Mouth and gut: how teeth, gums and mouthwash touch the gut flora

Healthy state

After the gut, the mouth is the most densely colonised place in the body, with hundreds of bacterial species on teeth, tongue, gums and in saliva. Fungi, viruses and bacteriophages Bacteriophages Viruses that only infect bacteria. In the gut there are probably about as many phages as bacteria, perhaps more, and they help determine which bacteria prevail. More in the chapter “Bacteriophages: the hunters in the gut”, that is, viruses that infect bacteria, are there too. Every site has its own community: the rough tongue hosts different species from the gum pockets, where hardly any oxygen reaches.

The mouth is also the first part of digestion. Chewing breaks the food down so that enzymes and acid have more surface to work on later. Saliva makes the bite slippery and starts starch digestion, and it contains substances that keep bacteria in check, including secretory IgA IgA (secretory IgA) The antibody of the mucous membranes. It is released into the gut in large amounts, where it holds germs in the mucus without causing inflammation. More in the chapter “IgA: the antibody that keeps the gut clean, and what vitamin A, D and omega-3 have to do with it”, lysozyme and lactoferrin. The salivary glands produce about one to one and a half litres a day; at night much less flows, which is why bacteria multiply overnight and breath often smells bad in the morning.

With saliva we constantly swallow an enormous number of mouth bacteria. It used to be thought that practically none of them arrive in the gut. A study of saliva and stool samples from 470 people in five countries showed otherwise: in healthy people the same bacterial strains were often found in mouth and gut. Most of these species could cross over, and in the researchers’ assessment many settle in the gut. So the mouth is a constant supply for the gut.

That this mostly stays harmless is due to several protective layers. Stomach acid kills many bacteria. Bile and the small intestine’s cleansing wave Migrating motor complex A wave of muscle contractions that sweeps the stomach and small intestine empty about every one and a half to two hours when fasting. Eating interrupts it. More in the chapter “Intermittent fasting and meal timing: 16:8, 5:2 and Ramadan” do the rest. And the established gut flora Microbiome, gut microbiota The community of all microbes in the gut, mainly bacteria, plus viruses, fungi and archaea. Most live in the colon. More in the chapter “Building microbial diversity” occupies the places, so newcomers find it hard to gain a foothold. IgA antibodies also keep mouth bacteria in check in the gut.

The mouth also has its own job for the circulation. Bacteria on the tongue convert nitrate from vegetables into nitrite. From this the body makes nitric oxide, which widens blood vessels. This pathway depends directly on the mouth flora.

Disrupted state: causes and effects

Dental plaque is a biofilm Biofilm A community of microbes that sticks to a surface and embeds itself in a self-made protective coat, such as dental plaque. More in the chapter “Biofilms in the gut and “biofilm busters””: bacteria stick to the tooth surface in a slime layer they make themselves, where they are protected from rinsing and defences. If it stays at the gum line, the gums become inflamed (gingivitis), bleed and swell. When the inflammation reaches the jawbone, it is called periodontitis. The community then shifts towards species that grow without oxygen and feed inflammation. Experts speak of a vicious circle: the inflamed gum supplies nutrients from the blood that favour exactly these species.

How much protection saliva offers depends on how much there is. Dry mouth is common, especially with age, and comes from many medications, for example against allergies, depression, weak bladder or high blood pressure, from mouth breathing and from autoimmune diseases Autoimmune disease A disease in which the immune system attacks the body's own tissue, for example the thyroid in Hashimoto's or the pigment cells in vitiligo. More in the chapter “Autoimmune diseases and the gut: Hashimoto's, vitiligo, type 1 diabetes and more” such as Sjögren’s syndrome. Without saliva, caries, fungal infection and gum inflammation increase. Smoking, diabetes and pregnancy also make the gums more vulnerable. People who cannot chew well, for example because of tooth loss or badly fitting dentures, often choose softer, lower-nutrient food and swallow coarser lumps.

In mice, periodontitis germs reached the gut and aggravated gut inflammation there, together with immune cells that had been primed in the inflamed mouth and migrated to the gut. In a Japanese study, Klebsiella strains from the saliva of people settled in the gut of germ-free mice. They were resistant to several antibiotics, prevailed mainly when the gut flora was disturbed, and triggered strong inflammation in susceptible animals.

In humans there are matching associations. People with inflammatory bowel disease Inflammatory bowel disease Umbrella term for Crohn’s disease and ulcerative colitis: long-term, relapsing inflammation of the gut that needs medical treatment. More in the chapter “Immune system and inflammation” have periodontitis more often. In liver cirrhosis, more than half of the increased gut species originally came from the mouth. In bowel cancer, researchers found the same strain of Fusobacterium nucleatum in the tumour and in the mouth. What is cause and what is consequence is open, and no study shows that better oral hygiene prevents these diseases.

Acid blockers open the gate further. In people taking proton pump inhibitors Proton pump inhibitors (PPIs) Acid blockers such as omeprazole or pantoprazole. They inhibit the acid pump in the stomach lining. More in the chapter “Acid blockers (PPIs): what they change in the gut and how to stop them properly”, significantly more mouth bacteria are found in stool, because stomach acid is missing as a filter. Something similar was seen in people with IgA deficiency.

Too much hygiene can also do harm. Antiseptic mouthwash with chlorhexidine reduced nitrite formation in the mouth by 90 per cent and the blood nitrite level by a quarter in one study. Blood pressure rose by 2 to 3.5 mmHg. After a week of chlorhexidine, the saliva was more acidic and the mouth flora clearly shifted. In an observational study, people who used mouthwash twice a day more often developed diabetes or a precursor within three years. The caveat remains: these are small studies and associations, not proof that mouthwash harms the gut or heart.