1. Does Fresh Sweat Have a Smell?
Have you ever smelled your arm right after exercising? It is said that sweat that has just come out has almost no smell. The MSD Manual Home Edition explains that substances from sweat glands do not produce a bad smell until they are broken down by bacteria and fungi that normally live on the skin [1].
This means the search for the source of the smell is not about the sweat itself, but about the bacteria living on the skin. A 2024 announcement by Osaka Metropolitan University and the Institute of Medical Science at the University of Tokyo says the same thing about armpit odor. The cause is the secretion from apocrine glands in the armpit sweat. It is odorless when it comes out, but when the normal bacteria on the skin metabolize it, smelly substances are formed [2].
According to this announcement, about 10% of Japanese people are said to have axillary osmidrosis, which is the medical name for strong armpit odor. This is not everyone, but about 1 in 10 people. However, the bacteria live on everyone’s skin. So, which of these bacteria are involved in making the smell?
2. Which Bacteria Live on the Skin of the Armpit?
The skin of the armpit is said to have mainly Staphylococcus, Corynebacterium, and Propionibacterium [3]. Micrococcus has also been investigated as one of the armpit bacteria. A study published in 2018 found that more abundant bacteria do not necessarily create more odor. Corynebacterium was related to the strength of the smell. On the other hand, a type of Staphylococcus called S. hominis (Staphylococcus hominis) had a strong relationship with bad odor production, even though there were fewer of them in the armpit [3].
There are also types of odors. The announcement from Osaka Metropolitan University and others says that armpit odors are most common in this order: milk-like (M-type), sour (A-type), and curry-spice-like (C-type). These three types make up about 90%. The C-type is the strongest, and the M-type is the weakest smell [2].
In this study, 20 healthy adult men (11 with C-type and 9 with M-type) cooperated by having their armpit bacteria examined. In the armpits of people with the C-type, Staphylococcus bacteria that produce odor-causing substances were clearly found in larger numbers [2]. However, because this was the result of only 20 people, it cannot be said to apply exactly to all people or women. So far, the main candidates are Staphylococcus and Corynebacterium. Where does Micrococcus fit in?
3. Does Research Support Blaming Micrococcus for Sweat Smell?
Micrococcus is a type of spherical bacterium. It is found in many places, such as soil, water, dust, and the skin of people and animals. One of its relatives, M. luteus, makes yellow pigment [6]. Since it is definitely on the skin, it is not strange that its name comes up as the "culprit" for sweat odor.
However, a study on armpit bacteria published by James et al. in 2013 has a slightly different view. Micrococcus was found in the samples, but the paper states that its contribution to bad odor is probably unlikely because the detection rate and density of this genus were low [5]. The same paper states that the main pathway for producing short-chain fatty acids (like isovaleric acid), which are odor components, is through Staphylococcus changing the amino acid leucine.
It is important to note that they did not say it is "unrelated." The paper’s expression is "probably does not contribute," which is a claim that the contribution is "thought to be small." Also, this study examined armpit samples, so other parts of the body are a different matter. The position of the bacterium named as the "culprit" was slightly different when checked. So, how did researchers identify the bacteria involved in creating odor?
4. What Happened When a Micrococcus Gene Was Put into E. coli?
To identify the bacteria that create odor, researchers at the University of York used an experiment with genes. This was announced at the annual conference of the Society for General Microbiology in 2015 [4].
According to this research, a type of Staphylococcus called S. hominis has an enzyme that changes substances in sweat into thioalcohols, which are strong-smelling substances. When the gene for that enzyme was moved into E. coli, the E. coli produced body odor smell when it touched human sweat [4]. It is said that thioalcohols are noticeable to the nose even if there are only about one in a trillion parts in the air.
A study from 2018 explains the same process. The secretions from the armpit’s apocrine glands are odorless. When bacteria process the precursors (the source of the smell) in them, a volatile thioalcohol called 3M3SH is made. This is a smell that can be detected even at very low concentrations [3]. The announcement from Osaka Metropolitan University and others also lists this thioalcohol 3M3SH, along with volatile fatty acids 3M2H and HMHA, as the main bad odor components of the armpit [2].
Putting in a gene and seeing if a smell appears is one way to confirm not just the strength of the relationship, but the cause itself: "This action of this bacterium causes the smell." So, how are other bacteria involved in making the smell?
5. How Is Micrococcus Used in Research on Lysozyme?
Although Micrococcus may not be the main actor in odor, its name appears in the history of science in another place. In 1922, Fleming showed that mucus has the power to dissolve bacteria and named the substance lysozyme [8]. Fleming, later known for discovering penicillin, had focused on substances the body uses to protect itself from bacteria before that.
The cell walls of a relative of Micrococcus (sold as the reagent Micrococcus lysodeikticus) are still used as material to measure how lysozyme works [6]. When making a number for how much lysozyme dissolves bacteria, this bacterium, which dissolves easily, is useful.
Even if it is a supporting actor in creating odor, a bacterium has played a leading role in research on body-protecting substances. The familiar explanation that "the truth of sweat odor is [specific bacterium]" changes in impression when you look at which study and which part of the story it comes from.
6. How Does Armpit Smell on a Shirt Change from Morning to Evening?
Put on a freshly washed shirt. Smell the armpit area in the morning and in the evening. Note down whether the strength of the smell and the type (sour, milk-like, etc.) changed.
Record how the strength and type of smell changed between morning and evening. You cannot decide which bacteria are working just by the smell. Do not judge good or bad by comparing your smell with other people’s smells.
If you want to learn more, the "Body Odor" page of the MSD Manual Home Edition is an entry point you can read in Japanese.