Viruses are somehow even weirder than we thought. In a recent study published last month, scientists say they’ve discovered two new viruses that hijack the machinery of other bacteria-infecting viruses in order to replicate themselves. One of these vampiric viruses even acts the part, appearing to “bite” and latch onto their helper virus so that they can enter the bacteria at the same time.
In the simplest of terms, viruses are tiny packages of genetic material, either DNA or RNA. The key characteristic of viruses is that they can’t make more of themselves without the help of other organisms, with their hosts ranging from single-celled bacteria to our skin cells. Often this process isn’t great for the health of the host, which is why viruses are a common source of human disease. This inherently parasitic attribute is the major reason why scientists continue to debate whether viruses should be considered a form of life.
As it turns out, however, there are even bigger freeloaders out there in the microbial world. Satellite viruses have their own distinctive protein shell (called a capsid) and usually enter cells much like other viruses, but need another helper virus co-infecting that same cell to actually start their replication process. And satellite nucleic acids additionally use helpers to craft themselves the capsid and/or tail needed to escape their infected cell and spread further.
Scientists at the University of Maryland, Baltimore County, and elsewhere say they’ve now discovered two new satellite viruses that seem to have some unique features of their own. Their findings were published late last month in The ISME Journal.
According to the paper, the viruses were found in soil samples collected from Missouri and Maryland. They appear to be satellite viruses of bacteriophages—bacteria-infecting viruses—that target species of Streptomyces bacteria. The satellite-and-helper combinations have been named the Mulch and Flayer phage systems, respectively.
This discovery alone is interesting, since these would be the first satellite viruses known to rely on bacteriophages for replication (until now, only satellite nucleic acids have been found tethered to them). But it gets even freakier, since the Flayer system appears to feature some virus-on-virus nibbling.
Satellites typically have their own means of entering and staying inside a cell unnoticed while they wait for a helper virus to come around; often this entails integrating themselves into the host cell’s genome. But the flayer satellite (dubbed the MiniFlayer) seems to be lacking that ability. Instead, it has its own mini-tail that can attach to the “neck” of the helper virus (dubbed the MindFlayer). In most of the MindFlayer phages examined underneath a microscope, a MiniFlayer was found tightly bound to them, and even some of the free phages showed signs of stray fibers around the base of their tail, almost like “bite marks,” the researchers say.
Based on photographic evidence and the lack of another plausible explanation, the authors believe that the MiniFlayer uses this attachment to literally invade the host at the same time as their helper—which would be another novel discovery for this class of viruses.
“When I saw it, I was like, ‘I can’t believe this,’” said lead study author and UMBC biologist Tagide deCarvalho in a statement released by the university. “No one has ever seen a bacteriophage—or any other virus—attach to another virus.”
These discoveries, assuming the findings are validated by other scientists, would be the latest to expand our still limited knowledge of viruses and their evolutionary journey. And they were almost missed, since the initial analysis of the team’s phage samples flagged an unknown genetic sequence as likely contamination. Thankfully, they were able to ask deCarvalho for help, which allowed them to get a close look at the samples using a transmission electron microscope. The authors now hope they and others can further discover and better understand these strange viral passengers.