Since human microbiota study’s still in its infancy, strong, conclusive proof of symbiotic associations aren’t yet available. Rather, strong correlations are. Proof of symbiosis requires
- A specific bacterial species consistently associated with several humans across time.
- Abundance in health versus reduction in disease.
- Successful re-planting of such a species into a diseased body should fully or partially restore health.
Further, being genetically diverse, geographically widespread, and having extremely varied diets, human-microbe symbioses are probably redundant between bacterial species and even across genera. This makes the 3rd proposition difficult to prove. Decisive proof becomes even more difficult given bacterial species associated with humans are being identified at exponential pace. For e.g., in the GI tract alone, bacterial species identified have increased from ~300 in 1980 to ~1000 by 2010 due to molecular technologies (see figure below from).
Substantial data supports at least the first two conditions in the example of Faecalibacterium (formerly Fusobacterium) prausnitzii, associated with the colon.
- Presence confirmed by both culture ( ) and 16SrRNA ( , , , ), F. prausnitzii is one of the most abundant anaerobic bacteria in human colon and feces.
- Possible symbiotic function? One of the main sources of colonic butyrate ( , ), generally considered beneficial to intestinal health as well as a preferred energy source for colonic epithelial cells, the colonocytes ( , , 10).
- Reduced presence in a variety of inflammatory bowel diseases ( , 12, , ) including Crohn’s disease ( , , ) and colorectal cancer ( , ).
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