The fact that blackberry extract at concentrations ≥ 56 μg/ml) greatly reduced virus yield (>
99%) when present throughout the inoculation and replication period but had no effect on
yield at concentrations ≤ 280 ng/ml when added following virus entry (Fig. 2) indicated that
blackberry extract may either inactivate the virus directly or block virus entry. To determine
in blackberry extract inactivates HSV-1, the extract was added to cell-free virus suspensions,
at the indicated concentrations, and incubated at 37°C for 1 hr to mimic the adsorption and
entry phase. Blackberry extract treated virus suspensions were diluted beyond the antiviral
concentration (1:100) with culture media and then virus infectivity was quantified by direct
plaque assay. In these experiments, even at the highest concentrations used the extract did
not affect the number of viable epithelial cells (data not shown). As indicated in Figure 3,
the amount of infectious HSV-1 virus remaining after 1 hr incubation at 37°C with 280 μg/
ml blackberry extract was significantly reduced compared to control cultures. Taken
together, these findings indicate a dose dependent antiviral effect of blackberry extract and
that this effect was most pronounced at the level of inactivation of cell-free virus.