**Core Concept**
Periodontosis refers to a chronic inflammatory disease affecting the periodontal tissues surrounding the teeth, often characterized by the destruction of the periodontal ligament and alveolar bone. The etiology of periodontosis is multifactorial, with both bacterial and host factors playing significant roles.

**Why the Correct Answer is Right**
In periodontosis, the key pathogens involved include *Porphyromonas gingivalis*, *Aggregatibacter actinomycetemcomitans*, and *Fusobacterium nucleatum*. These bacteria are often found in the subgingival plaque of patients with periodontosis and contribute to the disease's progression through the production of virulence factors such as lipopolysaccharides, hemolysins, and proteases. The presence of these bacteria can lead to inflammation, tissue destruction, and ultimately, the loss of teeth.

**Why Each Wrong Option is Incorrect**
**Option A:** *Streptococcus mutans* is a key organism in the development of dental caries, but it is not typically associated with periodontosis.
**Option B:** *Escherichia coli* is a common gut commensal and opportunistic pathogen, but it is not frequently found in periodontosis.
**Option C:** *Fusobacterium nucleatum* is indeed a pathogen commonly found in periodontosis, making this option incorrect.

**Clinical Pearl / High-Yield Fact**
The presence of *Aggregatibacter actinomycetemcomitans* in periodontal pockets is often associated with a more aggressive form of periodontitis, characterized by rapid attachment loss and bone destruction.

**Correct Answer: C. Fusobacterium nucleatum**