**Question:** Electron microscopic examination of the bacterial flora of necrotising ulcerative gingivitis indicates the presence of microorganisms within non-necrotic tissues in advance of other bacteria. The organisms involved are:

A. Bacteroides species
B. Streptococcus species
C. Prevotella species
D. Actinomyces species

**Correct Answer:**

**Core Concept:** Necrotizing ulcerative gingivitis (NUG) is a severe form of gingivitis characterized by necrotic tissue destruction and widespread inflammation involving the gingiva, periodontal ligament, and alveolar bone. In electron microscopy, this allows for detailed examination of bacterial flora within affected tissues.

**Why the Correct Answer is Right:**

Electron microscopic examination reveals that the microorganisms in necrotizing ulcerative gingivitis are localized within non-necrotic tissues before the onset of necrosis. This indicates a role for these organisms in the pathogenesis of the disease. The correct answer highlights the involvement of Actinomyces species, which are anaerobic gram-positive bacteria. They are known to cause gingival abscesses and are often found in dental plaque and periodontal pockets.

**Why Each Wrong Option is Incorrect:**

A. Bacteroides species: While Bacteroides species are involved in periodontal disease, they are not as strongly associated with NUG as Actinomyces species.
B. Streptococcus species: Streptococci are generally considered commensal organisms and are not primarily involved in this severe gingival inflammatory condition.
C. Prevotella species: Prevotella species are also involved in periodontal diseases, but they are not as strongly associated with NUG as Actinomyces species.

**Clinical Pearl:** In necrotizing ulcerative gingivitis, the primary role is played by Actinomyces species. These organisms are often found in dental plaque and periodontal pockets and can cause gingival abscesses. Electron microscopy helps in identifying these organisms within non-necrotic tissues, highlighting their involvement in the pathogenesis of the disease.