Detection of cysteine-induced salivary H₂S to evaluate the H₂S-producing capability of oral bacteria, using a simple and sensitive colorimetric assay: A preliminary study

The cysteine challenge test can be used to evaluate the ability of oral bacteria to produce malodor. But it involves administration of high concentrations of cysteine directly into the mouth, together with requiring special and expensive instruments. In this study, we detected cysteine-induced hydrogen sulfide (H₂S) in saliva using two colorimetric H₂S assays: a microplate cover-based assay and a silver nanoprism (Ag NPR)-coated paper-based assay. Based on the results of the microplate cover-based H₂S sensing assay, salivary H₂S varied with cysteine concentration, treatment time, and bacterial composition. The Ag NPR-coated H₂S sensing paper assay was used to quantitatively analyze cysteine-induced H₂S concentrations in 21 saliva samples treated with 20 mM cysteine for 3 min. As a result, the cysteine-induced salivary H₂S level was within a range from 1.43 to 3.60 μM. And it was positively correlated with the DNA copy number of Porphyromonas gingivalis in the saliva. Therefore, we suggest that cysteine-induced salivary H₂S be used as an alternative to the oral cysteine challenge test for evaluating the H₂S-producing capability of oral bacteria.