Which substances listed are not metabolized by caries bacteria?

The correct answer identifies sorbitol and xylitol as substances not metabolized by caries bacteria. To understand why this is significant, it's important to recognize the typical behavior of dental caries bacteria, such as Streptococcus mutans, which primarily thrive on fermentable carbohydrates such as sugars.

Sorbitol and xylitol are sugar alcohols that do not contribute to caries in the same way that regular sugars do. In fact, xylitol has been shown to have a protective effect against tooth decay. It cannot be fermented effectively by the bacteria that cause dental caries, leading to a reduced acid production in the oral environment. This lack of fermentation means that these substances do not fuel the bacterial activity that leads to tooth decay.

In contrast, the other options contain carbohydrates that are readily metabolized by dental caries bacteria. Sucrose, glucose, starch, and maltose are typical fermentable substrates that can be broken down effectively by these bacteria, resulting in acid production that demineralizes tooth enamel and leads to caries development. Thus, understanding the distinct metabolic pathways of these carbohydrates highlights why sorbitol and xylitol are unique in their relationship with dental health.