Why do our armpits and breath smell?

Raul Rivas Gonzalez

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“The streets stank of manure, the courtyards stank of urine, the stairwells stank of rotten wood and rat droppings, the kitchens stank of rotten cabbage and mutton grease; the unventilated rooms stank of moldy dust; the bedrooms, greasy sheets, damp duvets and the pungent sweet smell of urinals. The chimneys stank of sulfur, the tanneries of caustic lye, the slaughterhouses of coagulated blood. Men and women stank of sweat and dirty clothes.

This is how the German writer Patrick Suskind begins the novel “Perfume”, describing the aromas that raged in 18th century France. Well yes, from time to time, the animals give off Bad smell. And humans are no exception. Go ahead, soap and water are usually the solution, but sometimes it is not enough.

Various microorganisms are responsible for the smelly aroma that we emit when we are drenched in sweat. The human body odor is produced by the bacterial transformation of some odorless precursor molecules that are secreted onto the surface of the skin by apocrine glands, a type of sweat glands, typical of Homo sapiens, which are located in specific skin areas of the body such as the armpit, the nipple and the external genital region.

The bad smell of the armpit

The human axillary malodor is composed of a combination of volatile organic compounds with the volatile fatty acids and the tioalcoholes as the main ingredients. Volatile fatty acids with a higher carbon number have a lower odor detection threshold. It is generally accepted that short-chain volatile fatty acids (C2-C5) are among the molecules that cause axillary malodour.

Bacterial genera Staphylococcus, Cutibacterium (formerly Propionibacterium) and Corynebacterium they are part of the dominant microbiota that colonizes the armpit. And they are capable of fermenting glycerol and lactic acid into volatile short chain fatty acids (C2-C3) such as acetic acid and propionic acid.

In addition, staphylococci are capable of converting branched aliphatic amino acids, such as leucine, into highly odorous short-chain methyl branched volatile fatty acids (C4-C5), such as isovaleric acid, which has traditionally been associated with the sour note of evil. armpit odor.

Thioalcohols, despite being present in minute quantities, are the most incipient volatiles. Different thioalcohols have been detected in axillary secretions, with 3-methyl-3-sulfanylhexan-1-ol (3M3SH) being the most abundant.

Some species of staphylococci present in the skin have the capacity to generate 3M3SH from the odorless precursor Cys-Gly-3M3SH, secreted on the surface of the skin by the apocrine glands. Therefore, it is evident that the axillary microbiota plays an important role in the generation of human body odor.

Halitosis and bacteria

As with body odor, on many occasions the halitosis it is determined by volatile compounds produced mainly by bacteria that colonize the oral cavity.

The bacterial genera most related to halitosis are Actinomyces spp., Bacteroides spp., Dialister spp., Eubacterium spp., Fusobacterium spp., Leptotrichia spp., Peptostreptococcus spp., Porphyromonas spp., Prevotella spp., Selenomonas spp., Selenomonas spp., Selenomonas spp. spp., Tannerella forsythia and Veillonella spp. In this sense, Bacteroides forsythus, Porphyromonas gingivalis, Actinobacillus actinomycetemcomitans and Prevotella_intermedia have been associated with the production of cvolatile sulfur compounds from substrates that contain sulfur and that may be present in food or saliva.

In the oral cavity, they have been detected near 700 different volatile compoundsSome of the predominant malodorous compounds being methyl thioacetate, dimethyl disulfide, dimethyl trisulfide, dimethyl tetra sulfide, dimethyl pentasulfide, dimethyl sulfone, allyl thiocyanate, allyl isothiocyanate, S-methyl, thiolan-2-one, methylbenzene, tetramethylbutane, ethanol, putrescine, cadaverine, indole, and trimethylamine.

For example, the cadaverine, which gives off the typical smell of putrefaction, is produced by bacterial fermentation of the amino acid L-lysine. Anaerobic bacteria present in the oral cavity can break down sulfur-containing amino acids such as L-cysteine ​​to volatile compounds such as hydrogen sulfide, which have a characteristic smell of rotten eggs and helps us to exhale a fetid breath.

Other examples of malodorous microorganisms are the bacterium Proteus mirabilis, which produces a smell of fish very characteristic, and Eikenella corrodens which smells of bleach.

The smell of wet earth and saltpeter

However, not all microorganisms cause a bad odor. There are also those who smell that they fall in love. This is the case of some species of the genus Streptomyces that in humid conditions produce a very odorous sesquiterpenoid called geosmina, essential to create the petricor aroma that causes the typical and evocative wet soil’s smell.

Even the distinctive fragrance of the seashore, which is due in large part to the presence of dimethyl sulfide, is caused mainly by microbial activity.

The bacteria Pseudomonas aeruginosa can generate a sweet aroma similar to grape juice in burn patients. Wound dressings are impregnated with a distinctive odor because Pseudomonas aeruginosa produces 2-aminoacetophenone which is the molecule responsible, for example, for the smell of honey and white flowers in white wines.

Regarding the typical aroma of yogurt is mainly characterized by the presence of acetaldehído. The most important pathway for acetaldehyde formation is the breakdown of the amino acid threonine into acetaldehyde and glycine. The enzyme responsible for this catalysis, threonine aldolase, is present in Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus subsp. salivarius, the two essential bacterial species for the production of this dairy product.

As a final example, we cannot forget Streptococcus milleri, which produces diacetyl, which gives it the typical and pleasant smell of Butter, caramel or toffee.

In short, when you smell some peculiar aroma, be it pleasant or one of those that tan the cerebral hemispheres, think that it is possible that some microorganism is responsible.

Raúl Rivas González. Professor of Microbiology, University of Salamanca.

This article was originally published on The Conversation.

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