A Model for Statistical Evaluation of Precision Parameters of Microbiological Methods: Application to Dry Rehydratable Film Methods and IDF Reference Methods for Enumeration of Total Aerobic Mesophilic Flora and Coliforms in Raw Milk

Abstract

A new statistical approach for collaborative study data of microbiological methods Is proposed. This Includes a confirmatory test to the Polsson distribution of the number of colonies. In addition, 2 new statistical parameters are used to express precision as a percent of the original unit: the geometric relative standard deviation (GRSD) and the critical relative difference between 2 measurements (RD95). This statistical approach was applied to an Interlaboratory study to assess and compare the precision of both dry rehydratable film (PetrlfUrn® SM and Petrifllm® VRB) methods and International Dairy Federation (IDF) reference methods [total aerobic mesophilic plate count (TAMPC) and violet red bile lactose agar (VRBL) methods] for estimation of total bacteria and collform, respectively, in raw milk. Each of the 14 laboratories In the study analyzed 40 laboratory samples (20 different materials In blind duplicates) for total bacteria and collform counts by both the Petrifllm and standard methods. Repeatability standard deviations (In log10 unit) of TAMPC, Petrifllm SM, VRBL, and Petrifllm VRB were 0.106, 0.089, 0.219, and 0.171, respectively; their reproducibility standard deviations were 0.170,0.167,0.348, and 0.199, respectively.

An Evaluation of Raw Milk Microorganisms as Markers of On-Farm Hygiene Practices Related to Milking

“…A more appropriate comparison may be to compare the GRSD r we measured with those reported previously (11). For TVCs using Petrifilm plates, our values are lower than those reported previously with log-transformed bacterial measurements (11,33). Previous studies have, however, used larger numbers of up to 14 different laboratories (33).…”

“…For TVCs using Petrifilm plates, our values are lower than those reported previously with log-transformed bacterial measurements (11,33). Previous studies have, however, used larger numbers of up to 14 different laboratories (33). As might be expected, the BactoScans, which have automated liquid handling, had one of the lowest GRSD r values measured.…”

“…Adaptations to the method to ensure its suitability for bacterial counts were undertaken. In keeping with the practices established by a number of previous studies (33), uncertainty was calculated from log-transformed bacterial counts (11,36). Samples of raw milk (n ϭ 20) from bulk milk tanks on 20 different farms were analyzed in duplicate by two different technicians at the same time but in physically different laboratories using media and diluents from different manufacturers, made as different batches in different autoclaves wherever possible.…”

Avaliação de métodos para enumeração de microrganismos aeróbios mesófilos e coliformes em leite cru

“…Desde que foram desenvolvidas, as placas Petrifilm Ainda com respeito ao leite cru, (Piton & Grappin, 1991) considerando que os estudos realizados até então haviam sido com amostras artificialmente contaminadas, realizaram um estudo de comparação entre o Petrifilm ® AC e o método preconizado pela IDF, com a participação de 14 laboratórios. Os resultados revelaram que a enumeração de aeróbios mesófilos em Petrifilm ® AC foi 10% inferior à convencional.…”

“…Bishop & Juan (1988), analisando coliformes em leite bovino cru, também encontraram bons parâmentros estatísticos na comparação entre Petrifilm ® e convencional: r 0,963; inclinação da reta 1,017 e intercepto com eixo y = -0,024. Piton & Grappin (1991) apresentaram um modelo estatístico para avaliação dos métodos Petrifilm ® para contagem de aeróbios mesófilos e coliformes em leite cru em comparação com os métodos convencionais. No referido estudo houve participação de 14 laboratórios e foi concluído que o Petrifilm ® apresentou performance satisfatória mostrando forte correlação com as técnicas de referência.…”