The lowest pH achieved depends greatly on the microbial composition of the dental plaque the nature of the fermentable carbohydrate source the rate of diffusion of substrates and metabolites into and out of the plaque. The presence of significant numbers of aciduric, acidogenic bacteria in plaque developing in a sheltered site with a low diffusion rate coupled with a readily fermentable carbohydrate such a sucrose or glucose would produce the lowest pH. Under these conditions a pH in the region of 4.5, or even lower, might be attained. Conversely, a challenge with a carbohydrate which is metabolised more slowly to a plaque community with fewer aciduric, acidogenic microbes would result in less acid production and a higher terminal pH. The rate of diffusion of material into and out of plaque is governed by the density of the plaque and access by saliva. Thus less dense plaque fully exposed to saliva flow will more rapidly exchange metabolites with the surroundings. This will enable substrates to diffuse into the plaque rapidly and at the same time allow microbial by-products to diffuse out.
Molar fissures are the most caries-prone sites because they are very sheltered from saliva flow.
The deepest parts are often inaccessible to toothbrushes which means that fissures frequently contain impacted food for extended periods of time. This lower resting pH will encourage the growth of aciduric microbes such as S. mutans . In fact, research has shown that fissure plaque has a greater proportion of aciduric microbes including S.mutans and lactobaccilli. The microbial composition of dental plaque affects the lowest pH attained. In addition, the low pH produced will inhibit microbial metabolism and thereby slow the rate of acid production.