Rapid detection of respiring Escherichia coli O157:H7 in apple juice, milk, and ground beef by flow cytometry.

BACKGROUND Rapid and simple methods to detect viable pathogenic microbes in foods and drinks are required. Flow cytometry was used for the rapid detection of respiring Escherichia coli O157:H7 cells in apple juice, milk, and ground beef. METHODS CTC (5-cyano-2,3-ditolyl tetrazolium chloride) was used to estimate the respiratory activity of bacteria. Fluorescein isothiocyanate (FITC)-labeled anti-E. coli O157:H7 direct antibody (FA) was used for the specific detection of target cells. Food samples were inoculated with starved E. coli O157:H7 and E. coli K-12 cells, and analyzed by both fluorescent microscopy and flow cytometry after double staining with FA and CTC. RESULTS Respiring E. coli O157:H7 cells in food samples showed strong fluorescence of both FA (green) and CTC (red); thus, they could be clearly and specifically distinguished from respiring E. coli K-12 or inactive cells. A good correlation was achieved in flow cytometric analysis between the numbers of inoculated viable E. coli O157:H7 and those detected in milk and apple juice. The detection threshold for this flow cytometry for E. coli O157:H7 in milk, apple juice, and ground beef was 10(3) cells/ml (milk and apple juice) or 10(3) cells/g (ground beef) of sample when the total bacterial number in the sample was 10(6) cells/ml. CONCLUSIONS Respiring E. coli O157:H7 in food samples can be detected specifically within a few hours. Flow cytometry with FA-CTC double staining can be used to examine food contamination with various pathogenic microbes demonstrating physiologic activity through the use of a suitable fluorescent antibody.