Enterococci are hardy microbes that thrive in the gastrointestinal tracts of nearly all land animals, including our own, and generally cause no harm. So the discovery of a botulinum neurotoxin-like toxin in a strain of Enterococcus called E. faecium is raising scientific eyebrows.
Dubbed BoNT/En, the new toxin was found in an Enterococcus faecium strain isolated from cow feces in South Carolina. It is the ninth botulinum toxin to be described.
“This is the first time a active botulinum neurotoxin has been found outside of Clostridium botulinum and its relatives — and not just the toxin, but an entire unit containing the toxin and associated proteins that prevent the toxin from being degraded in the gastrointestinal tract,” said lead author Dr. Min Dong, from Boston Children’s Hospital and Harvard Medical School.
“Its discovery has implications in several fields, from monitoring the emergence of new pathogens to the development of new protein therapeutics — it’s a game changer,” added co-author Professor Andrew Doxey, from the University of Waterloo.
“This is a unique discovery of a botulinum neurotoxin in a bacterium that is both ubiquitous in animals and a serious problem in human health,” said co-author Dr. Francois Lebreton, from the Massachusetts Eye and Ear Infirmary, Harvard Medical School, and the Broad Institute of MIT and Harvard.
“Enterococcus faecium is in the gut of nearly every human; it is extremely tough and survives a lot of stresses, often including efforts to disinfect hospital surfaces. A hospital-adapted, antibiotic-resistant, hard-to-kill bug carrying a neurotoxin would be a worst-case scenario.”
The team sequenced the BoNT/En-producing Enterococcus faecium strain as part of a much wider search for the origins of enterococcal antibiotic resistance and disease-causing ability.
“We were not looking for a neurotoxin in Enterococcus faecium. There was no reason to suspect its existence,” Dr. Lebreton noted.
The newly sequenced Enterococcus faecium genome was then run through computer programs, which found the gene for botulinum toxin in the bacterial strain.
“The way that we discovered this toxin using computational methods is different from how toxins used to be identified in the past, and may become a standard approach in biomonitoring,” Dr. Doxey said.
“It represents scientific collaboration and data sharing at its best.”
The researchers concluded that BoNT/En was likely transferred from Clostridium botulinum bacteria in the environment into Enterococcus faecium in the cow’s gut, showing that the toxin can be transferred between very different species.
“Should we be scared? No, at least not yet,” said co-author Dr. Sicai Zhang, from Boston Children’s Hospital.
“The enterococcal isolate carrying the toxin luckily remains susceptible to key antibiotics. It was found only once from a single animal, and no signs of botulism disease were observed.”
When the scientists tested BoNT/En in rodents in the lab, it had little or no effect. Only when they manipulated the toxin to better target mouse and rat neurons did it become potent.
“The botulinum toxin is a powerful and versatile protein therapeutic,” said co-author Dr. Michael Mansfield, from the University of Waterloo.
“By finding more versions of the toxin in nature, we can potentially expand and optimize its therapeutic applications even further.”
The team’s results appear in the journal Cell Host and Microbe.
Sicai Zhang et al. Identification of a Botulinum Neurotoxin-like Toxin in a Commensal Strain of Enterococcus faecium. Cell Host and Microbe, published online January 27, 2018; doi: 10.1016/j.chom.2017.12.018