Evolutionarily, horizontal gene transfer between bacteria is generally not so groundbreaking. In this case however, it’s a little more newsworthy. A lot of bacteria carry plasmids, which are mobile little DNA structures that are not part of the bacterial chromosome and can be transferred to other bacteria. Often plasmids contain genes that may provide some selective advantage (otherwise why bother keeping them?), such as a toxin or an antibiotic resistance gene. Enterococcus happens to be frequent plasmid-carrier, which makes it an especially good candidate for frequent horizontal gene transfer. But first, Clostridium.
Clostridum is an especially fun group of bacteria including some really famous germs:
- C. tetani (Tetanus),
- C. difficile (the bacteria in your gut that gives you stomach issues after you take antibiotics as it swarms in on all the spaces other bacteria used to be, before they were killed by antibiotics),
- C. perfrinigens (perhaps the nastiest of all, gas gangrene),
- the less famous C. sordelli (which is a rare–but plenty horrific, pregnancy and abortion-associated infection almost always resulting in death by toxic shock syndrome)
- And of course, C. botulinum, the primary culprit of botulism.
What is Botulinum toxin?
C. botulium is known for releasing botulinum toxin (BoNT), a toxin which interferes with the release of the neurotransmitter—acetylcholine, from axon terminals at the neuromuscular junction resulting in flaccid paralysis (you can’t breathe).
The toxin (a protein) works by binding to receptors at the neuromuscular junction. It is then taken into the cell by receptor-mediated endocytosis. A conformational change occurs inside the early endosome (due to a pH change). A piece of the protein (the light chain) is translocated to the cytosol when another piece of the protein (the heavy chain) forms a sort of channel. This light chain can then cleave these important complex-forming “SNARE” proteins, which are vital to normal vesicular transport and neurotransmitter (which in this case is acetylcholine) release. So without these SNARE proteins, the little acetylcholine vesicles don’t get to where they need to be. In normal neurotransmitter release, SNARE proteins would work to tether the vesicles to the membrane so the cell and vesicle membranes could fuse to allow the release of neurotransmitters.
Botulinum toxin is the deadliest toxin known requiring just two billionths of a gram to kill someone person. It’s so dangerous that when type H (one serotype of the toxin) was discovered and failed to be controlled by typical botulinum antibodies, the type H toxin became the only example of a genetic sequence that was hidden from public databases due to security concerns. I say “may be” because how would we really know?
Not just in C. botulinum anymore
While we used to take comfort in knowing we could probably avoid Clostridium botulium by just avoiding puffed up canned food, rotten meat, and organic honey, that may no longer be the case. For the first time, the botulinum toxin has been found in a completely different (and very distantly related) bacteria: Enterococcus.
This new variant of the toxin called BoNT/En, was found in South Carolina, in an E. faecium strain isolated from cows. And it wasn’t just the toxin that was found. Several associated proteins that prevent the toxin from being degraded were also found. That being said, the BoNT/En variant didn’t give the cow it was found in, botulism, and it was not actually very harmful initially in mice. Researchers had to manipulate the toxin to better target mice before it was able to actually kill them (I know. Trying to give mice botulism is upsetting, and the reason why I stick to cells and microbes for my research). They are testing the toxin on human neurons to see how toxic it is to humans.
What makes this particular jump so terrifying is that Enterococcus is a highly abundant group of bacteria all over our bodies. It is ubiquitous in animals and actually a significant human pathogen. E. faecium is a very hardy bacterium easily evolving antibiotic-resistance and actually pretty difficult to get rid of in hospital surfaces. The thought of a dangerous neurotoxin being easily transferred to a ubiquitous and antibiotic-resistant bacteria is somewhat frightening. That being said, plasmids containing incredibly dangerous neurotoxins would likely need to provide a selective advantage to the host to stick around (and it’s possible in E. faecium’s case, it could even provide a slight disadvantage). BoNT can be carried in a bacteriophage, in a plasmid, or on a chromosome, but carrying a sizable gene cluster you don’t need in evolution has usually resulted in a “you don’t use it, you lose it” outcome. As plasmids by themselves are slightly disadvantageous to carry, what keeps them around is any selective advantage the genes they contain provide.
Where did these toxins come from?
Researchers found that the homolog of BoNT in Clostridium, seemed to be a flagellin gene. These flagellins contain collegenase which breaks down peptide bonds in collagen, so they are also a proteolytic toxin family like the Clostridia neurotoxins. They believe that the neurotoxin and adjacent genes, evolved from an ancestral collagenase-like gene cluster. This gene cluster was likely duplicated and evolved separately to become the BoNT we know today.
My case against Botox: amazing, but maybe take it easy?
Botox can be useful for treating cerebral palsy, urinary incontinence, wrinkles, sweating, spasms, headaches, and tinnitus. While no one’s reportedly died from cosmetic Botox use (presumably because people are probably extra careful with such a terrifying substance), there’s a risk of the toxin spreading beyond the injection site and causing respiratory paralysis. While ordinarily I’m totally accepting of people’s desire to do strange or expensive things in the name of beauty, my (probably illogical) fear is that all it takes is one new guy thinking a smudged decimal point is a comma and you’re dead of flaccid paralysis.
Consider that a muscle with a cut nerve will quickly result in atrophy which will definitely make you look worse, not better. Not using your face muscles isn’t going to prevent you from wrinkling because the issue is not the muscles, but the soft tissue decline that comes with age. Neuromodulators like Botox are very expensive, usually require a high enough dose to actually be effective and wear off in a few months, meaning you would have to get several injections a year to stay smooth and shiny, and the repeated injections are when people tend to see more negative side effects and require a higher dose.
Paralyzing muscles over and over again is not great for you long-term so maybe try sunscreen, a nice skincare routine, and drink plenty of water instead? Mistakes happen, people misplace decimal points, I’d rather not be near it personally. Just a friendly PSA to remind you to definitely do your research if you want to mess around with botulinum toxin!
- Sicai Zhang et al. Identification of a Botulinum Neurotoxin-like Toxin in a Commensal Strain of Enterococcus faecium. Cell Host and Microbe, 2018
- P.K. Nigam, Anjana Nigam. Botulinum Toxin. Indian Journal of Dermatology. 2010 Jan-Mar; 55(1): 8–14.
- Doxey AC, Lynch MD, Müller KM, Meiering EM, McConkey BJ. Insights into the evolutionary origins of clostridial neurotoxins from analysis of the Clostridium botulinum strain A neurotoxin gene cluster. 2008.