All oral bacteria produce acidic byproducts when they metabolize sugar, and this causes a drop in plaque pH. Although there are numerous bacteria present in plaque, there are two specific types of bacteria most associated with caries: Streptococci (most notably Streptococcus mutans) and Lactobacilli.
Streptococci are Gram-positive cocci that form chains, and constitute a relatively large proportion of plaque (~30% – 40%).10-13 They have very efficient sugar transport and storage systems and can produce large amounts of lactic acid when excess sugars are available, or they produce formic and acetic acids when they are utilizing their energy reserves. S. mutans is the strain most strongly implicated in acid production and caries.10-13 This acidogenic bacteria adheres to the biofilm on the tooth by converting sucrose into an extremely adhesive substance called dextran polysaccharide by the enzyme dextransucranase. (Note that S. mutans need not be present for caries to occur: Individuals without this strain can still get caries, since they may have other oral bacteria that create acid-demineralizing conditions).
Lactobacilli are Gram-positive rods that are only present in plaque in small numbers (~1%), but they are extremely aciduric, meaning that they can endure very acidic environments. It is quite possible that they do not significantly contribute to caries, but they are frequently isolated from caries lesions due to their ability to thrive at low pH.