Abstract:
Background: The combination of plant extracts has shown significant potential in enhancing
antimicrobial efficacy through synergistic interactions. Such combinations may offer improved
therapeutic outcomes, multitarget actions, reduced toxicity, lower dosage requirements, decreased
resistance development, and wider activity against many pathogens. Cinnamomum zeylanicum (Sri
Lankan cinnamon) and Curcuma longa (turmeric) are rich in phytochemicals and are renowned
for their antibacterial properties. However, the type of pharmacodynamic interaction between the
methanolic extracts from the bark of C. zeylanicum and the rhizome of C. longa grown in Sri
Lanka, has not yet been investigated.
Objective: To evaluate the synergistic antibacterial activity of methanolic extracts of Sri Lankan
C. zeylanicum bark and C. longa rhizome
Methods: Minimum inhibitory concentrations (MICs) of methanolic extracts were evaluated
using the broth microdilution method, using Muller Hinton broth in a 96 well plate across the
concentrations ranging from 100 mg/mL to 0.195 mg/mL with each concentration in triplicates.
The synergistic antibacterial activity was determined using the checkerboard microdilution
method and quantified by calculating the fractional inhibitory concentration index (FICi) against
clinical isolates of Staphylococcus aureus and Escherichia coli.
Results: The MIC of C. zeylanicum extract for both S. aureus and E. coli was 3.125 mg/mL. For
C. longa, the MIC was 25 mg/mL against S. aureus and 50 mg/mL against E. coli. Synergistic and
additive effects were observed with FICi ranging from 0.25 to 0.625. The MICs at which
synergistic effects observed were at 0.781 and 0.097 mg/mL of C. zeylanicum for both bacteria;
for C. longa, the MICs were 25 and 12.5 mg/mL for E. coli and 12.5 and 6.25 mg/mL for S.
aureus. Further, additive effects were observed at 0.195, 0.390 and 1.562 mg/mL of C. zeylanicum
for both bacteria; for C. longa, the MICs were 25 and 6.25 mg/mL for E. coli and 12.5 and 3.125
mg/mL for S. aureus.
Conclusions: The combination of methanolic extracts of C. zeylanicum bark and C. longa
rhizome demonstrated both synergistic and additive antibacterial effects against both S. aureus
and E. coli.
