Antimycobacterial potential of an extract of the red alga Polysiphonia virgata C. Agardh (Rhodomelaceae)
The red marine alga (Polysiphonia virgata C. Agardh, Rhodomelaceae), was investigated for antimycobacterial activity in the hope of finding novel compounds for the treatment of tuberculosis. Two purified fractions were tested against Mycobacterium smegmatis using direct bioautography. The minimum inhibitory quantity (MIQ) determined to be 15.62µg (zone size=4mm) and 7.4µg (zone size=4mm) for sample C1730 and 1730/11, respectively. This activity was comparable with those of Ciprofloxacin, which showed a 12mm diameter zone at an amount of 12.5 micrograms on the thin-layer chromatography plate. Further fractionation resulted in the isolation of a mixture of well known long chain fatty acids, namely oleic, linoleic, lauric, and myristic acids as the major antimycobacterial compounds. Oleic acid showed the greatest inhibition of the growth of M. smegmatis with a MIQ-value of 0.8µg; linoleic acid and lauric acid had MIQ-values of 1.56 and 3.125µg, respectively. Stearic acid, palmitic acid, and myristic acid did not inhibit the growth of M. smegmatis. Using the Bactec-460 radiometric method, oleic acid showed 100% inhibition of the growth of M. tuberculosis at a minimum inhibitory concentration (MIC) of 25µg/mL; lauric acid, myristic acid and linoleic acid showed 100% inhibition at 50µg/mL. Myristic acid and lauric acid showed 90% and 76% inhibition at 50µg/mL. Linoleic acid showed moderate inhibition of the growth of a clinical strain of multi-drug resistant M. tuberculosis at a concentration of 50µg/mL.
Acknowledgements: National Research Foundation for financial assistance. Dr Néstor M. Carballeira from the Department of Chemistry at the University of Puerto Rico for his invaluable guidance. Ms. Jean Mckenzie for the NMR spectra and Dr. Stephan Louw for the LC-MS and GC-MS analysis from the Central Analytical Facility, University of Stellenbosch.