Skin Microorganisms as Mediators of Nonverbal Communication
Our bodies are not a closed system. Our skin, the largest organ of the human body, exchanges more than just gases and water with the environment. It also exudes a number of volatile compounds that can be detected by our nose. However, we are not the direct producers of these odorous substances. Our scent is determined by the microorganisms that populate our skin. The sweat and sebum that we excrete through our glands are their growth medium, and thanks to the differences in secretion, there are different nutrients for them on different parts of our body. In some places, it contains more water, sometimes more salt or lipids, and somewhere the temperature is higher. Every body part offers different living conditions. Depending on the preferences of the microorganism, different strains or families populate these areas. Since some of their metabolic pathways and enzymes differ, they also produce different odorous substances. The composition of various odours and their ratio is innate to everyone, and we can recognise the resulting cocktail as the “scent of a particular person”.
Sweat: A breeding ground for microorganisms
In order to grow and reproduce, microorganisms need water, the right temperature, nutrients, and some also oxygen. This is why the environment on our skin is ideal for their life. Their food arrives at the skin through the two types of sweat glands that we have, and also through sebaceous glands that are found in every hair follicle. There are eccrine glands on the entire surface of the body, with the highest concentration on our palms, soles of our feet, forehead, and underarms. The body cools itself through these glands during sport and other physical activities. The sweat produced by eccrine glands consists of 99% water, with dissolved salts, amino acids, glycerol, lactic acid, proteins, and other electrolytes (1, 2).
Stress, the same as arousal, can be smelled in the air
During puberty, the apocrine sweat glands are activated. These are found mainly in the armpits, genitals, and scalp. These glands secrete a thick milky fluid with a much higher concentration of dissolved substances – lipids, fatty acids, proteins, steroids (such as androstenone and testosterone), vitamins, and electrolytes (1, 2). Secretion is triggered by emotional strain, stressful situations, or excitation of various kinds. From a biochemical point of view, the difference between apocrine and eccrine secretion is that during apocrine secretion, a portion of the internal content of cells – cytosol and a part of the membrane – is also excreted. During eccrine secretion, the cell excretes only some selected compounds in a process called exocytosis, without any loss of cytoplasm; this is why eccrine sweat is poorer (3).
Without microorganisms, sweat is odourless
We all remember the teen smell of high school locker rooms all too well – it is often called the smell of testosterone. Although this hormone has a chemical structure with a steroid nucleus and is completely odour-free, if the microorganisms decompose it to dihydrotestosterone using reductase, it contributes to the characteristic scent of a person. Testosterone as a precursor is not the main source of human body odour, however – even though it is definitely an important ingredient of the mix. Other interesting ingredients may be the compounds derived from androsterone which is metabolised to androstenone with a scent similar to urine and musk.
Our nose is able to detect one odorous molecule in a trillion
The principal originators of human body odour can be divided into three main categories. Corynebacteria metabolise skin lipids into medium-chain fatty acids (for example, they transform palmitic acid into isobutyric acid). These bacteria are thought to contribute the most to our smell. On the contrary, propionibacteria produce odorous metabolites the least, while staphylococci are somewhere in between (2).
Corynebacteria are the most active in all the metabolic pathways that produce odorous molecules. They produce the smelliest compound, 3-hydroxy-3-methylhexanoic acid (HMHA), which we can detect at concentrations as low as 4 ppt (part per trillion) which classes corynebacteria among the main producers of human body odour. Staphylococci produce medium-chain fatty acids out of amino acids and, not having such active lipases that participate in the reaction, do not produce HMHA in the same amounts as corynebacteria (2).
Sulphurous compounds (thiols) are another group of smelly substances. These are usually structures with a shorter carbon chain containing sulphur, such as can be found in onion or garlic. Thiols work similarly in human sweat. The most intense is 3-sulfanyl pentanol (3SP) which smells like onion with a dash of grapefruit. 2-methyl 3-sulfanyl butanol (2M3SB) has a distinctly meaty and fruity scent. We detect these compounds in the air at very low concentrations (1-2 ppt) and they have a very sweet smell with a hint of onion (2). I am sure you know this scent of the skin.
When did we start perceiving body odour as something undesirable?
For animals, scent is one of the main means of communication, and it also helps them find a mate. Sexual attraction is still influenced by a person’s scent nowadays, although our personal hygiene habits have caused the differences to be less pronounced than they once were. A hundred years ago, people smelled very different in our territory than we are used to today. Body odour was not as fought against and scent had its place in nonverbal communication – the smell of self-confidence differs from that of nervousness or fear. In the modern world, every culture has its own standards regarding the use of deodorants and frequency of washing oneself, but what is even more interesting is that human races also differ on a biological level, statistically speaking. For example, compared to Europeans, Asians have lower levels of apolipoprotein D which is responsible for transporting volatile fatty acids through the skin. Asians thus lack the precursor for the production of one of the smelliest substances – HMHA.
And where do you stand on body odour? Have you noticed that some people smell better than others to you? If you are looking for a way to prevent the production of smelly compounds but do not wish to harm your skin microbiome, try a natural deodorant based on coconut oil and soda.