiss.fu-berlin.de/institute/iso/mitarbeiter/wilrich/index.html. Conclusions: The application of the revised approach to the determination of MPN parameters permits those who wish to use tabulated values, and those who require access to a simple spreadsheet to determine values for nonstandard test protocols, to obtain the same output values for any specific set of multiple test results. The concept of ‘rarity’ is a more easily understood parameter to describe test result combinations that are not statistically valid. Provision of the SD of the log MPN value permits derivation of uncertainty parameters that have not previously been possible. Significance and Impact of the Study: A consistent approach for the derivation of MPNs and their parameters is essential for coherence between International Standard Methods. It is intended that future microbiology standard methods will be based on the procedures described in this paper.

Bacteriophage Cocktail for Biocontrol of Salmonella in Dried Pet Food

“…Samples (25 g) were added to 225 ml of buffered peptone water and then stomached for 2 min at 260 rpm, and the Salmonella population was quantified with the rapid mini modified semisolid Rappaport-Vassiliadis most-probable-number (MPN) assay as described by Shashidhar et al (34), modified so that positive wells were confirmed with the MicroSEQ 16S rDNA identification system for Salmonella detection (Life Technologies, Carlsbad, CA) (6, 7). Estimation of the MPNs and log reductions for the inoculated dry pet food and the contaminated undistributed commercial lots data was completed using the Poisson binomial model formulation as described in Jarvis et al (25). Statistical significance and confidence limits were determined using the alpha = 0.05 level.…”

Impact of storm runoff onSalmonellaandEscherichia coliprevalence in irrigation ponds of fresh produce farms in southern Georgia

“…Estimates of Salmonella MPN were calculated based on the presence of PCR-confirmed Salmonella in each of the nine replicates analysed per composited water sample (Jarvis et al 2010; Andrews et al 2014). The lower and upper limits of Salmonella detection were 0.0548 and 11 MPN 100 ml −1 , respectively.…”

Alkane-degrading bacteria at the soil-litter interface: comparing isolates with T-RFLP-based community profiles

“…Briefly, bacteria were detached from 1 g soil or 0.25 g litter by 1 h rotary shaking with a 10fold (w/v) amount of HC-mineral medium (Kloos et al, 2006). All values are calculated as log 10 MPN and respective log 10 standard deviations, and log-normal distribution is assumed (Jarvis et al, 2010). Heterotrophic bacteria were enumerated after growth on diluted Luria Broth (1 : 5 with HC-mineral medium).…”