What’s that smell?

Did you know that bacteria make us humans and other animals smell the way we do? On top of that, animals use that smell to communicate with each other.

The “fermentation hypothesis of chemical recognition” says that bacteria in the scent glands of mammals generate metabolites with specific odors that animals use to communicate with each other. What’s more, this hypothesis explains how variations in these chemical signals are actually due to variations in those populations of bacteria.

Let’s start with humans, Corynebacterium is responsible for that distinct “body odor” that emanates from our armpits. That odor is caused by 3-methyl-2-hexenoic acid (3M2H) and 3-hydroxy-3-methylhexanoic acid (HMHA). It turns out that the precursors for these chemicals are cleaved by a zinc-dependent bacterial aminoacylase.

In one study, researchers isolated bacteria from the human axilla (a fancy word for armpit). They identified 19 strains of Corynebacterium and 25 strains of Staphylococcus. Curiously, only isolates of Corynebacterium, not Staphylococcus, produced 3M2H or HMHA from the precursors 3M2H-Gln or HMHA-Gln, indicating that these strains produced an Nα-acylglutamine aminoacylase.

So, what’s the use of human body odor? It’s safe to say that most people think body odor smells bad – it’s definitely not attractive to other humans. Some researchers think that may be the point – or at least it was at some point in our evolution. Body odor may have been used by early humans as a way to assert dominance or repel rivals.
Things are a little more clear cut when it comes to mice, however. Mice use odors to attract mates of the same species and to repel rats. One of these odor chemicals is trimethylamine, which is specific for the olfactory receptor TAAR5. Trimethylamine synthesis requires two steps, one of which involves bacteria. We’ve all smelled trimethylamine before, it’s the stinky smell of bad breath and spoiled food!

In humans, trimethylamine is the byproduct of bacteria metabolizing dietary choline, and researchers wanted to know if the same held true for mice. To figure things out, they collected urine from mice that were fed a choline/methionine-free diet or that were treated with an antibiotic. Sure enough, these mice produced less trimethylamine in their urine than control mice.

Likewise, urine from wild type mice contained trimethylamine that activated its receptor TAAR5 (assayed with a reporter gene), but urine from mice on the choline-free diet or treated with antibiotics did not activate TAAR5. These findings suggest that bacteria produce trimethylamine from dietary choline.

While trimethylamine is produced by commensal bacteria, pathogens produce their own array of odors. Mice have receptors in their vomeronasal organ that recognize formylated peptides that are produced by tissue damage or bacterial infection. There are also chemosensory cells in the respiratory epithelium that detect bacterial quorum sensing molecules called acyl-homoserine lactones.

Next are the meerkats. These members of the mongoose family are native to South Africa. They produce a smelly paste from a pouch beneath their tails. They use this paste to mark their territories, applying it to plants, rocks, and their meerkat pals. Researchers found over 1,000 types of bacteria and some 220 odorous chemicals in the stinky paste. The key finding was that specific odors are produced by specific microbial communities – a specific meerkat family smells the way it does because of its own specific microbes.

The group detected five main phyla of bacteria in the meerkats’ anal pouch – Proteobacteria, Firmicutes, Bacteroidetes, Actinobacteria, and Fusobacteria. Specific genera of bacteria found in the paste were Porphyromonas, Fusobacterium, Anaerococcus, and Campylobacter.

The researchers also found that subordinate and dominant male meerkats had different bacterial communities in their pouches. However, there were no significant differences among dominant and subordinate females.

Finally, like meerkats, hyenas, deposit a scented paste from anal scent pouches to communicate with other hyenas. In one study, researchers used scanning electron microscopy to look for bacteria in the scent pouches of both spotted and striped hyenas (two groups that differ rather significantly in their lifestyles and behavior).

They found that the bacterial communities differed between spotted and striped hyenas, but all communities were largely made up of fermentative anaerobes – specifically, species from the order Clostridiales. For the spotted hyenas, they identified Clostridiales from the genera Anaerococcus, Clostridium, Fastidiosipila, Finegoldia, Murdochiella, Peptoniphilus, and Tissierella. The bacterial communities differed, however, based on sex and female reproductive status.
Next time you reach for your deodorant, remember all those little bacteria living in your armpits!

The gut Microbiome can Influence the Brain

You may have heard of the microbiome, the community of microbes that lives in our gastrointestinal tract, as it is getting a lot of research attention since genetic techniques advanced in recent years. That gave researchers the power to assess what species were living in our guts and how they were influencing our health. It turns out that they are having a huge impact, and while that is not surprising, it has been interesting to find that they may also be affect our moods and behaviors.

The phenomenon even has a name – the gut-brain axis. There are nerves that go directly from your gut, which has an extensive nervous system of its own, to your brain. The brain is protected from the rest of the body by the blood-brain barrier, so it had been simply assumed that microbes in the gut had no effect on the brain. But the microbes actually end up producing an effect that does transmit to the brain, through the molecules they release. The breakthrough researchers made in that area which showed how microbes in the gut did indeed cause a reaction in the brain, although it’s still not known exactly how that happens, and additional research has indicated that the relationship is quite complex.

Scientists have gone on to show that transplanting the microbiome of one mouse into another can induce behavioral changes.

Fungus can Easily Release Toxins Into the air

It’s known that some common molds can be harmful to human health; researchers have confirmed that several kinds of fungi that grow indoors on wallpaper produce toxins that can easily go airborne. When they are present in the air, people just inhale them, at which point they can cause illness. Some of these fungi, which can cause so-called sick building syndrome, are described in the video below, and the new work has been published in Applied and Environmental Microbiology.
“We demonstrated that mycotoxins could be transferred from a moldy material to air, under conditions that may be encountered in buildings,” explained the corresponding author of the work, Jean-Denis Bailly, DVM, PhD, a Professor of Food Hygiene, at the National Veterinary School in Toulouse, France. “Thus, mycotoxins can be inhaled and should be investigated as parameters of indoor air quality, especially in homes with visible fungal contamination.”

The investigators wanted to gather more data on how mycotoxins from indoor fungi present a threat to human health. For this work, the scientists created a model environment in which the flow of air over contaminated wallpaper was modeled, and the speed and direction of the airflow could be controlled by researchers. They then assayed the bioaerosols generated in the experiment.

“Most of the airborne toxins are likely to be located on fungal spores, but we also demonstrated that part of the toxic load was found on very small particles – dust or tiny fragments of wallpaper – that could be easily inhaled,” noted Bailly.
Three species of fungus were used in this study: Aspergillus versicolor, Penicillium brevicompactum, and Stachybotrys chartarum. These species are well-characterized, as they are known to be common food contaminants. In addition, they “are frequent indoor contaminants,” said Bailly. He explained that they synthesize different mycotoxins. Fungi have mycelia, which are projections that function to harvest nutrients from their environment. The mycelia of these various fungi are also different, which could be leading to the variation in mycotoxins.

There are new questions raised by this research, said Bailly. “There is almost no data on toxicity of mycotoxins following inhalation,” he noted, likely because most research has investigated how they contaminate food. These different fungi also appear to contaminate air at different rates, releasing varied levels of mycotoxins.

Bailly suggested that the demand for homes that are very energy efficient could be aggravating this problem. Such homes “are strongly isolated from the outside to save energy,” while appliances that use water, like coffee makers “could lead to favorable conditions for fungal growth,” he explained.

“The presence of mycotoxins in indoors should be taken into consideration as an important parameter of air quality,” concluded Bailly.