Interlaboratory collaborative study on a flow cytometry method for lactic acid bacteria quantification in starter cultures, probiotics and fermented milk products according to ISO 19344/IDF232 / Sandra Casani, Ken Flemming Hansen and Stéphane Chartier.

An international collaborative study of ISO 19344│IDF 232, Milk and milk products - Starter cultures, probiotics and fermented milk products - Quantification of lactic acid bacteria by flow cytometry was conducted to determine precision figures for the quantification of lactic acid bacteria and probiotic strains by flow cytometry in starter cultures, probiotics and fermented milk products. Except for yoghurts, the repeatability limit (r) and reproducibility limit (R) values were calculated to be 0.06 and 0.45 for log10 active fluorescent units (AFU) per gram and 0.07 and 0.38 for log10 total fluorescent units (TFU) per gram. The corresponding values for yoghurts were 0.08 and 0.76 for log10 AFU per gram and 0.10 and 1.17 for log10 TFU per gram. The latter reproducibility value illustrates the challenges encountered in analysis of TFU in yoghurts. Furthermore, equivalence of AFU results between the three different staining protocols was evaluated and demonstrated for most of the strains included in this study.