Microbial melanin physiology under stress conditions and gamma radiation protection studies

Abstract

Any life-threatening alteration in environmental circumstances from the optimal imposes stress on an organism. Microorganisms possess various strategies, including pigment production, to resist stress conditions in their surrounding environments. In the present study, yeast form of fungus Aureobasidium pullulans either melanized or albino was exposed to different stresses, such as salinity and antifungals (fluconazole, itraconazole, and terbinafine) as well as gamma irradiation. Results revealed that at 800 μg/ml or less of antifungal terbinafine, a positive impact was achieved on the viability of A. pullulans yeast cells, in the cultivation medium wasn't supplemented with melanin precursor (L-3,4 dihydroxyphenylalanine; L-DOPA). This enhancement in the growth yield was accompanied by increases in both internal and external melanin yields by eight and six folds, respectively. In addition, melanin is synthesized during terbinafine stress in a pathway independent from L-DOPA, but based on the excess of acetyl-CoA to tolerate squalene accumulation by feedback inhibition. Moreover, the melanized yeast cells show greater viable stability against gamma irradiation as well as pigment productivity than albino cells. The main objective of this study is to create new composites to use them as shielding materials. Theses composites based on fungal melanin's chelating activity with nanoparticles of bismuth, lead, and silver. The mass attenuation coefficient of Melanin-Bismuth composite is nearly double the mass attenuation of lead at energy = 0.662 MeV; a higher increase in mass attenuation is observed with higher gamma intensities; 1.17 and 1.33 MeV. Coupling synthesis of composites containing natural microbial products with nanotechnology is a promising field to manufacture new radiation shielding materials.