Which bacterium is primarily responsible for initiating caries?

Streptococcus mutans is recognized as the primary bacterium responsible for initiating dental caries due to its strong association with the formation of dental plaque and its ability to metabolize sugars, producing acid as a byproduct. This acid contributes to the demineralization of enamel, which is a critical process in the development of caries.

S. mutans has several key characteristics that make it particularly effective in establishing caries. It can adhere strongly to tooth surfaces, forming biofilms that provide a protective environment for the bacteria. In the presence of fermentable carbohydrates, S. mutans produces lactic acid, which lowers the pH in the oral cavity, further enhancing enamel demineralization.

The other options do not primarily initiate dental caries. Lactobacillus, while associated with the progression of caries, is not the initial pathogen; it tends to flourish in more advanced stages of dental decay. Streptococcus aureus and Bacillus fusiformis are not primarily involved in the caries process; S. aureus is more commonly associated with systemic infections and is not a significant contributor to dental caries, while Bacillus fusiformis is not specifically implicated in caries formation. This highlights the unique role S.