Antimycobacterial activity of the red algae gelidium pristoides, plocamium corallorhiza and polysiphonia virgata

Doctoral Thesis


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University of Cape Town

In 1993, the World Health Organisation declared tuberculosis a global health emergency. Currently, efforts are underway to improve the way the disease is managed and to find more effective treatments that would combat the problem of long treatment periods, toxicity, drug-resistance and HIV-coinfection. In the process, natural product chemistry continues to play an important role in the search for new compounds to treat tuberculosis. Terrestrial plants have been investigated for antimycobacterial activity, while marine plants are yet to receive as much attention. In this project, three South African marine plants were drawn into the search for novel anti-tuberculosis compounds. One of the seaweeds is already part of the local seaweed industry, namely Gelidium pristoides, while Plocamium corallorhiza and Polysiphonia virgata have economic potential. These three red algae were extracted extensively and fractionated using preparative layer chromatography and preparative centrifugally accelerated radial thin-layer chromatography (Chromatotron). The crude extracts of the algae showed no inhibitory activity to growth of the causative agent of human tuberculosis, Mycobacterium tuberculosis. However, when the purified fractions were tested against M. tuberculosis in the BACTEC-460 radiometric method at a concentration of 125 μg/mL, fractions 322, 323 and 333 of P. virgata showed 100% inhibition, while two fractions of G. pristoides showed 91.7% and 79.2% inhibition, respectively. Two fractions of P. corallorhiza demonstrated 41.2% and 73.5% inhibition. The bioactive fractions of P. virgata were further purified and resulted in the isolation of a known compound namely, 2-methoxyethyl-2-methacrylate (MEMA). When MEMA was tested by radiometric assay against M. tuberculosis, it showed anti-tuberculosis activity at a MIC-value of 100 μg/mL and no cytotoxicity against Chinese hamster ovarian cells. However, in a re-investigation into the bioactive compounds of P. virgata it was established that MEMA was not the major bioactive compound. Long chain fatty acids were responsible for the antimycobacterial activity of the algal extract particularly oleic acid, linoleic acid, dodecanoic acid, and myristic acid. Oleic acid inhibited the growth of M. tuberculosis at and MIC-value of 25 11 g/rnL, while dodecanoic acid, myristic acid and linoleic acid all had MIC-values of 50 μg/mL. Stearic acid and palmitic acid was also isolated from the seaweed, but only moderate inhibition of M. tuberculosis was observed for at MIC-values of 50 μg/mL. Oleic acid showed moderate inhibition at 50 μg/mL against the multi-drug resistant isolate of M. tuberculosis, while myristic acid and dodecanoic acid showed significant inhibition against the same at 50 μg/mL and moderate inhibition at 25 μg/mL. Linoleic acid also inhibited the growth of the multi-drug resistant strain at 50 μg/mL. Oleic acid showed the most inhibition of the growth of M. smegmatis in direct bioautography with an MIC-value of 0.8 μg/mL, while linoleic acid and dodecanoic acid had MIC-values of 1.56 μg/mL and 3.125 μg/mL., respectively. Stearic acid, palmitic acid, and myristic acid did not inhibit the growth of M. smegmatis.

Summary in English.