Fleagrass, a herb known for its pleasant aroma, is widely used as a mosquito repellent, antibacterial agent, and for treating colds, reducing swelling, and alleviating pain. The antifungal effects of the essential oils of fleagrass and carvacrol against Candida albicans were investigated by evaluating the growth and the mycelial and biofilm development of C. albicans. Transmission electron microscopy was used to evaluate the integrity of the cell membrane and cell wall of C. albicans. Fleagrass exhibited high fungicidal activity against C. albicans at concentrations of 0.5% v/v (via the Ras1/cAMP/PKA pathway). Furthermore, transmission electron microscopy revealed damage to the cell wall and membrane after treatment with the essential oil, which was further confirmed by the increased levels of β-1,3-glucan and chitin in the cell wall. This study showed that fleagrass exerts good fungicidal and hyphal growth inhibition activity against C. albicans by disrupting its cell wall, and thus, fleagrass may be a potential antifungal drug.
Xanthorrhizol (XTZ), isolated from Curcuma xanthorrhiza,
has potent antifungal and antibacterial activity. It shows
very strong activity against Gram-positive bacteria, such as
Streptococcus mutans and Staphylococcus aureus, but is generally
not active against Gram-negative bacteria. In this study,
we explored the possibility of using a combination strategy
for expanding the antimicrobial spectrum of XTZ against
Gram-negative bacteria. To take advantage of XTZ being a
food-grade material, 10 food-grade or generally recognized
as safe (GRAS) antimicrobial compounds with low toxicities
were selected for combination therapy. In addition, polymyxin
B nonapeptide (PMBN), which is less toxic than polymyxin
B, was also selected as an outer membrane permeabilizer.
The antibacterial activity of various double or triple
combinations with or without XTZ were assayed in vitro
against four Gram-negative bacterial species (Escherichia
coli, Salmonella enterica serovar Typhi, Salmonella enterica
serovar Typhimurium, and Vibrio cholerae), with synergistic
combinations exhibiting clear activity subjected to further
screening. The combinations with the greatest synergism
were XTZ + PMBN + nisin, XTZ + PMBN + carvacrol, and
XTZ + PMBN + thymol. These combinations also showed
potent antimicrobial activity against Shigella spp., Yersinia
enterocolitica, and Acinetobacter baumannii. In time-kill
assays, the three combinations achieved complete killing of
E. coli within 2 h, and S. Typhi and V. cholera within 15 min.
This is the first report on expanding the activity spectrum
of XTZ against Gram-negative bacteria through combination with PMBN and food-grade or GRAS substances, with
the resulting findings being particularly useful for increasing
the industrial and medical applications of XTZ.
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