Computer illustration of the mimivirus particle. Credit Jose Antonio Penas.

Mimiviruses are viruses so big they can actually be seen with a simple light microscope. European scientists have now learned that these bizarre organisms have their own immune system that makes them virtually invulnerable to predatory viruses, suggesting these creatures may actually represent a new branch in the tree of life.


Mimiviruses, which measure just a micrometer across, were first discovered infecting amoebae. These microscopic creatures have more genes than some bacteria, but they’re more closely related to viruses like smallpox. But unlike most viruses, they can actually make amino acids, DNA letters, and complex proteins. In this sense, they more closely resemble prokaryotes, which are microbes that lack nuclei.

As Nature News now reports, these organisms have another unique biological feature: They have their own working immune system. In this case, they’ve evolved a way to combat a virophage known as Zamilon that’s known to infect prokaryotes. These infections eliminate the mimivirus’s ability to replicate itself, so to fight back, it evolved a defense system based on the same principle as CRISPR—a DNA cut-and-paste tool currently in use by geneticists to modify organisms. Ewen Callaway explains in Nature News:

In bacteria and another kind of prokaryote, called archaea, CRISPR systems store a library of short DNA sequences that match those of phages and other invading DNA. When a foreign DNA sequence with matching sequences in this library attacks a cell, specialized ‘Cas’ enzymes unwind the intruder DNA and chop it into pieces, stopping an infection. Biologists have now repurposed CRISPR as a technology to edit genomes.


Incredibly, this system works similarly to antibodies; as Carl Zimmer noted in STAT,

When a virus infects bacteria, the bacteria capture bits of its genetic material and lodge it in their own DNA. Later, they can consult this library of captured DNA to recognize invading viruses. That recognition lets them guide enzymes to the viruses and destroy their DNA while sparing their own.

The researchers were able to identify the presence of this immune system, dubbed MIMIVIRE (short for mimivirus virophage-resistance element), in mimiviruses by analyzing the genomes of 60 mimivirus strains.

According to lead researcher Didier Raoult from Aix-Marseille University in France, mimiviruses represent a fourth domain of life, alongside bacteria, archaea, and eukaryotes. The presence of the MIMIVIRE defence system, says Raoult, is an ancient adaptation that warrants its own branch on the tree of life. The details of this work can now be found in Nature.


Correction: An earlier version of this article stated that mimiviruses can be seen with the naked eye. Mimiviruses are about one micrometer, which is not visible by the human eye (the human eye’s resolution is about 100 micrometers). They are, however, visible by a light microscope, which is unusual for a virus (most viruses require an electron microscope to be seen).

[Nature News]