More than just disgusting Why mucus is important for our health

Whether it's snot from the nose, sputum from the lungs or secretions from an oozing wound: many people think "Yuck!" when they see mucus. But these bodily fluids play an important role in our health.

It acts as a first line of defense against pathogens, a habitat for our microbiome and a conveyor belt for our digestion, so that everything literally runs like clockwork.

Even in ancient times, mucus was ascribed a central role: as one of the four bodily fluids alongside blood, black bile and yellow bile, which should be in balance for health and well-being. Nowadays, the sticky secretions tend to be regarded as a disgusting carrier of disease.

However, mucus is much more than just an excretory product of the body: as a natural lubricant, it is of great importance for swallowing and digestion, for example, prevents the penetration of pathogens in the form of nasal secretions or cervical mucus, protects the stomach wall from stomach acid and is home to a large part of the human microbiome, for example in the intestinal flora.

More than one liter of mucus a day

On average, the human body produces around one to one and a half liters of mucus per day. The composition of the gel-like substance, which mainly consists of water, proteins, antibodies and enzymes, varies depending on the region of the body.

Certain glycoproteins, known as mucins, are responsible for the composition of the mucus. In interaction with their respective environment, they determine whether the mucus is thin, as in the lungs, or thicker, as in the stomach.

These mucins probably also play a major role in rendering harmless dangerous germs that try to enter the body via the nose. Contrary to what was previously thought, the mucins may not act as a barrier - rather, the sugar molecules in the mucins could interfere with the signal transmission of the germs and thus render them harmless. At least that's what a study published in the journal Nature Microbiology in 2019 by German biophysicist Katharina Ribbeck, who researches mucus at the Massachusetts Institute of Technology in the USA, suggests.

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According to the study, it is not yet clear exactly how mucins tame germs. However, if the corresponding bioactive sugar molecules could be identified, this could probably allow the development of a "class of therapeutics for the treatment of persistent bacterial infections".

Bio-ink from mucus

This is not the only conceivable application: In a study published in the journal "ACS Applied Bio Materials" in June this year, an Indian research group presented a mucin-based bio-ink for the 3D printing of lung tissue. Such an ink could one day be used to produce 3D lung models to study lung diseases and test possible treatments.

Research into medical applications of mucins is also being conducted at the Technical University of Munich under the direction of Oliver Lieleg, including biotinte made from mucins.

Lieleg, who has been working on the topic for 15 years, attributes the great potential of the complex mucus molecules to their diverse properties: for example, they can bind a lot of water, adhere to many surfaces and suppress the adhesion of other objects quite well. "But there is also the opposite case: viruses, for example, are bound very well by mucins and that is part of the job of mucus, so that we are not constantly lying in bed with a cold."

Lubricant for contact lenses

Mucins are also well tolerated by the body as an endogenous material - an important property for potential medical applications, such as special coatings for contact lenses or intubation tubes.

For these, Lieleg's team has taken advantage of the fact that mucins are a good lubricant: "Contact lenses and intubation tubes are hard materials which, in the worst case, can leave damage on soft tissues." Mucin-based coatings could minimize such irritations - and in the case of contact lenses also ensure that lipids from the tear film of the eyes do not deposit on the lenses.

Lieleg's group obtains the mucus molecules from pig stomach mucus, which is diluted and chemically purified until pure mucin with a cotton candy consistency is obtained.

The researchers have also developed a wound healing patch specifically for injuries to soft tissue such as the tongue or intestines. The prototype contains a mucin-based side that has an antibacterial effect and thus prevents the wound from becoming contaminated.

Lieleg and his team are also investigating possible disorders of the mucus system and have been able to show that fine dust particles can influence the barrier effect of mucus. The group is currently conducting similar studies on microplastic particles. "Mucus intercepts a lot of what reaches our mucous membranes from the external environment," summarizes Lieleg. "If contaminants weaken this barrier, things may get through that shouldn't."

Unjustified bad image

The scientist emphasizes: "I think we simply underestimate how important mucus is for us." Without mucins in the tear film, blinking, for example, would become difficult over time. Certain diseases are characterized by reduced mucin production, for example in saliva. "If it doesn't lubricate properly, then speaking and swallowing is painful," says Lieleg.

The fact that we don't get sick all the time is thanks to the mucus molecules, which catch a lot: "What comes out of the nose may not look so nice, but that's because the fine dust particles or most of my office neighbor's flu viruses are luckily stuck in there." Slime has far too bad an image. "I think that many people don't know how important it is for our everyday well-being."

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