Tuesday, March 1, 2016

How bacteria and fungi can poison the air

One of the main reasons we study how bacteria and fungi work is to minimize their negative effects on our health. These effects usually stem from being munched on (in other words, an infection) and/or being damaged by a toxic substance (being poisoned). While poisonings due to bacteria and fungi predominantly occur either in association with infections (e.g. diphtheria and tetanus) or via eating contaminated food (e.g. botulinum toxin and aflatoxins) or a misidentified mushroom (e.g. amatoxins), it's also possible to become sick after breathing in harmful gases or aerosols produced by these groups of organisms.

Our cells can acquire energy by taking electrons from certain bits of organic carbon and transferring them via a series of steps to oxygen, producing water as a byproduct. Sulfur-reducing bacteria do something similar, but instead of oxygen they dump their electrons on sulfate and other electron-deficient forms of sulfur, producing hydrogen sulfide. It's a poisonous (to human cells), corrosive, flammable, AND explosive gas. Hydrogen sulfide lacks colour but is nevertheless detectable at very, very low concentrations because of its potent rotten egg odour. However, our noses quickly adjust to the smell, rendering it odourless.

There's an old saying in toxicology, which is the dose makes the poison. Hydrogen sulfide is no exception. It's present in our bodies in very small amounts, where it helps ensure our nerves and blood vessels work properly. Being exposed to low concentrations of the gas isn't too much of a problem, since we're equipped with enzymes able to detoxify limited amounts of it. However, at high concentrations, the gas is a lethal fast-acting poison. It acts much like hydrogen cyanide to prevent our cells from extracting energy from organic carbon using oxygen. Due to its effects on the central nervous system, it quickly knocks people unconscious. Thereafter, death typically occurs as a result of the gas suppressing the region of the brain where breathing is controlled (central respiratory arrest).

Hydrogen sulfide is heavier than air, so it can accumulate in low-lying places with poor airflow. The gas tends to show up wherever you have sulfur-reducing bacteria, lots of organic carbon and sulfur (for the bacteria to live off of), no oxygen (otherwise other bacteria would take over), and warm conditions (>20 °C) (to ensure appreciable bacterial growth). These conditions can be found in outhouses, sewers, septic tanks, and manure pits, particularly in the summertime. People can be fatally poisoned by hydrogen sulfide if they enter these confined and poorly ventilated spaces without their own separate air supply.

The harmful nature of hydrogen sulfide was reported as early as 1700 by the Italian physician Bernardo Ramazzini, who noted cases of poisoning among those employed to clean bacteria-filled cesspits and latrines. During the 18th and 19th centuries, a number of people working in the sewers of Paris died when they were exposed to hydrogen sulfide. Victor Hugo described how dangerous the situation was in Les Misérables (1862), even mentioning sulphuretted hydrogen (an old way of saying hydrogen sulfide) by name.

A section of the sewers beneath Paris (Source)

Another airborne poison worth mentioning is clouds of fungal spores. Many of the fungi hiding out as networks of threads inside plants and soils reproduce by creating mushrooms, which serve to spread their spores (seeds) around. In the case of puffballs and earthstars, pushing on them causes a large number of spores (we're talking billions to trillions) to be shot into the air through a central hole. It looks a bit like smoke. The push can happen via falling raindrops or a trampling foot. Puffballs and earthstars often grow in dense groups, so a lot of spores can end up in the air all at once.

Animals who breathe in a bunch of fungal spores can end up with a nasty lung-based illness known as hypersensitivity pneumonitis. Essentially, the spores trick the immune system into overreacting, causing a damaging inflammation deep within the lungs. Lycoperdonosis is a form of hypersensitivity pneumonitis specifically caused by inhaling spores from puffballs of the genus Lycoperdon.

White blood cells (indicating inflammation) and a fungal spore (arrow) from the lung of a dog (Source)

In one particularly unfortunate case, a group of eight apparently sober teenagers decided it would be a good idea to inhale the spores squeezed from some puffballs they had acquired. After a couple of days, they started to cough, had trouble breathing, ran a fever, and felt generally crappy. Things got so bad that five of the teens were hospitalized, two of them needing to be intubated to help them breathe. Their inflamed lungs were treated with corticosteroids and they all eventually recovered from their illness.

Lycoperdonosis has also been seen in people who snorted puffball spores as a folk remedy for a nosebleed. Both puffballs and earthstars have killed or seriously injured dogs who disturbed them and inhaled their spores as they ran about or dug holes.


Alenghat T et al. 2010. Lycoperdonosis in two dogs. Journal of Veterinary Diagnostic Investigation 22(6):1002-1005. [Full text]

Nogué S, Pou R, Fernández J, Sanz-Gallén P. 2011. Fatal hydrogen sulphide poisoning in unconfined spaces. Occupational Medicine 61(3):212-214. [Full text]

Policastro MA, Otten EJ. 2007. Case files of the University of Cincinnati fellowship in medical toxicology: Two patients with acute lethal occupational exposure to hydrogen sulfide. Journal of Medical Toxicology 3(2):73-81. [Full text]

Whitney J, Beijerink N, Martin P, Talbot J, Barrs V. 2013. Hypersensitivity pneumonitis in a dog associated with Geastrum triplex spores. Medical Mycology Case Reports 2:122-124. [Full text]


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