Bacteria-killing virus essential to marine ecosystems may provide insights for understanding the human gut

Bacteria and viruses are microscopic organisms that are found practically everywhere on the planet. Humans even have bacteria thriving inside their bodies. Despite their abundance, many of these microorganisms are yet to be discovered, much less studied. Now, researchers from the Massachusetts Institute of Technology (MIT) and the Albert Einstein College of Medicine have recently discovered an elusive family of viruses that have escaped scientific analysis due to their characteristics.

This previously unknown family of viruses thriving in the world’s marine habitats may help shed some light on how microorganisms maintain the ocean’s ecology, and may play an important role in understanding the human microbiome, as reported by a new study published in Nature on January 24, 2018.

In conducting the new study titled, “A major lineage of non-tailed dsDNA viruses as unrecognized killers of marine bacteria,” lead author Kathryn M. Kauffman of MIT collected daily water samples off the Massachusetts coast for three months. Viruses found in the samples were incubated for two weeks in cultures of Vibrionaceae, a common family of marine bacteria. For the analysis, the scientists randomly selected more than 200 viruses that successfully infected and multiplied in the Vibrionaceae bacteria. Out of these viruses, 18 were found to belong to a new family of small, non-tailed dsDNA (double-stranded DNA) viruses.

The elusive Autolykiviridae

The researchers named the new family of viruses Autolykiviridae, after Autolycus, a character in Greek mythology known for being an elusive trickster.

To identify bacterial viruses like Autolykiviridae, scientists typically look at the behavior and overall body shape of known viruses, most of which have distinctive “tails.” With Autolykiviridae being non-tailed dsDNA viruses, the scientists observed during sequencing that these viruses have significantly shorter genomes than other viruses. Autolykiviridae were found to have about 10,000 bases, compared to the typical 40,000 to 50,000 for tailed viruses.

This might explain why they have been overlooked in previous studies of ocean viruses, Kauffman said according to a Live Science report.

In experiments involving more than 300 strains of Vibrionaceae bacteria, the researches found that the Autolykiviridae viruses preyed on many more strains of Vibrionaceae than the tailed viruses in the water samples.

“They caused about 40 percent of the bacterial killing observed, despite comprising just 10 percent of the viruses that we isolated,” said study co-leader Libusha Kelly, Ph.D., assistant professor of Systems & Computational Biology and of Microbiology & Immunology at Einstein College.

This suggests that viruses related to Autolykiviridae may be key players in the predator-prey balance in marine ecosystems as they can infect many diverse groups of ocean bacteria, and may even prey on other currently unidentified bacterial groups, according to Dr. Kelly.

The study findings revealed that Autolykiviridae were members of an ancient viral lineage that possess specific types of capsids – the protein shell encasing the viral DNA. These viruses were commonly observed to infect animals and single-celled organisms, but not bacteria.

Dr. Kelly believes that the presence of Autolykiviridae-like viruses is not limited to the oceans. “We’ve found related viral sequences in the gut microbiome,” she said. This significant finding may give way to further studies to determine how these related viruses may influence microbial communities in the human gut or how they may relate to health.

Tailed and non-tailed viruses

The most abundant viruses in the oceans (and on the entire planet) are double-stranded DNA (dsDNA) viruses, which comprise of tailed and non-tailed varieties. The tailed viruses have been extensively analyzed in studies, while the non-tailed viruses are less well-understood mainly because they are more difficult to culture.

Current environmental models of virus-bacteria interactions are based on the well-studied tailed viruses, since their tails are used to infect bacteria. The non-tailed viruses were shown to act more slowly – and the killed-off regions only show up after several days have passed – so their presence was never noticed in most studies. The new study on Autolykiviridae may open doors to more research focused on elusive non-tailed viruses and their roles in marine ecology.

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