The stimulating effect of extracts of South African plants on melanin production and their antibacterial activity against Cutibacterium acnes

Abstract

Progressive macular hypomelanosis (PMH) is a hypopigmented skin disorder, which is identified by the presence of hypopigmented macules on the trunk and upper extremities. It is caused by the bacteria, Cutibacterium acnes which decreases melanogenesis in the affected areas resulting in hypopigmented macules. Current treatments include a combination of ultra-violet radiation and antibiotics, however, these antibiotics cause side effects such as severe rashes, blistering and dryness. Conversely UV radiation leads to increased oxidative stress which can cause premature ageing and susceptibility to skin cancer development. Therefore, there are currently no consistently effective or adequate treatment options for PMH. Furthermore, there are only a few studies which have evaluated the potential of plant extracts or natural products as potential treatments for PMH. In this study, thirty-three extracts, prepared using ethanol, water and dichloromethane from the leaves and twigs of eleven South African medicinal plants, were investigated for their antibacterial activity as well as tyrosinase and melanin stimulatory activity as possible targets for the treatment of PMH.

The Sideroxylon inerme L. ethanolic extract showed a noteworthy minimum inhibitory concentration (MIC) of 125 µg/mL against C. acnes (ATCC 6919) and had an additive effect when combined with the positive control, tetracycline. The Bulbine frutescens (L.) Willd. ethanolic extract (at 200 µg/mL) increased the monophenolase activity of tyrosinase by 31.44 ± 1.41%. It was furthermore, able to increase melanin production by 8.55 ± 1.66% (at 200 µg/mL), which was compared to α-melanocyte stimulating hormone (α-MSH), which showed a 13.39 ± 1.44% increase at 100 µM. Furthermore, B. frutescens and S. inerme ethanolic extracts showed fifty percent inhibitory concentration (IC50) values > 400 µg/mL against human melanoma (UCT-Mel-1) and human keratinocyte (HaCaT) cells, indicating low toxicity against keratinocytes and melanocytes.

Consequently, the ethanolic leaf and stem extract of B. frutescence and S. inerme should be considered for further evaluation as potential treatments of hypopigmentation due to their melanin production and antibacterial activity. Further investigation would include evaluating the irritancy potential of these extracts to determine cosmetic safety as topical treatments and to conduct in vivo hypopigmentation trials to determine their efficacy.