Because only a small number of bacteria were invasive within the intestinal mucosa, the researchers quantified bacteria in the remaining three mucosal compartments. Which compartments were used?

• A. Free mucus, adherent mucus, attaching to surface epithelia
• B. Lamina propria, submucosa, muscularis
• C. Luminal contents, crypt mucus, goblet cells
• D. Epithelial surface, crypt lumen, muscularis

Answer: (A)

The key idea is measuring bacteria that reside in non-invasive, mucosal niches rather than penetrating tissue. When bacteria are not invading the mucosa, they can still associate with the mucous environment in three distinct places: the free mucus that floats over the epithelium, the adherent mucus that remains attached to the mucosal surface, and bacteria that are actually attaching to the surface epithelial cells. Quantifying bacteria in these three compartments lets researchers map how bacteria are distributed right at the mucosal front without entering the tissue proper.

This is why those three compartments are the best choice. The other options describe tissue layers (like lamina propria, submucosa, muscularis) or mix luminal contents with cells or spaces that aren’t the defined mucosal niches used for this non-invasive assessment, so they don’t fit the goal of capturing bacteria associated with the mucosal surface but not invasive.