Why did this study utilize FISH on mucosal biopsies rather than 16S rRNA sequencing of fecal samples to identify these associations?

• A. To reduce cost
• B. To assess the spatial distribution of the microbiota and specifically identify bacteria in the adherent mucus layer
• C. Because 16S cannot detect Fusobacterium
• D. Because FISH is faster

Answer: (B)

The question tests understanding of why spatial context of the microbiota matters and how different methods reveal different information. FISH on mucosal biopsies was used because you can actually see where bacteria are in relation to the gut lining, specifically within the adherent mucus layer and along the epithelium. Fluorescent in situ hybridization uses probes that bind to bacterial rRNA directly in the tissue, so you get a visual map of which bacteria are sitting right at the mucosal surface and how they are distributed in that niche. This mucosa-associated or adherent community can have a different composition and host interactions than bacteria in the lumen, and these relationships are often most relevant to disease associations.

In contrast, sequencing 16S rRNA genes from fecal samples reports who is present and how abundant they are in stool, but it loses information about where those bacteria are located. It cannot distinguish mucosa-adherent communities from luminal ones, and thus may miss associations tied to bacteria occupying the mucus layer or interacting with the epithelium. While 16S can detect taxa like Fusobacterium, the key limitation here is the lack of spatial context, not the ability to detect a given organism.