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Scientists Make A Giant Virus Discovery
In 1892, the first virus was discovered. The tobacco mosaic virus was found by Dmitri Ivanovsky, who had set out to determine the cause of a disease plaguing tobacco plants across Europe. It wasn’t until the late 1930s that scientists, using a technique called electron microscopy, discovered the size of the virus: approximately 300 nanometers in length and 18 nanometers in diameter. While the tobacco mosaic virus would turn out to be a relatively small virus, for years, most viruses were found to be tiny, usually under 400 nm.
But, recently scientists at the University of Vienna have recently discovered a giant virus that infects a deadly human parasite. The parasite Naegleria fowleri, often called the brain-eating amoeba, is notorious for causing a rare but fatal brain infection. The parasite is found in warm freshwater and can be extremely difficult to treat, with a fatality rate in humans of 98%.
The virus, which was given the name Catovirus naegleriensis, belonging to the Mimiviridae family of other similarly large, infectious viruses, was isolated from a wastewater treatment plant in Klosterneuburg, Austria. Researchers found that the virus’s genome was surprisingly large, with a genome size of 1,163,307 nucleotides, larger than some bacterial species like the human pathogen mycoplasma.
While Catovirus naegleriensis infects several different Naegleria species, it was initially found by researchers replicating in an amoeba called Naegleria clarki. This amoeba is often found in freshwater environments, it is harmless to humans and instead feasts on bacteria.
When the researchers took a closer look at the giant virus genome, they found many typical features of the Mimiviridae family, including genes for replication, transcription, and translation.
Catovirus naegleriensis also had typical capsid and core proteins. Capsid proteins are what give the virus its structure; they keep the virus’s genetic material safe and contained. While core proteins are found inside the virus’s shell and help with virus replication.
Researchers found that the virus had genes that might be involved in virus defense. Virophages are viruses that can infect other viruses. Catovirus naegleriensis viral defense genes could be used to defend against virophage attacks, and help them to survive and proliferate within their amoeba hosts.
The researchers next wanted to look at how Catovirus naegleriensis infects the amoebal host Naegleria clarki. Once the amoeba engulfs the giant virus, a process called phagocytosis, the virus makes something called viral factories, which are specialized structures that viruses use to replicate their genetic material and assemble new viral particles. Within 8 hours of the giant virus’s initial infection, it was able to lyse the amoeba, releasing new viral offspring.
Viruses often acquire genetic elements from their host, and this giant virus was no exception. The genome of Catovirus naegleriensis had 37 genes that were very similar to protist genes, 16 of which were not related to the viral host species, Showing that while the giant virus does infect Naegleria, it may also infect other protist species.
Naegleria amoeba can be very diverse, and different species thrive in very different temperatures, but it was unknown if the giant virus could survive in these different environments. Researchers tested the virus’s ability to infect a range of Naegleria species and found that the virus could infect every species of Naegleria that they tested, including Naegleria fowleri, the deadly human pathogen. However, the virus could not infect other amoeba species, indicating the virus is likely pretty specialized to infecting Naegleria.
Catovirus naegleriensis’ ability to infect the lethal Naegleria fowleri could make it a promising biocontrol agent and offer a promising new avenue to reduce populations of the brain-eating amoeba. For decades, scientists have believed viruses were tiny and compact; however, with more discoveries of giant viruses, we are learning viruses may be more diverse than we think.
Link to the original post: Arthofer, P., Panhölzl, F., Delafont, V. et al. A giant virus infecting the amoeboflagellate Naegleria. Nat Commun 15, 3307 (2024).
Featured image: Image made by the author with Canva.com
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