What is the mechanism of action of a caries detector dye?

Correct answer: D. It binds to denatured collagen in irreversibly damaged dentine

• A. The dye does not stain bacteria themselves; it stains the degraded matrix they leave behind.
• B. It is not a pH indicator and does not react with bacterial acid.
• C. Fluorescence-based detection is the principle behind DIAGNOdent, a different technology altogether.
• D. CORRECT. Caries detector dyes — typically 1% acid red in propylene glycol — bind to DENATURED COLLAGEN. In infected dentine the collagen cross-links have been irreversibly degraded by bacterial enzymes, exposing binding sites the dye attaches to. In affected dentine the collagen matrix is intact and capable of remineralisation, so it does not stain.

Note: ⚠️ Use with caution. These dyes also stain the naturally less mineralised dentine at the enamel-dentine junction and around the pulp, so following them slavishly leads to OVER-EXCAVATION and unnecessary pulp exposure.