Antimicrobial compounds from Alpinia conchigera

Abstract

Ethnopharmacological relevance: The rhizome of Alpinia conchigerahas been used as a condiment in the northern states of Peninsular Malaysia and occasionally in folk medicine in the east coast to treat fungal infections. In some states of Peninsular Malaysia, the rhizomes are consumed as a post-partum medicine and the young shoots are prepared into a vegetable dish. This study aimed to investigate the chemical constituents of the pseudostems and rhizomes of Malaysian Alpinia conchigera and to evaluate the antimicrobial activity of the dichloromethane (DCM) extracts of the pseudostems, rhizomes and the isolated compounds against three selected fungi and five strains of Staphylococcus aureus.

Materials and methods: The dried and ground pseudostems (0.8kg) and rhizomes (1.0kg) were successively extracted in Soxhlet extractor using n-hexane, dichloromethane (DCM) and methanol. The n-hexane and DCM extracts of the pseudostem and rhizome were subjected to isolation and purification using column chromatography on silica gel using a stepwise gradient system (n-hexane to methanol). Briefly, a serial two fold dilutions of the test materials dissolved in DMSO were prepared prior to addition of 100μl overnight microbial suspension (108 cfu/ml) followed by incubation at 37°C (bacteria) or 26°C (dermatophytes and candida) for 24h. The highest concentration of DMSO remaining after dilution (5%, v/v) caused no inhibition to bacterial/candida/dermatophytes' growth. Antibiotic cycloheximide was used as reference for anticandidal and antidermatophyte comparison while oxacilin was used as reference for antibacterial testing. DMSO served as negative control. Turbidity was taken as indication of growth, thus the lowest concentration which remains clear after macroscopic evaluation was taken as the minimum inhibitory concentration (MIC).

Results: The isolation of n-hexane and DCM extracts of the rhizomes and pseudostems of Alpinia conchigera via column chromatography yielded two triterpenes isolated as a mixture of stigmasterol and β-sitosterol: caryophyllene oxide, chavicol acetate 1, p-hydroxy cinnamaldehyde 2, 1'S-1'-acetoxychavicol acetate 3, trans-p-coumaryl diacetate 4, 1'S-1'-acetoxyeugenol acetate 5, 1'-hydroxychavicol acetate 6, p-hydroxycinnamyl acetate 7 and 4-hydroxybenzaldehyde. The DCM extract of the rhizome of Alpinia conchigera indicated potent antifungal activity against Candida albicans, Microsporum canis and Trycophyton rubrum with MIC values of 625μg/ml, 156μg/ml and 156μg/ml, respectively. It also showed significant inhibitory activity with MIC values between 17.88 and 35.75μg/ml against the mutant Staphylococci isolates MSSA, MRSA and Sa7. Amongst the isolated compounds, the lowest inhibition observed were of 1'S-1'-acetoxyeugenol against the dermatophytes (MIC 313μg/ml) followed by trans-p-coumaryl diacetate against both dermatophytes and candida (MIC 625μg/ml). The compound p-hydroxycinnamyl acetate strongly inhibited Staphylococcusaureus strain VISA (MIC 39μg/ml) followed by trans-p-coumaryl diacetate and 1'-hydroxychavicol acetate with MIC value of 156μg/ml.

Conclusion: In conclusion, the observed antibacterial, anticandidal and antidermatophyte activity of the extracts and compounds obtained from the rhizome confirm the traditional use of Alpinia cochigera rhizome in the treatment of skin infection.