Resazurin

When a solution containing resorufin is submitted to reducing conditions (E_h < -110 mV), almost all resorufin is reversibly reduced to the translucid non-fluorescent dihydroresorufin (also known as hydroresorufin) and the solution becomes translucid (the redox potential of the resorufin/dihydroresorufin pair is -51 mV vs. standard hydrogen electrode at pH 7.0). When the E_h of this same solution is increased, dihydroresorufin is oxidized back to resorufin, and this reversible reaction can be used to monitor if the redox potential of a culture medium remains at a sufficiently low level for anaerobic organisms.

Resazurin solution has one of the highest values known of Kreft's dichromaticity index. This means that it has a large change in perceived color hue when the thickness or concentration of observed sample increases or decreases.

Usually, resazurin is available commercially as the sodium salt.

Cell viability applications

Resazurin is reduced to resorufin by aerobic respiration of metabolically active cells, and it can be used as an indicator of cell viability. It was first used to quantify bacterial content in milk by Pesch and Simmert in 1929. It can be used to detect the presence of viable cells in mammalian cell cultures. It was introduced commercially initially under Alamar Blue trademark (Trek Diagnostic Systems, Inc), and now also available under other names such as AB assay, Vybrant (Molecular Probes) and UptiBlue (Interchim).

Resazurin based assays show excellent correlation to reference viability assays such as formazan-based assays (MTT/XTT) and tritiated thymidine based techniques. The low toxicity makes it suitable for longer studies, and it has been applied for animal cells, bacteria, and fungi for cell culture assays such as cell counting, cell survival, and cell proliferation. In antimicrobial assays, resazurin is commonly utilized to assess the minimum inhibitory concentration (MIC) or minimum bactericidal concentration (MBC) of antimicrobial agents.

To take the place of a standard live/dead assay, resazurin also be multiplexed with chemiluminescent assays, such as cytokine assays, caspase assays to measure apoptosis, or reporter assays to measure a gene or a protein expression.

The irreversible reaction of resazurin to resorufin is proportional to aerobic respiration.

Resazurin reduction test

Resazurin can be used as one of a series of rapid tests to determine the quality of a milk sample. In this test, resazurin is added as a violet redox dye which turns mauvish-pink due to conversion to resorufin and then to colourless dihydroresorufin. This happens due to lowering of the oxidation-reduction potential in the milk sample caused by presence of bacteria which utilize available oxygen present in the milk for aerobic respiration. The rate of the colour change is used as an index for the number of bacteria present in the milk sample.