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The Appendix Rescues the Gut-brain Axis in Times Of Crises

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Shin
Shin
 2021-05-08

Evolution keeps the appendix safebox for good reasons.

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Remove the appendix to stay safe? Now we know that‘s not good advice. Appendectomy is a surgical removal required to treat appendicitis that affects 1 in 20 people in the United States.

Despite life-saving, appendectomy has been associated with increased risks of colorectal cancer, recurrent C. diff infections, and mood disturbances.

Appendectomy and Mood

A 1984 study by a trio of Italian psychiatrists found that patients who underwent appendectomy behaved like depressed individuals soon after. “Both [appendectomy and depressed] groups displayed more loss, socially undesirable events, and uncontrollable events than the nonpatients,” the psychiatrists reported.

Put it simply, appendectomy groups developed helplessness — a behavioral tendency that gives up on trying to change a situation — like those in the depressed groups.

A 1992 study from Royal North Shore Hospital, Australia concurred — finding that appendectomy patients were nearly 3x more likely to develop symptoms of clinical depression than controls with an intact appendix.

The appendix has something to do with the brain, they must have thought. But how?

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Appendix Biofilm/Microbiome

Examing the ‘fresh’ colon from a deceased organ donor “revealed that biofilms were most prominent in the appendix, both in terms of bacterial density and biofilm continuity along the epithelial surface,” writes Randal Bollinger, Professor Emeritus of Surgery and colleagues at Duke University Medical Center in 2007.

Biofilm refers to the community of microbes sticking to each other and to a surface. In a sense, biofilms are microbiotas which also mean microbial communities colonizing a given environment.

Indeed, microbial contents of the appendix biofilm are similar to the gut microbiome — comprising phyla such as Firmicutes, Proteobacteria, Bacteroidetes, etc. “This organ harbors a diverse microbiota and that although it shares a substantial amount of microbes with the intestinal tract, it has its own defined microbiome,” the 2007 study authors wrote.

And to the surprise of many, “our studies have indicated that the immune system protects and nourishes the colonies of microbes living in the biofilm,” says William Parker, Assistant Professor and a colleague of Prof. Bollinger.

Why would the immune system safeguard the appendix biofilm from outside pathogens? Because it serves as a “safe house” for symbiotic (or friendly) bacteria to live in the meantime. It serves as a backup that restores microbial populations in the gut when the gut contents are flushed out — perhaps during antibiotics usage or pathogenic infections.

“Once the bowel contents have left the body, the good bacteria hidden away in the appendix can emerge and repopulate the lining of the intestine before more harmful bacteria can take up residence,” Parker told ScienceDaily.

This helps explains why appendectomy has a linkage to diseases of gut dysbiosis such as C. diff infections (commonly caused by over-wiping of gut bacteria by antibiotics), colorectal cancer (a gut inflammatory disorder), and mood (partly attributed to a dysregulated gut-brain axis).

No appendix means no support of the gut microbiome. No appendix means that the gut microbiome is all on its own in situations when its contents are “purged” — such as during antibiotics usage, diarrhoeal infections, laxatives, etc. And disturbed gut microbiome is known to play tricks on the brain owing to a rapid gut-brain neurological pathway called the vagus nerve.

The appendix biofilm/microbiota supports the gut microbiome and gut-brain axis in times of crisis.

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Evolution Selects the Appendix

Anatomist Heather F. Smith and her colleagues wanted to understand the evolutionary basis of the appendix. Is it really a vestigial organ, they wonder. Vestigial organs once served a purpose in ancestral species, but they have lost their function as species evolved. They are thus residual structures — such as wings of flightless birds or pelvic bones of snakes.

In 2017, Smith and her team collected data on the presence of an appendix in 553 mammalian species to construct a putative evolutionary tree called phylogeny. To their astonishment, different lines of ancestry have independently evolved and maintained the appendix. “The finding that there have been three separate evolutionary gains of the appendix within the……suggests that the appendix has a deep evolutionary history in mammals,” Smith et al. wrote.

Their results remain significant after controlling for diet and other environmental factors. This means that the appendix has evolved not from environmental or dietary evolutionary pressures. This means that the appendix evolved for other necessary reasons — for example, to support the gut-brain axis by repopulating the gut microbiome after diarrhoeal infections.

Mammalian evolution maintains the appendix safebox for good purposes— one of them being a wingman for the gut-brain axis in critical situations.

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This article was originally published in Microbial Instincts with minor modifications.