3.4 Flow Cytometry
FCM analysis of bacteria stained with this kit typically allows differentiation of cell sub-populations into a
profile of intact/viable or permeabilized/damaged (Berney et al., 2007). Therefore, changes in bacterial
membrane permeability of food-borne microorganisms treated with herb and spice extracts displaying
antimicrobial properties were evaluated at the range of concentrations (1.25-40 mg ml-1) using PI/Syto 9 staining
coupled to FCM analysis. Sample representative results of FCM analysis are presented with percentage of
sub-populations marked within each region (Figure 2). The fluorescence profiles obtained were based on
reference to two control populations, one of untreated cells indicative of intact/live cells and the other of
heat-treated cells indicative of permeabilized/damaged cells. For each of the strains studied, untreated control
cells were stained as Syto 9 positive and mainly located in lower and upper right quadrants (Figure 2a), and
viability was confirmed by both absorbance measurement at 600 nm and plate count on TSA plates. For control
cells inactivated by heating, populations were stained as PI positive and were located in the upper left quadrant,
these cells were unable to recover and grow on TSA plates (Figure 2b). As an illustration, Figure 2 (c-h) shows
the evolution of the various physiological states of E. coli cells, following treatment with an ethanolic extract of
sage. Cells treated with the lowest extract concentration of 1.25 mg ml-1 did not appear affected, as the levels of
intact/live cells did not differ significantly from that of untreated live controls (Figure 2c). When concentration
of herb extract was increased to 2.5 mg ml-1 an increased percentage of cells was evident in the upper right
quadrant (Figure 2d). Cells in this quadrant stained both Syto 9 and PI positive. The presence of this
double-stained population may indicate cells with damaged membranes allowing penetration of PI into the cell
interior.Following treatment with 5 mg ml-1 a clear shift from green fluorescence towards red fluorescence was
observed with ~28% of cells located in the upper left quadrant indicating damage of cells induced by the extract
(Figure 2e). Treatment with the extract at the MIC level (10 mg ml-1) drastically changed the previous profile
with the appearance of a high level of permeabilized/damaged cells located in the upper left quadrant (~94%)
with only ~3% of viable cells located in upper right quadrant (Figure 2f). Further increasing the extract
concentration to 20 mg ml-1 (MBC) or 40 mg ml-1 led to a more defined population of permeabilized cells with
cytograph dot plot patterns corresponding to that of heat treated control cells (Figure 2g-h). In agreement with
plate count data, at these two levels of added extracts, cells were unable to recover viability on agar media.
Although, MIC and MBC levels of the active extracts were different for various strains of food-borne bacteria
used in this study, FCM results clearly indicated that activity of these extracts was dose dependent, and agreed
with trends noted for microdilution and kill time assays. The cytometric data shown for sage extract is
representative of the anti-microbial effects noted for other extracts using this technique.