u/Natural_Science_Doc

So far in this series we've focused on bacteria in terms of which species grow, what they eat, and why polyester feeds them better. However, there is a second half to the odor equation that has nothing to do with bacterial growth. It's about what happens to the smelly molecules after bacteria produce them.

This matters because two fabrics can generate similar amounts of odor compounds and still smell completely different to your nose. What matters is whether those compounds stay locked inside the fiber or escape into the air.

This comes down to a property called moisture regain which can be described as how much water a fiber holds onto after it feels dry to the touch.

When researchers at Aarhus University measured the residual water content in cotton and polyester after both fabrics had fully dried under identical conditions, cotton retained about 2.2% of its weight in water. Polyester retained ten times less than that at only 0.23% (Møllebjerg et al. 2021).

That thin film of residual moisture in cotton isn't just sitting there. It's doing something useful: dissolving and trapping volatile fatty acids (the odor molecules that bacteria produce). Those compounds are water-soluble, and as long as they're dissolved in the fiber's retained moisture, they aren't evaporating into the air and reaching your nose. The fiber is effectively sequestering or holding in the stink till you can wash it out in the laundry.

Polyester holds almost no residual moisture. So, as soon as bacteria produce odor compounds, those molecules have nowhere to hide. They sit on a dry, non-absorbent surface and volatilize freely into the air. The result is faster, more intense odor release, even if the total amount of odor compounds produced is similar.

This mechanism also explains one of the more counterintuitive findings in the textile odor literature. Wool is a hydrophobic fiber, its surface actually repels water, similar to polyester. You'd expect it to have a similar odor problem. But wool consistently scores among the lowest fabrics for perceived odor intensity after wear, despite supporting high bacterial counts (McQueen et al. 2007).

The explanation lies in wool's unusual structure. While the fiber surface is hydrophobic, the fiber interior is highly hygroscopic, meaning it absorbs and retains moisture within its core. This gives wool a high moisture regain despite its waxy outer layer. That internal moisture reservoir traps odor compounds the same way cotton, bamboo, and hemp do, preventing them from becoming airborne. Wool is essentially a stealth moisture trap with water-repellent on the outside, odor-absorbing on the inside (Møllebjerg et al. 2021).

So, what does this mean for the overall odor picture?

Polyester gets hit from both sides. It selectively grows the smelliest bacteria (#2), feeds them more efficiently through better sebum access (#3), and does nothing to trap the odor molecules those bacteria produce (#4). It's a three-stage amplifier: more of the wrong bugs, better-fed bugs, and no odor containment.

Cellulosic fibers like cotton, hemp, bamboo, lyocell, and other plant-based or regenerated-cellulose materials have the opposite profile at each stage. They don't preferentially grow odor-causing species, they don't coat themselves in bacterial food, and they retain enough moisture to sequester volatile odor compounds before they reach the air.

But we're not done yet. There's one more mechanism that makes polyester's odor problem compound over time and it explains why your oldest gym shirt is always your smelliest, no matter how many times you wash it.

That's coming in #5. Till then, let me know if you have any comments or questions about what we've covered so far. I think the wool paradox is pretty interesting - anyone else?

References: Møllebjerg, A. et al. (2021). The Bacterial Life Cycle in Textiles is Governed by Fiber Hydrophobicity. Microbiology Spectrum, 9(2), e01185-21.

McQueen, R.H. et al. (2007). Odor Intensity in Apparel Fabrics and the Link with Bacterial Populations. Textile Research Journal, 77(7), 449–456.

In #2 we established that polyester doesn't grow more bacteria than cotton, rather it grows the smellier ones.
But that raises a question: why? What is it about polyester that gives odor-causing Micrococcus a 10-fold growth advantage?

The answer isn't really about the bacteria at all. It's about the food.

The primary nutrient source for bacteria living on your clothing isn't sweat; it's sebum. Sebum is the oily, waxy substance produced by your skin's sebaceous glands. It's a mix of triglycerides, fatty acids, cholesterol, and wax esters, and it's the raw material that odor-causing bacteria metabolize into the volatile compounds you smell. Sweat carries sebum into the fabric during exercise, and from that point on, how the fabric handles that sebum determines how well bacteria can feed.

This is where fiber chemistry takes over.

Researchers at Aarhus University designed an in vitro model that mimics what happens when you sweat into fabric. It used artificial sweat mixed with real human sebum, inoculated with the five bacterial species most associated with textile odor and applied that lovely mixture to cotton and polyester swatches under controlled conditions. (Møllebjerg et al. 2021).

What they found was striking but makes complete sense from a materials science standpoint. --> Polyester absorbed significantly more sebum than cotton. That alone might not seem like a big deal, but the distribution of that sebum told the real story.

Using confocal laser microscopy with a lipid-specific fluorescent dye, the researchers visualized exactly where sebum ended up inside each fabric. On cotton, sebum formed isolated spherical droplets scattered throughout the textile meaning little pockets of oil not really absorbed on the fiber and separated from each other cellulose fiber. On polyester, which is more compatible with oily substances, sebum spread out into a thin, continuous coating along the entire length of each fiber, bridging the spaces between fibers and creating a uniform layer of bacterial food across every available surface.

Picture the difference like this: cotton serves sebum as individual appetizer plates at separate tables - no sharing. Polyester lays it out as an all-you-can-eat buffet line along all of the fiber.

The consequence for bacterial growth is straightforward. More sebum, spread more evenly, means more surface area where bacteria can access nutrients. Bacteria adhered to polyester had higher initial metabolic activity than those on cotton meaning they were eating more and metabolizing faster.

But here's the part that really pins it down. The same researchers ran a critical control experiment: they inoculated both fabrics with bacteria in a simple salt buffer — no sweat, no sebum. Just bacteria and fabric. Under those conditions, bacterial activity on polyester and cotton was statistically identical. No difference at all (Møllebjerg et al. 2021).

That's the key finding.
The natural fibers themselves aren't killing or preventing bacteria and polyester itself isn't what is promoting the faster growth of odor causing bacteria. It's the inherent properties of those fiber and the subsequent relationship of the fiber surface with sebum that creates the difference. Polyester's hydrophobic (and oleophilic ("oil-loving") surface has a chemical affinity for the lipids in sebum -- "like attracts like." Cotton's hydrophilic (water-loving) cellulose surface doesn't have that same attraction, so sebum doesn't spread, doesn't coat, and doesn't become as available.

Remove the sebum, and the playing field levels completely. Add it back, and polyester becomes the better restaurant.

This also helps explain why the Ghent University team found Micrococcus selectively enriched on polyester (short #2). Micrococci are known to be efficient metabolizers of a broad range of skin lipids. A fabric that provides more lipid fuel in a more accessible format is going to favor the species best equipped to use it. It's not that polyester has some mysterious property that attracts Micrococcus. It's that polyester creates a nutritional environment where the best lipid-metabolizers outcompete everyone else.

So, now we have two pieces of the puzzle: polyester selects for the smelliest bacteria (#2), and it does so by providing them with better access to their primary food source (#3).
But there's another side to the odor equation that has nothing to do with bacteria at all. The next piece of the puzzle is about what happens to the odor molecules after they're produced, and why some fabrics release them into the air while others trap them.

That bit is coming in #4.
Till then, let me know your thoughts or questions on any of this. Want to know more about confocal microscopy and how we can tell how oil spreads on a surface?

Also...is anyone else starting to feel like the word "sebum" should be in the same category as how some people feel about the word "moist" ? ...se-bum...

References: Møllebjerg, A. et al. (2021). The Bacterial Life Cycle in Textiles is Governed by Fiber Hydrophobicity. Microbiology Spectrum, 9(2), e01185-21.

Callewaert, C. et al. (2014). Microbial Odor Profile of Polyester and Cotton Clothes after a Fitness Session. Applied and Environmental Microbiology, 80(21), 6611–6619.

In #2 we established that polyester doesn't grow more bacteria than cotton, rather it grows the smellier ones.
But that raises a question: why? What is it about polyester that gives odor-causing Micrococcus a 10-fold growth advantage?

The answer isn't really about the bacteria at all. It's about the food.

The primary nutrient source for bacteria living on your clothing isn't sweat; it's sebum. Sebum is the oily, waxy substance produced by your skin's sebaceous glands. It's a mix of triglycerides, fatty acids, cholesterol, and wax esters, and it's the raw material that odor-causing bacteria metabolize into the volatile compounds you smell. Sweat carries sebum into the fabric during exercise, and from that point on, how the fabric handles that sebum determines how well bacteria can feed.

This is where fiber chemistry takes over.

Researchers at Aarhus University designed an in vitro model that mimics what happens when you sweat into fabric. It used artificial sweat mixed with real human sebum, inoculated with the five bacterial species most associated with textile odor and applied that lovely mixture to cotton and polyester swatches under controlled conditions. (Møllebjerg et al. 2021).

What they found was striking but makes complete sense from a materials science standpoint. --> Polyester absorbed significantly more sebum than cotton. That alone might not seem like a big deal, but the distribution of that sebum told the real story.

Using confocal laser microscopy with a lipid-specific fluorescent dye, the researchers visualized exactly where sebum ended up inside each fabric. On cotton, sebum formed isolated spherical droplets scattered throughout the textile meaning little pockets of oil not really absorbed on the fiber and separated from each other cellulose fiber. On polyester, which is more compatible with oily substances, sebum spread out into a thin, continuous coating along the entire length of each fiber, bridging the spaces between fibers and creating a uniform layer of bacterial food across every available surface.

Picture the difference like this: cotton serves sebum as individual appetizer plates at separate tables - no sharing. Polyester lays it out as an all-you-can-eat buffet line along all of the fiber.

The consequence for bacterial growth is straightforward. More sebum, spread more evenly, means more surface area where bacteria can access nutrients. Bacteria adhered to polyester had higher initial metabolic activity than those on cotton meaning they were eating more and metabolizing faster.

But here's the part that really pins it down. The same researchers ran a critical control experiment: they inoculated both fabrics with bacteria in a simple salt buffer — no sweat, no sebum. Just bacteria and fabric. Under those conditions, bacterial activity on polyester and cotton was statistically identical. No difference at all (Møllebjerg et al. 2021).

That's the key finding.
The natural fibers themselves aren't killing or preventing bacteria and polyester itself isn't what is promoting the faster growth of odor causing bacteria. It's the inherent properties of those fiber and the subsequent relationship of the fiber surface with sebum that creates the difference. Polyester's hydrophobic (and oleophilic ("oil-loving") surface has a chemical affinity for the lipids in sebum -- "like attracts like." Cotton's hydrophilic (water-loving) cellulose surface doesn't have that same attraction, so sebum doesn't spread, doesn't coat, and doesn't become as available.

Remove the sebum, and the playing field levels completely. Add it back, and polyester becomes the better restaurant.

This also helps explain why the Ghent University team found Micrococcus selectively enriched on polyester (short #2). Micrococci are known to be efficient metabolizers of a broad range of skin lipids. A fabric that provides more lipid fuel in a more accessible format is going to favor the species best equipped to use it. It's not that polyester has some mysterious property that attracts Micrococcus. It's that polyester creates a nutritional environment where the best lipid-metabolizers outcompete everyone else.

So, now we have two pieces of the puzzle: polyester selects for the smelliest bacteria (#2), and it does so by providing them with better access to their primary food source (#3).
But there's another side to the odor equation that has nothing to do with bacteria at all. The next piece of the puzzle is about what happens to the odor molecules after they're produced, and why some fabrics release them into the air while others trap them.

That bit is coming in #4.
Till then, let me know your thoughts or questions on any of this. Want to know more about confocal microscopy and how we can tell how oil spreads on a surface?

Also...is anyone else starting to feel like the word "sebum" should be in the same category as how some people feel about the word "moist" ? ...se-bum...

References: Møllebjerg, A. et al. (2021). The Bacterial Life Cycle in Textiles is Governed by Fiber Hydrophobicity. Microbiology Spectrum, 9(2), e01185-21.

Callewaert, C. et al. (2014). Microbial Odor Profile of Polyester and Cotton Clothes after a Fitness Session. Applied and Environmental Microbiology, 80(21), 6611–6619.

Let's start with the thing most people don't know: sweat doesn't smell. Fresh sweat is about 99% water, with some salts, amino acids, and a small amount of lactate and urea. If you collected it in a vial, you'd barely notice anything. The odor you associate with a hard workout isn't coming from what your body secretes. It's coming from what bacteria do with it.

Skin bacteria metabolize the organic compounds in sweat and sebum (the oily secretion from your skin's sebaceous glands) and they produce volatile fatty acids as metabolic byproducts. Those short-chain fatty acids range from acetic acid (vinegar) to isovaleric acid and those are what you're actually smelling. The musty, sour, sweaty notes that hit you when you open your gym bag? That's bacterial metabolism, not human perspiration.

So far, straightforward. Here's where it gets interesting.

You'd expect the bacteria on your shirt to be the same as the bacteria on your skin, after all, they transferred from one to the other during your workout, right? But when researchers at Ghent University collected T-shirts from 26 people after an hour-long spinning session and profiled the microbial communities using DNA fingerprinting (DGGE), they found something surprising: the textile microbiome was fundamentally different from the skin microbiome (Callewaert et al. 2014, Applied and Environmental Microbiology).

On your axillary skin (under arms), the dominant odor-producing bacterial family is Corynebacterium. These are the bacteria most responsible for the characteristic sharp, pungent body odor that develops in your armpits. They're well-adapted to the lipid-rich, low oxygen environment of the skin surface, and they're efficient at converting sweat precursors into the three main classes of bad-smelling underarm compounds: short branched-chain fatty acids, steroid derivatives, and sulfanylalkanols.

But Corynebacterium can't really thrive on fabric. In the Ghent study, corynebacteria couldn't be detected on any of the 26 T-shirts by DNA fingerprinting, couldn't be isolated by culture in petri dishes, and showed declining populations on every fabric type tested in controlled lab experiments — polyester, cotton, nylon, wool, viscose, acrylic, and fleece. Every single one. The species that dominates your armpit odor simply doesn't survive the transition to fabrics.

So, if it’s not corynebacteria smelling on your clothes, what's actually growing on your shirt?

The textile microbiome turned out to be dominated by Staphylococcus and Micrococcus species. Staphylococci were found on practically every shirt regardless of fabric type — they're generalists. However, Micrococcus showed a striking pattern: it appeared on nearly every synthetic shirt and was almost completely absent from 100% cotton shirts. This wasn't a subtle statistical trend. The DGGE banding patterns showed clear, visible preferential enrichment of micrococci on polyester and other synthetic fibers.

Why does this matter for odor? Because Micrococcus species are known to be potent odor generators. They can fully catabolize several kinds of fatty acids into volatile malodor compounds. They're related to corynebacteria and share some of the same metabolic toolkit for producing stink, but unlike corynebacteria, they thrive in the oxygen-rich, fiber-surface environment of clothing rather than the oily, oxygen-poor environment of underarm skin.

So, the picture that emerges is this: your shirt doesn't smell like your armpit, and it's not supposed to. The odor on your shirt is produced by a different microbial community that assembles itself based on the conditions the fabric provides. Your skin seeds the initial population during sweating, but the fabric environment selects which species flourish and which die off. That selection, which turns out to be heavily dependent on what your shirt is made of, is what determines whether your post-workout laundry smells merely lived-in or genuinely offensive.

That raises the obvious next question: if polyester and natural fibers can host similar total numbers of bacteria, why does polyester consistently smell so much worse?

That's coming in #2.
Before I finalize that, let me know what you think about this first piece in terms of length and depth of language so I can adjust - and of course, if you have any questions, let me know in the comments.

References: Callewaert, C. et al. (2014). Microbial Odor Profile of Polyester and Cotton Clothes after a Fitness Session. Applied and Environmental Microbiology, 80(21), 6611–6619.

 

Most people assume post-workout odor is just "sweat smell" --- "it's inevitable, the same on every fabric, and the only variable is how much you sweat."

It turns out, almost none of that is true. The bacteria on your gym shirt are entirely different species from the bacteria on your skin, and which ones thrive depends on molecular-level properties of the fiber they're living on.
So, why does a polyester shirt smell worse than a cotton one after the same workout, even when both carry similar amounts of bacteria? I'm a polymer scientist with a background in fiber chemistry and textile R&D, and that question is where this series starts.

There's a surprising amount of peer-reviewed research on this topic, and the real answers are more interesting than most of what circulates online. The science involves microbial ecology, surface chemistry, and some counterintuitive findings that are worth understanding-- especially if you're someone who cares about what you're putting against your skin every day.

Each post in this series covers one piece of the puzzle, explained in plain language with the studies cited so you can dig deeper if you want - or post a question and I'll respond. Here's what we'll work through:

#1 Are the bacteria on your shirt even the same ones on your skin?

#2 If two fabrics carry similar amounts of bacteria, why does one smell dramatically worse?

#3 What are textile bacteria actually eating, and why does one fiber type serve it up better?

#4 How do some fibers trap odor molecules while others let them fly?

#5 Why does your old gym shirt smell worse than your new one — even right out of the wash?

#6 Is there one physical property that ties all of this together?

#7 And what does all of this actually mean for choosing fabrics that work?

Each of these will be a short easy read (~3 minutes?) but super informative…and sometimes surprising. The first one is coming soon.

Spoiler for #1: the bacteria making your clothes smell aren't even the same species as the ones in your armpits!