Final answer:
To initiate caries, bacterial plaque pH must reach a level of 5.5 or lower. It is the acidic environment created by the metabolic activity of bacteria like Streptococcus mutans fermenting sugars that leads to the demineralization of tooth enamel.
Step-by-step explanation:
To initiate caries, bacterial plaque pH must reach levels that are acidic enough to dissolve the minerals of the tooth, including enamel. The pH at which tooth decay begins is generally around the acidic value of 5.5 or lower. This is due to the metabolic activity of microbes such as Streptococcus mutans, which forms a biofilm on the teeth and ferments sugars into acids like lactic acid, consequently leading to caries.
The pH scale, ranging from 0 to 14, indicates that any pH level below 7.0 is considered acidic, whereas levels above 7.0 are alkaline. The bacteria within dental plaque thrive in acidic conditions, specifically in a pH window where enamel demineralization can occur, eventually causing caries if proper dental hygiene is not maintained.