Prediction of Aerobic Plate Count of Escherichia coli in Green Tea Beverage by Flow Cytometry and Multivariate Analysis

Abstract

Flow cytometry (FCM) and aerobic plate count (APC) by the culture method were performed on green tea samples spiked with Escherichia coli type strain NCTC 9001 solutions of different concentrations. In FCM, fluorescence signals from multiple stained bacteria and other fluorophores were detected using detector channels A, B and C, and recorded as events with a voltage at each channel. As a conventional FCM, the number of events with voltage larger than the threshold values at channels A and C with zero voltage at the channel B, which corresponded only to dead cells, was regarded as the predicted APC. In addition, voltage histograms of channels A and C were developed with the optimized bin width and merged horizontally. Then partial least squares regression (PLSR) was applied to the histogram data to develop a calibration curve by which APC could be predicted. The root mean square error (RMSE) between measured APC and that predicted by conventional FCM was 1.09-1.15 [log10 cfu/ml]. The APC predicted by the PLSR and that measured were in good agreement with RMSE of 0.51 [log10 cfu/ml], which verified the potential of the proposed method for the accurate APC prediction by FCM.