Towards an understanding of bacterial metabolites prodigiosin and violacein and their potential for use in commercial sunscreens

Synopsis
Objectives

To exploit the microbial ecology of bacterial metabolite production and, specifically, to: (i) evaluate the potential use of the pigments prodigiosin and violacein as additives to commercial sunscreens for protection of human skin, and (ii) determine antioxidant and antimicrobial activities (against pathogenic bacteria) for these two pigments.

Methods
Prodigiosin and violacein were used to supplement extracts of Aloe vera leaf and Cucumis sativus (cucumber) fruit which are known to have photoprotective activity, as well as some commercial sunscreen preparations. For each, sunscreen protection factors (SPFs) were determined spectrophotometrically. Assays for antimicrobial activity were carried out using 96-well plates to quantify growth inhibition of Staphylococcus aureus and Escherichia coli.
Results
For the plant extracts, SPFs were increased by an order of magnitude (i.e. up to ~3.5) and those for the commercial sunscreens increased by 10–22% (for 4% w/w violacein) and 20–65% (for 4% w/w prodigiosin). The antioxidant activities of prodigiosin and violacein were approximately 30% and 20% those of ascorbic acid (a well-characterized, potent antioxidant). Violacein inhibited S. aureus (IC_{506.99 ± 0.146 μM) but not E. coli, whereas prodigiosin was effective against both of these bacteria (IC50 values were 0.68 ± 0.06 μM and 0.53 ± 0.03 μM, respectively).}

Conclusion
The bacterial pigments prodigiosin and violacein exhibited antioxidant and antimicrobial activities and were able to increase the SPF of commercial sunscreens as well as the extracts of the two plant species tested. These pigments have potential as ingredients for a new product range of and, indeed, represent a new paradigm for sunscreens that utilize substances of biological origin. We discussed the biotechnological potential of these bacterial metabolites for use in commercial sunscreens, and the need for studies of mammalian cells to determine safety.