Microbial Origins of Body Odor | ASM.org (2024)

"Odors have a power of persuasion stronger than that of words, appearances, emotions or will."

These are the remarks of Patrick Süskind in his popular novel "Perfume: The Story of a Murderer." His words, although used to describe how the sense of smell is tied to human feelings about an object or a person,are widely confirmed in nature. Flowers produce fragrances as an evolutionary strategy to attract pollinators and ensure reproduction, and many species of insects, fish and mammals emit peculiar odors to attract mates. In fact, male brown lemmings can tell whether a female has ever mated simply by its odor.

Humans also emanate a range of smells, not all of which are pleasant. Take, for instance, the smelly armpits or the stinking foot! Body odor (BO) is a fairly common issue that affects people at some point in their lives, and more often than not, microbes are the root of the issue. Commensal microbes on the skin metabolize certain compounds in sweat and can produce foul-smelling odors. One can mask BO with deodorants and antiperspirants, but sometimes the odor can be an indication of an underlying disease.

Causes and Biochemistry of Body Odor

Humans have3 types of sweat glands—apocrine, eccrine, sebaceous. While eccrine sweat glands are present in all skin types on the body, apocrine and sebaceous are restricted to certainlocations. Body odor is primarily caused byapocrine sweat glandsthat become activated during puberty.These sweat glandsdevelop in hairy regions like the armpits, genitalsand scalp, where they secrete an oily fluidcomprised ofproteins, lipidsand steroids.Contrary topopular belief, this viscous fluid(sweat)isnaturallyalmostentirelyodorless. It is only when members of the skin microbiota metabolize thesesecretions thatthey producethemalodorousbyproducts, which cause body odor. In humans, armpitsoffer a moist,warm environmentwheremicrobescan thrive, making them a microbial hotspot.

The composition of the skin microbiota varies from one individual to another, and between locations on the same host; sometimes, even the left armpit can have a vastly different flora compared to the right. However, the major bacterial players that colonize the skin and produce body odor are similar. Some common skin bacteria that produce body odor include members of Corynebacterium, Staphylococcus and Cutibacterium genera.

The biochemistry behind the microbial conversion of sweat to malodorous products is still notcompletely understood. However, the odor can be largely attributed to the production of volatile organic compounds (VOCs), including volatile fatty acids and thioalcohols.

Key volatile fatty acids that contribute to body odor include 3-methyl-2-hexenoic acid (3M2H), which has a ‘goat-like’ odor, and3-hydroxy-3-methylhexanoic acid (HMHA), which has a ‘cumin-like’ odor. Theseodorants are producedby some members ofCorynebacterium, includingCorynebacteriumstriatum,CorynebacteriumjeikeiumandCorynebacteriumbovis. Other medium- and short-chain fatty acids also contribute to odor. If you havesmelly feet, it is probably becauseStaphylococcus epidermishas degraded the leucine in your sweat to isovaleric acid, acheesy-smelling compound.

Thioalcoholsget their stinky odor from sulfurand,despite being present inonlytrace amounts,aresome of the most pungentVOCsproduced.3-methyl-3-sulfanylhexan-1-ol (3M3SH) isathioalcohol, produced byStaphylococcus hominis,which makes the underarmssmell like rotten onions or meat.S. hominisencodes aproton-coupled oligopeptide transporterthat importsthethioalcohol-conjugatedprecursorS-Cys-Gly-3M3SHinto the cell, and subsequent catabolism results in the foul-smelling 3M3SH.

FactorsInfluencingBodyOdor

Several factors, including sex, genetics, age and diet can influence the type of odor that an individual emits.In fact, it has been suggested that, similar to a fingerprint, every individual’s body odor is unique and may be partly determined by genetics.Men have larger sweat glands and generally produce more sweat than women. This typically results in larger populations ofCorynebacteriumspp.and intensified cheese-like odor, due to the production of higher quantities of volatile fatty acids.

The underarm body odorhas beenlinked toa gene calledABCC11, which encodesa protein that transports molecules across cellular membranes, including molecules in the sweat.IftheABCC11geneisnon-functional,sweat molecules are unable to cross the membrane barrier to reach the armpit.Thisstarves bacteriaon theother side of theskin surface, as they areunable toaccess ormetabolizethe organic compounds inthe sweat. As aresult, odorant substancesare not produced.Loss-of-function ABCC11 mutation is fairly common inEast Asian populations(80-95%).

The chemical nature of body odor has also been suggested to change with age. The characteristic"nursing home smell" of elderly peopleis thought to be associated with the presence of an unsaturated aldehyde, 2-nonenal. Thiscompound has an unpleasant greasy and grassy odor and is produced upon oxidative degradation ofω7 unsaturated fatty acids in skin surface lipids.

De-odorizingBodyOdor

There aren’t many evidence-based guidelines to manage body odor, but deodorants and antiperspirants are commonly used to reduce or prevent it. Deodorants contain chemicals thatkill skin floraand block production of stinky byproducts. Antiperspirants reduce the amount of sweat produced by clogging sweat glands. Many contain aluminum chloride, which creates a gel-like substance that forms a plug at the sweat ducts in the skin. Manydeodorants and antiperspirants also contain antimicrobials,like propylene glycol, triclosan andbenzalkonium chloride, which decrease bacterial abundance and result in an altered skin microbiome in the armpits. However, the modified microbiome can have unintended consequences as well. For instance, astudyshowed that the use of antiperspirants resulted in an increase in odor-producingActinobacteriain some individuals.

While deodorants and antiperspirants dotheminimumjob of reducing bacterial load and their byproducts, respectively, a growing body of work is focusing on the use ofprobiotics and artificial microbial communitiestoreduce malodor. The field is still in its infancy,but onetechnique beingdevelopedisarmpit microbial transplantation.In preliminary work,scientistsusedantibacterialstoremovethe armpit microbiome from apersonwith BOand replaced it withbacteriafrom the armpit microbiomeof a healthy (related) donor. Comprehensive results on the efficacy and success of the procedure have yet to be published.

BodyOdor andDiseases

Certain medical conditions associated with metabolic imbalances can be diagnosedfromodors that areemitted from the skin. For instance,trimethylaminuriais associated witha strong fish-like body odor.Phenylketonuriais associated witha musty odor, andhypermethioninemiais associated within an odor akin to that of boiled cabbage.

Body odor profilescan also be used to diagnose diseases like malaria.Scientistscollected samples of skin volatiles from more than 400 school children in malarial areas in Western Kenya and usedthem,in combination with predictive modeling,toidentify asymptomatic malarial infections with 100% sensitivity. Thesestudiesstillneed to be replicated indiverse population settings,butthe dataofferhope of establishing skin volatile biomarkers as a robust, noninvasive strategy to identify asymptomatic malaria infections.

Additionalresearchdemonstrated thatindividuals with malaria have a unique blend of skin odorsthat makes them more attractive to mosquito vectors. Here, scientists examined the VOCs associated with socks collected from school children in Western Kenya. Data showed that children with malaria had elevated levels of aldehydes heptanal, octanal andnonal, compared to parasite-free individuals. Furthermore, the volatile compounds were detected by mosquito antennae, making malaria-infected children more prone to further attack by these insects.Whether these compounds are produced by the parasite or the skin microbiota has yet to be determined.

Conclusion

It is now largely recognized that many compounds contributing to body odor originate from the skin microbiome, although we have a limited understanding of the underlying biochemistry. Defining the structural and molecular basis of odorless precursors, and the manner by which they are subsequently converted to odorant chemicals, can help inform the design of strategies to inhibit malodor formation.

Innovative therapies along the lines of armpit microbiota transfers may offer respite and psychological relief to people with chronic body odor conditions. At the same time, scientists are unraveling the role of odor in certain diseases.Leveraging this understanding will help in the development of rapid diagnostic tests, as well as treatments for a variety of pathogens and diseases.