Gram-negative bacteria are generally more resistant to
hydrophobic antimicrobial compounds than gram-positive
bacteria because of the presence of an outer membrane that
acts as a permeability barrier (Nikaido 1976). Cho et al.
(2005) reported that gram-positive bacteria are more susceptible
to ethanol-extracted propolis than gram-negativebacteria, including Escherichia coli O157:H7, but several
other studies have reported that it exerts antimicrobial
activities against both gram-positive and gram-negative
bacterial species.