After incubation, 25 mL of
3-{4,5-dimethylthiazol-2-yl}-2,5-diphenyl tetrazolium bromide
(MTT) (Sigma Life Science), dissolved in DMSO (0.8 mg/mL) was
added to each of the wells and incubated for 1 h in order to allow
the viable microorganisms to metabolize the yellow MTT dye into
formazan (purple crystals).