How does your body know who the good bacteria are ?

For every 1 “you” cell, there are 10 bacteria cells.

They’re mostly found hanging out in your digestive tract – around 100 trillion of them. Fortunately, the majority of them are “good” guys.

Unfortunately, every now and again – you come face to face with a real nasty character like Salmonella. It is often a “moving” experience, but after a short period of time, your body takes back control and eliminates the bad “guy”.

Which leads to the trillion cell question – how does your body know who the good bacteria are ?

Good guys and bad guys look the same

To be perfectly honest, they don’t really look very different.

Just like human good guys and bad guys, what makes a good guy “good” and a bad guy “bad”, is what they do.

Good bacteria provide us with nutrients as they help to digest our food. Bad bacteria on the other hand, digest us in one way or another.

Scanning for bad bacteria

Initially, scientists thought that the body’s surveillance system just simply ignored the bacteria in our gut. They believed that the security camera’s weren’t looking at the huge crowd of bacteria hanging out in the gut, but only worried about the organisms which made an attempt to enter the cells.

But using sophisticated microscopes, scientists have found that many of the good “guys” actually knock on the doors – so it is highly unlikely the scanning equipment is unaware of their presence.

Bacteroides fragilis and the colon crypts

B .fagilis, is a good bacteria, that chooses to live deep inside the crypts of the colon. Despite its very close contact with the mucosal cells – the body’s security guards ignore it.

Scientists have discovered the reason that the security guards ignore it, is because they “think” it is actually self i.e. a YOU cell. The bacteria are able to create the deception with the help of special molecule.

The regulatory T cells deception

The security guards that are being fooled, belong to a group called the regulatory T cells (T_reg cells).

T_reg cells major function in the security system is to avoid any “friendly fire” i.e. it ensures the immune system doesn’t make a mistake and attack “you” cells. It does this by turning down certain immune responses.

Bacteroides fragilis produces a molecule that “tricks” the Treg cells in the gut, into thinking that it is really one of your normal cells.

The molecule the bacteria produces that does the trickery, is called polysaccharide A (PSA). The T cells pick up the PSA molecule, because they believe that the cell is not foreign, they stop other T cells (Th₁₇) from launching an attack.

Since the Th₁₇ cells are instructed to “stand down’, nothing happens to the potential invader. And the bacteria gets to live happily ever after in your gut.

Security clearance depends on correct codes

Removal of the PSA molecule causes the Th₁₇ cells to go into full attack mode and destroy the “friendly” bacteria as if it was a “bad” guy.

So to be a good guy, a bacteria must have the necessary security clearance.

Good health depends on packing your gut (and other bits as well) with the right bacteria. The “right” bacteria are clearly very special characters, who are actually a little more like “you” and a little less like regular bacteria.

The Toll-Like Receptor 2 Pathway Establishes Colonization by a Commensal of the Human Microbiota. Science, 21 April 2011. June L. Round, S. Melanie Lee, Jennifer Li, Gloria Tran, Bana Jabri, Talal A. Chatila, and Sarkis K. Mazmanian.

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