**Core Concept**
In dental plaque, the bacterial composition evolves over time. In early stages (within 24 hours), the microbiota is dominated by **early colonizers** that adhere rapidly to the tooth surface, with **Streptococci** being the most abundant.

**Why the Correct Answer is Right**
Within the first 24 hours of plaque formation, **Streptococcus mutans** and other **Streptococci** are the predominant bacteria. These organisms colonize the tooth surface quickly due to their ability to adhere to enamel and form biofilms. They are early colonizers and produce lactic acid, initiating demineralization. Bacteroides and Spirochetes appear later in more mature plaque, while Actinomyces are present in older, more complex biofilms.

**Why Each Wrong Option is Incorrect**
Option B: *Bacteroides* – These are **late colonizers** found in mature plaque (>48 hours), not in 2-day-old plaque.
Option C: *Spirochetes* – These appear in **later stages** of plaque and are associated with periodontal disease, not early biofilm.
Option D: *Actinomyces* – These are **not dominant** in early plaque; they become more prevalent in older, deeper biofilms and are linked to infections like actinomycosis.

**Clinical Pearl / High-Yield Fact**
Remember the **"Streptococci first, then Bacteroides"** rule: early plaque (48 hrs) = Bacteroides and anaerobes. This sequence is critical in understanding dental caries progression.

✓ Correct Answer: A. Streptococci