The application of flow cytometry in determining the bacteriological quality of raw sheep's milk in Slovakia

Abstract

In this study enumeration of bacterial cells in raw sheep's bulk milk by an automated flow cytometry method (Bactoscan FC) was evaluated. The results were discussed with respect to the results obtained by the analysis of cow's milk. Within-laboratory reproducibility, which depended on the counting level, was slightly worse than that of raw cow's milk (valued in the interval > 50 Individual Bacterial Cell (IBC·μL-1). In routine procedure, if preceding samples had counts > 20 000 IBC·μL-1, then this was observed to increase the count of the following sample significantly in many cases ("carry-over"). Bactoscan counts were moderately enhanced with increasing numbers of somatic cells in sheep's milk. However, this trend was indicated only in samples with lower plate counts (< 10 5 colony-forming units (CFU)·mL-1). The regression equation between the results of the anchor plate method and the Bactoscan method was computed from the sample set which represented sheep's milk production in Slovakia during 2004 (data between percentiles 1 and 99, n = 877): log 10 CFU·mL-1 = 1.088 log10 IBC·μL-1 + 2.292; with an accuracy of the estimate, expressed as standard deviation of regression sy,x = 0.305 log 10 CFU·mL-1. The actual regression for raw cow's milk was verified, introducing both raw cow's milk samples and raw sheep's milk samples. The transformation of IBC·μL-1 to CFU·mL-1 realised by various published conversions was evaluated by testing of the parallelism of the regression lines and also by comparison of obtained differences on a model data set. The conclusions support the requirement to set up a special conversion for the intended field of application at any time. © INRA, EDP Sciences, 2006.