Mucus found to harbor a previously unknown human immune system


May 26, 2013

Bacteriophage attacking an infectious bacterium

Bacteriophage attacking an infectious bacterium

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Though not something people like to ponder, the purpose of mucus as a protective barrier that keeps underlying tissues moist and traps bacteria and other foreign organisms is well known. However, researchers at San Diego State University (SDSU) have now discovered that the surface of mucus is also the site of an independent human immune system that actively protects us from infectious agents in the environment.

Mucus is produced by tissue that lines the inner surfaces of the body which come into contact with the outer environment. The mouth, nose, sinuses, throat, lungs, and gastrointestinal tract all possess mucus-producing tissue. The mucus layer has long been identified as providing protection against the outside world, but in a rather passive manner.

No longer. An SDSU research team led by biology post-doc Jeremy Barr has discovered that a previously unsuspected immune system is active on the surface of the mucus layer. This immune system consists of a layer of bacteria-infecting viruses (bacteriophage), that actively attack and kill infectious (and harmless) bacteria as the viruses multiply.

Bacteriophage adheres to mucus layers and provides immunity against invading bacteria (Image: Jeremy Barr)

When bacteriophage comes in contact with the surface of a layer of mucus, they bond to sugar molecules, which causes the viruses to adhere to the mucus surface. When such a layer was exposed to E. coli bacteria, the SDSU researchers found that the bacteriophage immediately attacked and killed the E. coli. The bacteriophage layer acts as a powerful anti-microbial barrier that protects animals from infection and disease.

To double check this hypothesis, cells not coated by mucus were also exposed to E. coli. The cell death rate was three times greater for the non-mucus coated cells than for those coated with mucus.

Many varieties of bacteriophage are collected on the mucus layer of animals from the environment. These take on a role to defend the animal from infection by killing infectious intruders. The bacteriophage find the mucus layer to be a pleasant environment, where they find comfortable living conditions and plenty of food and hosts for replication of the bacteriophage: a win-win situation for all.

“This discovery not only proposes a new immune system but also demonstrates the first symbiotic relationship between phage and animals,” Barr said. “It will have a significant impact across numerous fields.”

Source: San Diego State University

About the Author
Brian Dodson From an early age Brian wanted to become a scientist. He did, earning a Ph.D. in physics and embarking on an R&D career which has recently broken the 40th anniversary. What he didn't expect was that along the way he would become a patent agent, a rocket scientist, a gourmet cook, a biotech entrepreneur, an opera tenor and a science writer. All articles by Brian Dodson

Interesting, though a bit surprising to me for some reason that at this late date of medical investigation that researchers are only now discovering the active bacteriophage in mucus.


I was told, a couple of years ago, by an octogenarian, that babies pick their nose and eat the mucus because it has anti bacterial properties. Perhaps there could be some research into the longevity of people who have carried on their early predilection?


Wow. This is big in many ways, not the least of which is how science really cannot assume anything that isn't actually backed by methodic investigation. We all assumed we knew what mucus' function was, so we didn't look at it anymore. Oops. This should be headlines in the mainstream news.

TJ Lambert

That makes one wonder about the use of toothpaste or even worse mouthwash, especially when dealing with gum infections. Are they a case of 'bad' mucus? Is there bad and good mucus?


I've always suspected saliva/mucus had a antibacterial effect. If an animal (most mammals at least) gets a cut or receives some other bleeding wound it will tend to lick it liberally. I'm guessing that that behavior serves several functions, cleaning, maybe increasing blood clotting, and as this study shows it also applies a antibacterial coating. So next time you cut yourself, forget the Neosporin, just spit on it ;-)

Siegfried Gust

I always wondered why mating couples kiss.

Douglas Knapp

We will be answering questions about the work at AskScience on Reddit tomorrow if you guys have questions!


It seems blowing your nose into a handkerchief not only clogs your sinuses it robs the body of valuable bacteriophage. After nearly 40yrs free of sinus problems I may also be in line for a very long life. I'll let you know in another 40 yrs.


Older people have numerous problems, one very serious one is to produce enough mucus to protect their lungs from infections. This demonstrates the further importance of mucus.

@TJ Lambert: You are the one that is assuming too much here. There is no one method of investigation that will automatically lead to the 'truth'. Numerous investigations do help towards having an answer, but don't think they have stopped looking at it.

We only know how certain we are of an answer or assumption and we come up with experiments that test that answer. There is a hierarchy of answers (with a certainty that comes with each) that we have made. In this example look at the function of mucus.

We have come up with that mucus traps bacteria and viruses. Then we go down in the hierarchy and say that trapped viruses kill bacteria. If we go further go down, we find out that the killed bacteria releases foul odours and cause bad breathe! (made that up for example)

The hypothesis of the anti-bacterial properties of mucus must have made certain people, though they haven't been able to verify that until now.

Fretting Freddy the Ferret pressing the Fret

If you find you can't see the TV too well - or anything else - try this: Liberally lick your fingers then gently rub your closed eyes for a few seconds. Lick your fingers again and notice the wetness of the returned wet finger now seems thick, with mucus. Repeat the process. Things start to look clearer. (Or is it all in my mind?) Might this be how cataracts form - the mucus over the eye, staying put and slowly drying out?


Someone please investigate thunderbird4's idea

Andrew Zuckerman

Hmm.....what about MRSA which several doctors and nurses tell me lurks in the nose? Time to rethink the booger picking and eating theory of good health, Thunderbird4

Sue Keller
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