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Why do your sneakers and active-wear smell bad?

On sweat and sweat odour - microbial ecology, biochemistry and genetics

Sweat does not smell - in its original form. It is only when bacteria on the skin start to break down the sweat that odour occurs. The fight against sweat odour has now developed into a billion-dollar industry.

The problem arises on the skin; the interface where the human body meets the microorganisms in the environment. Several factors influence the end result: human genetics, the microbial ecosystem that colonises the body and also microbial genetics - the tools microorganisms have at their disposal to break down the nutrients in sweat.

Odour, in turn, can be explained by biochemistry. Odour is made up of molecules suspended in the air. Some of them are perceived as very unpleasant, even in very small quantities. When sweat smells bad, it is because odourless molecules have been broken down by micro-organisms and converted into malodorous substances - often volatile fatty acids or substances containing Sulphur.

Microorganisms - a carpet of life that covers everything

Microorganisms are very small, single-celled life forms. Because they are invisible to the naked eye, we need tools to record them at all. Until recently, our scientific tools only allowed us to study those organisms that could be (successfully) grown in a laboratory. We have therefore believed that these were the only ones that existed.

However, rapid scientific and technological developments have made it possible to read the genetic code of all organisms in a given environment - without the need for successful cultivation in a laboratory environment. Metagenomics has brought a transformative realisation: we are in fact surrounded by vast quantities of micro-organisms we had no idea existed; a biological 'dark matter' that completely dominates the planet.

We now know that less than one percent of all life forms present in a random sample - from any environment - are those that can be cultivated. Without the new technology, these would have remained invisible.

Now we also know that the visible world - plants, animals, fungi - is a vanishingly small part of life on Earth. The real diversity and richness of species is outside our field of vision, in the world of micro-organisms.

MICROBIAL ECOSYSTEMS

Microbial ecology is the study of the interaction between living microorganisms and their environment. Microorganisms create microscopic ecosystems, which are governed by the same laws as any other ecosystem on Earth. Which species thrive, benefit or are disadvantaged is determined by the resources available. Natural selection occurs even at the microscopic level. Micro-organisms interact, compete and war with each other. They break down substances together, step by step, in cooperation, forming networks of interdependence.

A microbial ecosystem establishes itself rapidly in all environments and under all possible conditions. Microorganisms are everywhere, all the time, covering every surface on earth. This of course includes the human body - both inside and out.

THE BODY'S UNKNOWN BIOMASS - THE MICROBIOME

Relatively recently - in the 21st century - we have become aware of the billions of single-celled organisms living in and on the human body. In fact, every human being lives in symbiosis with a rich biomass composed mainly of bacteria - our microbiome. The vast majority of these live in the large intestine (a colony commonly known as the microbiota) and perform a variety of vital tasks.

The healthy human body is completely dependent on bacteria to ensure the normal functioning of many of the body's systems - in particular the immune system. Many disease states are characterised by marked changes and abnormalities in bacterial populations. Different species increase or decrease, or their interrelationship shifts.

The outside of the body is also protected by allied microorganisms. They colonise the skin, prevent pathogenic micro-organisms from taking hold and secrete acidic substances that lower the pH. A more acidic environment inhibits the growth of unwanted bacteria. But they also secrete anti-microbials to fight off their competitors through chemical warfare.

The body is like a vast continent with a variety of environments. Microbial ecosystems therefore also vary with the conditions of the place. For example, the bacteria living in the mouth and nose are completely different from those living on the scalp.

Under the arms, conditions are different. A very specific set of microorganisms therefore live on this skin surface. And there are many of them: the armpit contains as many single-celled organisms as there are people on the planet - at least.

Together, the micro-organisms and the environment create the well-known problem we are constantly fighting: sweat odour.

The skin interface

Sweat odour occurs when sweat encounters the microorganisms of the environment. Both of these factors also contribute to sweat odour varying from person to person. On the one hand, each person's sweat is unique in its composition and quantity. Secondly, each person has a personal microbiome.

Metagenomics has of course made it possible to map the specialised ecosystem that normally inhabits the skin surface of human armpits. It has been found that it varies from person to person, but within certain limits: a few families of bacteria and their subspecies recur frequently, but in different proportions. Some microorganisms - mainly from the Propionibacterium, Staphylococcus and in particular

Corynebacterium - produce a distinct, unmistakable and unpleasant odour. And they do so because sweat contains nutrient-rich molecules that they can break down in their metabolism.

TWO TYPES OF SWEAT GLANDS

Humans have two different systems for producing sweat, and two different types of sweat glands associated with each system.

By far the largest number of sweat glands are merocrine. They are controlled by the neurotransmitter acetylcholine, from the hypothalamus in the brain, which senses and controls body temperature. When body temperature rises, large amounts of sweat are produced, mostly consisting of water and salts. These sweat glands are found virtually all over the body, and especially on the hands and feet. The function of the merocrine sweat glands is mainly temperature regulation, but sweating can also be triggered by stress and nervousness.

Sweat under the arms, on the other hand, is produced by apocrine sweat glands. They become active during puberty and open around hairs. This sweat has a different composition. It is milky, oily and full of lipids (fats), proteins and carbohydrates. This is excellent nourishment for the resilient bacteria that have made this place their habitat, and which seem to resist all our attempts to eradicate them. Sweating is controlled by noradrenaline, a neurotransmitter and blood-borne hormone, and is influenced by emotion and touch.

BIOCHEMISTRY - SOME MOLECULES SMELL WORSE THAN OTHERS

So what does the bad smell consist of? The answer is: mainly short, volatile fatty acids that result when odourless lipids are broken down by microorganisms. Another source of discomfort is substances containing sulphur - nature's surest bet for producing truly repulsive odours. This type of odour is generated naturally by the decay and breakdown of organic matter, but can also be produced deliberately. An example of this is the skunk's defence mechanism, but examples can also be found in the plant kingdom: smelly odours can be produced by flowers that want to imitate the smell of rotting flesh in order to attract and be pollinated by flies.

Sulphur is found in proteins (and their building blocks amino acids) secreted in sweat from the mesocrine sweat glands. When these larger molecules are broken down by micro-organisms, the end result is smaller, volatile, sulphur-containing molecules - and humans have a very well-developed ability to sense the presence of these substances even at very low concentrations.

Genetic background

GENETICS CONTROL SWEATING

Human sweat is controlled by underlying genetic factors. For example, they determine how many sweat glands you have, how they are distributed over your body and the composition of your sweat. Not everyone's sweat is identical. Men's sweat is composed differently from women's, which means that men's sweat smells different and even stronger. For example, steroids are present in male sweat - feeding another category of bacteria, and thus adding to the cocktail of smelly substances in sweat.

Sweat odour also depends on a person's age, as it is also controlled by hormones that change throughout life. It is also influenced by ethnicity: Europeans and Africans sweat in a similar way, while Asians have a genetic variant that makes their sweat much less malodorous.

MICROBIAL GENETICS, ENZYMES AND METABOLIC PATHWAYS

Since odour arises as a result of the breakdown work of microorganisms, it is also interesting to map what actually happens when sweat lipids, proteins and carbohydrates are transformed from 'raw materials' into malodorous metabolites.

The possible degradation pathways depend on the enzymes available to the microorganisms present. Enzymes are vital tools that all living things need for biological processes to take place in their cells. However, enzymes are highly specialised, and a specific enzyme is required for a particular reaction to take place. Thus, microbial genetics can reveal which tools a particular bacterium actually has access to - and thus what it can break down, and into what.

A gene is an instruction that describes how a protein, such as an enzyme, is to be built. If the gene is there, so is the protein, and vice versa. If the protein is present in a bacterium, the bacterium will also be able to carry out certain predetermined reactions along a degradation pathway. In this way, we can find out which bacteria are responsible for which odours, and why.

Smell, the future and research

DAILY ROUTINES IN THE BATHROOM

The fight against bad smells is a multi-billion dollar industry, whose value is increasing as we become increasingly sensitive to 'natural' smells - and increasingly intolerant of expressions of unbridled physicality. It is now socially obligatory to use all available means to avoid bad smells. The most common remedies, apart from personal hygiene, are the use of deodorants and antiperspirants. They have the same aim, to reduce the smell of perspiration, but use different agents.

Deodorants work by absorbing bacteria and odour molecules. The rate of bacterial growth is reduced by pH-lowering and antimicrobial substances. The odour is masked with fragrances. Antiperspirants instead block the sweat glands and prevent sweating, usually with the help of aluminium chloride. Whatever the method, the effect wears off fairly quickly and the smell returns.

BACTERIAL EXCHANGE

Since microorganisms play a crucial role in body odour, it is not surprising that attempts are being made to replace the microbial ecosystem that has established itself in the armpits. One of the most interesting experiments has involved transplanting an entire ecosystem from a "fragrant" identical twin to his smelly brother (carried out by Dr Chris Callewaert of the University of California, San Diego). The experiment is said to have been successful and the troubled brother escaped his bad body odour for up to a year.

However, the fact that the fragrant ecosystem established itself after the transplant can probably be attributed to the fact that the brothers share the genetics that control their sweating. The bacteria that thrived on one brother could therefore very likely also find a home on his identical brother.

'SMART WEAPONS' TARGETING SPECIFIC ENZYMES

At the University of York, a research team led by a molecular biochemist (Dan Bawdon, Ph.D) in collaboration with the company Unilever, has chosen instead to focus on specific enzymes along a metabolic pathway. Through their experiments, they have identified genes in the bacterial family Staphylococcus that encode enzymes necessary for the production of thioalcohols - sulphur compounds with a particularly unpleasant odour. They now hope that further research will lead to the discovery of a substance that can turn off the ability to convert the components of sweat into thioalcohols.

Text: Ester Nylöf, writer

Appendix:

THE INVISIBLE WORLD

How is Life on Earth really organized these days? Until the 1960s, all life forms on Earth were classified as either prokaryotes or eukaryotes. The division into two well-defined domains was based on the presence of the cell nucleus, i.e. a membrane that encloses the DNA molecule.

Eukaryotes, the domain that includes all multicellular organisms, have a cell nucleus. Prokaryotes, on the other hand, all of which appeared to be bacteria, do not. The bacteria/prokaryotes were in turn classified mainly on the basis of their appearance and the way they react with certain chemicals.

The American scientist Carl Woese began in the 1960s to systematically deduce, using simple and time-consuming methods, the code he believed must be the simplest common denominator of life: ribosomal RNA.

Ribosomal RNA, together with other proteins, forms the ribosome, a structure where the DNA code is "translated" into chains of amino acids - a fundamental process in all living things. Woese then discovered the existence of a group of microorganisms that did not appear to be related to bacteria. The codes were too different. But at the same time, these organisms were both single-celled and lacked a cell nucleus. So they were prokaryotes, but still not related to bacteria.

He had in fact discovered the third domain - archaea. This revolutionized the way life on Earth is related and organized. Since the 1970s, the Tree of Life has officially had three branches: bacteria, eukaryotes and archaea. 

ALMOST ALL LIFE ON EARTH IS SINGLE-CELLED

All bacteria are single-celled. All archaea are single-celled. Almost all eukaryotes are also single-celled. But among the eukaryotes is life's most fascinating exception: large, multicellular organisms - plants, animals and fungi - that is, all the things we see around us and think of when we talk about "life on Earth".

The remarkable archaea were first discovered in extreme environments, like hot sulphurous springs. Later, they have been found everywhere, from soil samples to the human gut. However, they seem to live completely parallel to our domain. But unlike bacteria, they don't even seem to interact with eukaryotes - and so they don't cause any known diseases.

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