Scanning Electron Microscope photograph of
Vibrio cholerae bacteria.
Credit: G. Knott, M. Blokesch, EPFL
What is not generally appreciated is that the coronavirus pandemic is not the only pandemic the human population of Earth is being subjected to. We are also in the midst of a cholera pandemic - the seventh such pandemic - caused by the bacterium
Vibrio cholerae, a waterborne pathogen that infects the gut of humans through contaminated water and food. When ingested,
V. cholerae colonizes the gut’s inner surface, causing a watery diarrhea, that if left untreated, can lead to severe dehydration and death. The form that infects humans differs from the 'wild' form in a number of ways, one of which is the low number of plasmids they carry, but quite why remained a mystery until now.
According the World Health Organization (WHO), the ongoing seventh cholera pandemic is responsible for up to four million infections, and up to 143,000 deaths each year.
Now, three researchers at the Laboratory of Molecular Microbiology, Global Health Institute, School of Life Sciences, Ecole Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland have discovered that the variant responsible for the current pandemic has been modified to make it better able to defend itself against viruses and ingression of genetic material from other bacteria via horizontal gene transfer of plasmids. Their paper was published a few days ago in
Nature
According to the EPFL new release:
Horizontal gene transfer
Only a few strains of V. cholerae can cause pandemic disease, with most being harmless aquatic organisms. This is because the pandemic strains have acquired specialized “toolboxes” of genes and other genetic elements called “pathogenicity islands”, which can turn the bacterium into a pathogen.
Strains that cause cholera pandemics have acquired pathogenicity islands through a process known as “horizontal gene transfer”, by which bacteria share genes both within and across species. Horizontal gene transfer is a powerful driver of bacterial evolution because it can quickly endow bacteria with new abilities that help them adapt and survive. But it is also indiscriminate, passing on genes that are unnecessary or even harmful to their new host.
Horizontal gene transfer often involves plasmids – self-replicating circular pieces of DNA found in bacteria that can carry up to hundreds of genes. But strains of V. cholerae that are causing the currently ongoing 7th pandemic of cholera only rarely carry plasmids while plasmids are abundant in related strains isolated from the environment instead of patients.
We wanted to find out why plasmids are so rare in the 7th pandemic clade of V. cholerae, shedding light on how bacterial pathogens evolve.
Professor Melanie Blokesch, lead author
Laboratory of Molecular Microbiology
Global Health Institute
School of Life Sciences
Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland
Mystery of the missing plasmids
First, the researchers introduced a small model plasmid into V. cholerae strains from the 6th and 7th pandemics, as well as non-pandemic strains isolated from different water bodies. They then tthracked the plasmid’s stability over the course of many generations. Surprisingly, the model plasmid persisted in all strains, but was quickly eliminated from the 7th pandemic ones.
Two DNA defense systems
Encouraged, the scientists used genetic engineering methods to identify the parts of the V. cholerae genome responsible for this loss. This approach led to the discovery of two novel defense systems that work together to eliminate plasmids, and are encoded within two distinct pathogenicity islands.
Publishing in Nature, the researchers named the systems ‘DNA defense modules’ (Ddm). The first one, DdmDE, is made up of two proteins that target and degrade small plasmids in a process helped by a second defense system, DdmABC.
This finding might explain why the recent pandemic strains mainly carry antibiotic resistance integrated in their genome and not on plasmids.
Professor Melanie Blokesch
The combined activity of these two defense systems solves the long-standing mystery of the missing plasmids in the 7th pandemic V. cholerae strains. Furthermore, our discovery suggests that the ability of the 7th pandemic strains to defend against mobile genetic elements has likely played a key role in their evolution and success.
The team also found that DdmABC targets large plasmids that often carry huge arrays of antibiotic-resistance genes, and can persist by jumping from one bacterium to the next, spreading multidrug resistance.
In summary then, it seems that the ability to pick up plasmids left around by other bacteria and thus acquire new traits is a two-edged sword because it can also insert harmful genes into the bacterium. The pathogenic form of
V. cholerae is sufficiently well adapted to living in the specialised environment in the human gut that, on balance, picking up more plasmids carries a greater risk of harm than it does of acquiring a benefit, so they have been modified to reject any such plasmids. The same mechanism also protects them against the phage viruses that abound in the human gut.
Of course, a biologist will explain this as the result of evolution, but a creationist is obliged by dogma to reject that explanation and instead attribute this to intelligent [sic] design by their favourite magic deity - which unfortunately leaves that deity looking responsible for making 4 million people sick and killing 143,000 of them every year, by designing a specialised form of
V. cholerae to do it with.
For some reason, the politically motivated extremist frauds behind the creationist movement would prefer their dupes to see their putative deity as some sort of evil, pestilential genius than to accept that the scientific method is the best tool we have for discovering the truth about the world. Could this be because the scientific method is also the best tool we have for exposing the fallacies that underpin their political ambitions, so they prefer an ignorant, superstitious and above all, fearful, population?
