Antimicrobial and drug-synergistic potential of Alpinia conchigera Griff.-derived phenylpropanoids against Mycobacterium smegmatis

Aims: This study aimed to evaluate the antimicrobial activity of naturally derived phenylpropanoids from Alpinia conchigera (A. conchigera) Griff. and its synthetic analogues, as well as interactions between selected compounds with first-line tuberculosis (TB) drug, rifampicin, against Mycobacterium smegmatis, a potential opportunistic nontuberculous mycobacterium (NTM) and a surrogate organism for TB. Methodology and results: Twelve phenylpropanoids of A. conchigera were evaluated for antimicrobial activity against M. smegmatis (ATCC 14468). The phenylpropanoid compound from A. conchigera with the lowest minimum inhibitory concentration and bactericidal (MIC, MBC) values were selected for checkerboard tetrazolium microplate assay (TEMA) with rifampicin to determine drug interactions. A majority of the compounds had antimicrobial activity, however, purified natural compound 1′S-1′-acetoxychavicol acetate (ACA) showed the highest antimicrobial activity with an MIC value of 62.5 µg/mL against M. smegmatis. The combination of ACA and rifampicin produced indifferent interaction with fractional inhibition concentration (FIC) index of 1.5, while the combination of rifampicin and ACA synthetic analogue 4-allyl-2,6-methoxyphenyl isobutyrate produced a synergistic interaction effect with FIC index of 0.5. None of the compounds tested were bactericidal but appear to be bacteriostatic. Conclusion, significance and impact of study: This study presents the first report on the antimicrobial potential of natural A. conchigera-derived ACA against M. smegmatis as well as the synergistic interaction of 4-allyl-2,6-methoxyphenyl isobutyrate with rifampicin which warrants further investigation.