Abstract:
Background: Thymol, a natural monoterpene phenol is not only relevant clinically as an anti-microbial, anti-
oxidant and anti-inflammatory agent but also holds the prospect as a natural template for pharmaceutical semi-
synthesis of therapeutic agents. It is a major component of essential oils from many plants. Evidence abound
linking overall bioactivity of thymol to its monoterpene nucleus, specifically, the hydroxyl (-OH) substituent on
carbon number one (C1) on the monoterpene nucleus. Other studies have posited that the overall bioactivity of
thymol is not substantially altered by chemical modification of - OH on the C1 of the monoterpene nucleus. In
view of this, it is still unclear as to whether removal or modification of the –OH on C1 of the monoterpene nucleus
relates generally or context-dependently to bioactivity of thymol.
Objective: The present study investigated anti-bacterial effects of ester-and-ether substituted derivatives of thymol
on S. aureus, P. aeruginosa and E. coli.
Materials and methods: twelve ester-and-ether substituted derivatives of thymol (6TM1s and 6TM2s) were syn-
thesized and characterized by using HPLC, Mass spectrometry, and IR techniques. Anti-bacterial activity of the 12
thymol derivatives was evaluated using broth macrodilution and turbidimetric methods against pure clinical
isolates (S. aureus, P. aeruginosa and E. coli). Standard anti-biotics used were Thymol Streptomycin and fluclox-
acillin, while DMSO was used as vehicle for thymol derivatives. MIC and MBC were determined.
Results: Thymol produced broad-spectrum growth inhibition on all isolates. At equimolar concentrations, thymol
and reference drugs produced concentration-dependent growth inhibition against the isolates (Staphylococcus
aureus, Pseudomonas aeruginosa and Escherichia coli) compared to DMSO. Although the growth inhibitory effects of
the ester-and-ether derivatives of thymol was significant (P 0.05) compared to DMSO, it was however insig-
nificant (P 0.05) compared to thymol and reference antibiotics. Comparatively, at equimolar concentrations,
ester-substituted derivatives of thymol, particularly the branched chain derivative (TM1C) produced more
effective growth inhibition on the isolates than the ether-substituted derivatives of thymol. Thymol was twice as
potent (MIC and MBC, 500 μg/ml) than both ester-and-ether substituted derivatives of thymol (MIC and MBC, >
1000 μg/ml) on all the three clinical isolates. Increase in side chain bulkiness of –OH moiety on the monoterpene
nucleus of thymol decreased growth inhibition on isolates.
Conclusion: Thymol has demonstrated broad-spectrum anti-bacterial effects attributable to the hydroxyl moiety on
C1 of the monoterpene nucleus. Structural modification of the hydroxyl moiety on C1 of the monoterpene nucleus
of thymol with either ether-or-ester substitutions yielded no significant anti-bacterial effects.
