2,6-Dichlorophenolindophenol: A Redox Indicator for Assessing Viability of Oral Bacteria

2,6-Dichlorophenolindophenol (DCIP) is a redox dye and colorimetric reagent previously shown to enable rapid quantification of laboratory cultures of oral microorganisms. DCIP is reduced by viable microbial cells, resulting in loss of its blue color that can be measured spectrophotometrically. Previous studies demonstrated that several strains of oral bacteria and yeasts grown in culture reduce DCIP, with significant correlations observed between increasing viable plate counts and DCIP reduction. The present investigation expanded upon these studies by evaluating DCIP as a method for assessing heterogeneous mixtures of oral microorganisms collected from human subjects. Oral rinse samples were obtained from 184 adults aged 18–70 years and analyzed for DCIP reduction and viable bacterial counts. DCIP reduction was observed in all oral samples, and viable bacterial counts spanning an approximately two-log range (~100-fold difference) demonstrated a statistically significant correlation with DCIP reduction (r = 0.74; p < 0.001). Nonviable organisms did not reduce DCIP, and no DCIP reduction occurred in the absence of bacteria. These results support DCIP reduction as a practical, low-cost platform for estimating viable oral microbial burden, with the additional advantage of a visually interpretable colorimetric readout.