Tailless DNA-containing bacteriophages could not be detected by standard methods - but now scientists believe that they dominate the world's oceans.
Each drop of water not too deep from the sea surface contains about 10 million viruses. It is hardly surprising that biologists are constantly discovering new ones, although the discovery of a whole new group is still very rare. It seems that standard techniques simply did not allow them to be noticed, although such viruses turned out to be extremely numerous - perhaps they are widespread not only in the ocean and simply eluded scientists.
Martin Polz of the Massachusetts Institute of Technology and his colleagues examined water samples taken off the North American coast of the Atlantic Ocean. They looked for viruses containing double-stranded DNA - such viruses are extremely widespread, including herpes, and the bacteriophage T4 (Caudovirales) familiar to everyone from school. Infecting cultures of marine bacteria Vibrionaceae, scientists discovered 200 different DNA viruses, of which 18 were attributed to the new order Autolykiviridae. They write about this in an article published in the journal Nature.
The group's name refers to the hero of Greek mythology Autolycus, a cunning thief who is as difficult to catch as these viruses. “The most common viruses on Earth are bacteria-infecting viruses containing double-stranded DNA,” write Poltz and his co-authors. “However, the Caudovirales with tailed viruses, which dominate the databases and collections of crops, are not representative of their presence in the environment. In fact, tailless viruses dominate the water samples."
Bacterial Cells Infected with Autolykiviridae / MIT Phages, Kauffman et al., 2018.
"Tailless" bacteriophages Autolykiviridae are distinguished by a small genome size - only about 10 thousand bases, 4-5 times less than in "tailed" relatives. Their ability to infect and kill bacterial cells was also unusual. If ordinary phages in experiments, as a rule, affected one specific species of vibrios, these infected on average four and led to a noticeably higher cell death. "They were found to be responsible for 40 percent of bacterial deaths, although they accounted for about 10 percent of the total number of viruses in the sample," explains one of the authors of the work.
Vibrio bacteria of different species are located along the perimeter of the diagram; the circles of viruses infecting them are associated with them: blue - "tailed" bacteriophages, orange - "tailless" Autolykiviridae / MIT, Kauffman et al., 2018.
These figures indicate the great role that the new group Autolykiviridae plays in the regulation of bacterial populations and in the biosphere as a whole. Scientists are confident that their technique will help find viruses that have previously escaped detection in the soil, and possibly in the human intestine. Similar genetic sequences are indeed found in the genomes of bacteria in the human microflora.
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Sergey Vasiliev