Just under a year ago, a study made a splash among CF news and advocacy groups. It was a paper out of Southern California that linked the chemical gas 2,3-Butanedione to certain types of bacteria that are commonly grown in the lungs of CF patients. 2,3-Butanedione, also known as diacetyl, is a found in many types of foods, and is particularly known for providing the butter flavor in microwave popcorn. The study, authored by Katrine Whiteson, et al., measured breath gas metabolites in seven CF patients and one healthy control. They noted an apparent relationship between the levels of certain bacterial colonization and level of 2,3-Butanedione detected.
Diacetyl, as I mentioned, is a buttery flavored molecule that is a byproduct of fermentation. Anyone who has brewed beer is familiar with diacetyl, as it is usually (with some exception) considered a defect. In microwave popcorn (at least in some brands), it is used to add the artificial butter flavor. When the study made the rounds of CF news outlets, there was often mention of diacetyl being implicated as the culprit for a condition known as ‘popcorn lung’,(or more properly Bronchiolitis Obliterans) named so for the common affliction of lung damage seen in workers at microwave popcorn plants who are exposed to high levels of diacetyl in the factory over time. This sounded very alarming, and many people began to question if CF patients should even eat microwave popcorn in order to limit their exposure to this toxic chemical.
While it is true 2,3-Butanedione can cause damage to the lung tissue, there’s nothing to suggest that casual consumption of microwave popcorn carries any risk aside from it just being junk food. To achieve the kind of scarring talked about in popcorn lung, you’d probably have to open the bag of freshly popped popcorn and huff it on a continual basis. Another misunderstood interpretation implied that diacetyl caused pseudomonas to grow, which is not quite true. What the study does say is this (put on your science hats, things are about to get tricky): 2,3-Butanedione was more present in the lungs of the CF volunteers than in those of the healthy control. One of the patients’ breath gas detected lower levels of 2,3-Butanedione after being treated with IV antibiotics for an exacerbation. This means the the level of 2,3-Butanedione could serve as a biomarker for infection.
In CF lungs, the tissue pH is slightly lower than in healthy lungs due to lack of oxygen, or pockets with mucus plugs. It is there in those pockets some of the microbes are driven to respirate anaerobically creating an environment for fermentation. Fermentation lowers the pH further, and in order to avoid lethal acidification, 2,3-Butanedione is produced to bring the environment to a more neutral pH.
2,3-Butanedione has the ability to harm lung tissue directly by reacting with the immune system’s guanidinium groups such as those found in arginine side chains. However, as the authors cited, recent work has shown that 2,3-Butanedione also mediated cross feeding between fermenting bacteria and Pseudomonas a.
At low-oxygen environments, molecules called phenazines, which are produced by bacteria such as Pseudomonas and Streptomyces, and usually (when oxygen is present) create deadly oxygen radicals that act as antibiotics, instead act as alternative electron receptors and enables anaerobic respiration for other member of the microbial community. So, there’s a synergism happening, particularly with the phenazine called pyocynin produced by Pseudomonas a. that contributes to its ability to colonize the lungs.
The sputum samples in this study were dominated by P. aeruginosa, Streptococcus spp., and Rothia mucilaginosa, and through metagenomic testing, they were able to assert that the genes encoding 2,3-Butanedione biosynthesis were disproportionately abundant in Streptococcus spp, where as the genes for the consumption of Butanedione pathway products were encoded by P. aeruginosa and R. mucilaginosa. In other words, it’s possible that strep is helping to produce it, while pseudomonas and others are feeding off it.
So this tells us some form of metabolism is happening by anaerobic bacteria in the lungs caused by low oxygen and low pH environments. It also means that there’s a synergism happening where the triggered byproduct 2,3-Butanedione is acting as a carbon source, or signaling molecule with other bacteria.
The authors suggest that the amount of 2,3-Butanedione detected in breath gas metabolites could potentially be a biomarker for upcoming infections. Many patients and their loved ones have said they’ve been able to smell an infection coming on their breath. Being able to link breath gas metabolite to the products unique to microbial metabolism, and then using metagenomic sequencing learn specifically which bacteria are colonizing, could potentially lead to new technology strategies in attacking an exacerbation before it does irreversible damage.
This study was small and stressed the need for further research with larger sample sizes. Considering that 2,3-Butanedione gas detection levels were measured to be lower when antibiotics were in use, that could give us a clue if the reason has to do with smaller population of the producing bacteria, a change in the metabolism of the bacteria, or a consumption of this metabolites.
Understanding a causal relationship between the gas and the pathogens could potentially be very significant to lung health.
It should be said that 2,3-Butanedione is one of an abundance of gases taken from the airways of the lungs that could be different or cause changes in CF lungs, and at this point it is really hard to assess the significance of any one of them. However some interesting work has been done with diacetyl as far as microbial interactions in host insects, mammals, and plants that has shed some light on how these diacetyl and acetoin producing microbes evolved. In mosquitoes, for example, 2,3-Butanedione can disrupt their CO2 receptors, so they can no longer detect the exhaling breath of the life that is their next blood source. Knowing this, it could lead to the prevention of many types of mosquito borne illnesses, and potentially save thousands of lives.
This has nothing to do with CF, but it is, of course interesting. And, hey, next time you’re out camping, you can bring some pre-popped microwave popcorn instead of Deet to protect you from the mosquitoes. (No promises about bears though, they’d probably love the smell of diacetyl). On second thought, bring the Deet.
Whiteson, Katrine L.; Meinardi, Simone; Lim, Yan Wei; Schmieder, Robert;
Maughan, Heather; Quinn, Robert; Blake, Donald R.; Conrad, Douglas; and Rohwer, Forest. “Breath gas metabolites and bacterial metagenomes
from cystic fibrosis airways indicate active pH-neutral 2,3-butanedione fermentation,” ISME Journal(2014) Vol. 8. January 9, 2014.