Adaptation of an [3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide] assay to evaluate the cytotoxicity of the extracellular products of micro-organisms pathogenic to fish

Cholesterol Interferes with the MTT Assay in Human Epithelial-Like (A549) and Endothelial (HLMVE and HCAE) Cells

Metabolically active cells are able to convert the MTT [3-(4,5-dimethythiazol-2-yl)-2,5-diphenyltetrazolium bromide] dye to blue formazan. This is the basis of the MTT assay, which is among the most widely used screening methods to evaluate cell viability and proliferation. When testing the effects of cholesterol products on the viability of human pulmonary epithelial-like A549 cells using trypan blue staining (cell numbers) and the MTT assay, results were inconsistent. The MTT assay indicated greater than 50% loss of viability with exposure of cells to cholesterol, whereas there was no decrease in viability indicated by trypan blue exclusion and propidium iodide uptake. A similar decrease in MTT reduction was obtained upon cholesterol treatment in human lung microvascular endothelial cells (HLMVECs) and human coronary artery endothelial cells (HCAECs) without loss of viability. This suggested a direct interference of cholesterol with the assay. However, using a cell-free system, there was no decrease in the reduction of MTT by ascorbic acid during incubation with a similar concentration of cholesterol. Light microscopy revealed enhanced exocytosis of formazan granules in presence of cholesterol. Incubation with apolipoprotein A-1 decreased cholesterol-mediated inhibition of MTT assay. These studies indicate decreased MTT reduction as a result of enhanced exocytosis of formazan due to cholesterol. A careful validation of viability assay procedures is therefore suggested in experiments where cholesterol is a constituent, to avoid a potential bias in concluding results of cytotoxicity studies.

Azoreductase and dye detoxification activities of Bacillus velezensis strain AB

Azo dyes are known to be a very important and widely used class of toxic and carcinogenic compounds. Although lot of research has been carried out for their removal from industrial effluents, very little attention is given to changes in their toxicity and mutagenicity during the treatment processes. Present investigation describes isolation of a Bacillus velezensis culture capable of degrading azo dye Direct Red 28 (DR28). Azoreductase enzyme was isolated from it, and its molecular weight was found to be 60 kDa. The enzyme required NADH as cofactor and was oxygen-insensitive. Toxicity and mutagenicity of the dye during biodegradation was monitored by using a battery of carefully selected in vitro tests. The culture was found to degrade DR28 to benzidine and 4-aminobiphenyl, both of which are potent mutagens. However, on longer incubation, both the compounds were degraded further, resulting in reduction in toxicity and mutagenicity of the dye. Thus, the culture seems to be a suitable candidate for further study for both decolourization and detoxification of azo dyes, resulting in their safe disposal.