Colorimetric alamarBlue assay as a bacterial concentration and spoilage index of marine foods

The alamarBlue (resazurin) assay incorporates a colorimetric and fluorometric growth indicator based on detection of the metabolic activity of cells. We investigated the relationship between results of the colorimetric alamarBlue assay using 96-well microplates and colony-forming units (CFU) on aerobic agar plates of Escherichia coli, Staphylococcus aureus, Bacillus subtilis strains and unknown bacterial flora in spoiled foods. As bacteria grew in a TYG broth containing alamarBlue (TYG/A), they converted the color from a blue oxidized state to a red reduced state. When 105 CFU of the mesophilic bacteria were placed in a well, the time for converting the color required from 2 to 4 h at 37 °C incubation. In the case of pork and saury fish meats stored at 4 °C, the alamarBlue reduction, value, performed by OD 550–595 nm in the TYG/A incubated for 4 h, increased with the CFU/g meat. These results suggest that the procedure using alamarBlue assay can provide a simple, rapid, low technology, low cost and compact experiment to determine bacterial growth and spoilage of foods including marine foods.