Final answer:
The bacterium Actinomyces viscosus is responsible for root surface caries, whereas Streptococcus mutans is notably involved in tooth decay due to its role in the formation of acid-producing biofilms on teeth.
Step-by-step explanation:
The microbe that causes root surface caries is Actinomyces viscosus. This bacterium, along with Streptococcus mutans, is known to be a major contributor to dental issues such as cavities and gum diseases. Streptococcus mutans particularly plays a significant role in tooth decay; it is a cariogenic species that thrives in the biofilms of plaque and tartar on teeth.
Streptococcus mutans is considered a key pathogen because it ferments sugars producing lactic acid, which in turn dissolves minerals in the teeth leading to decay. While gingivitis and periodontitis may involve a variety of microbes, including Streptococcus and Actinomyces species, it is the Actinomyces viscosus that is notably associated with root surface caries.