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Scientists discover largest bacteria-eating virus. It blurs line between living and nonliving.

An illustration of bacteriophages invading a bacteria.
(Image: © Shutterstock)

Huge bacteria-killing viruses lurk in ecosystems around the world from hot springs to freshwater lakes and rivers. Now, a group of researchers has discovered some of these so-called bacteriophages that are so large and so complex that they blur the line between living and nonliving, according to new findings.

Bacteriophages, or "phages" for short, are viruses that specifically infect bacteria. Phages and other viruses are not considered living organisms because they can't carry out biological processes without the help and cellular machinery of another organism.

That doesn't mean they are innocuous: Phages are major drivers of ecosystem change because they prey on populations of bacteria, alter their metabolism, spread antibiotic resistance and carry compounds that cause disease in animals and humans, according to the researchers in a new study, published Feb. 12 in the journal Nature

Related: Going Viral: 6 new findings about viruses

To learn more about these sneaky invaders, the researchers searched through a DNA database that they created from samples they and their colleagues collected from nearly 30 different environments around the world, ranging from the guts of people and Alaskan moose to a South African bioreactor and a Tibetan hot spring, according to a statement.

From that DNA, they discovered 351 huge phages that had genomes four or more times larger than the average genome of phages. Among those was the largest phage found to date with a genome of 735,000 base pairs — the pairs of nucleotides that make up the rungs of the DNA molecule's "ladder" structure — or nearly 15 times larger than the average phage. (The human genome contains about 3 billion base pairs.)

These phages are "hybrids between what we think of as traditional viruses and traditional living organisms," such as bacteria and archaea, senior author Jill Banfield, a University of California, Berkeley, professor of Earth and planetary science and of environmental science, policy and management, said in the statement. This huge phages' genome is much larger than the genomes of many bacteria, according to the statement. 

The authors found that many of the genes coded for proteins that are yet unknown to us. They found that the phages had a number of genes that are not typical of viruses but are typical of bacteria, according to the statement. Some of these genes are part of a system that bacteria use to fight viruses (and was later adapted by humans to edit genes, a technique called CRISPR-Cas9). 

Scientists don't know for sure, but they think that once these phages inject their DNA into bacteria, the phages' own CRISPR system strengthens the CRISPR system of the bacteria. In that way, the combined CRISPR system could help to target other phages (getting rid of the competition).

What's more, they found that some of the phages had genes that coded for proteins necessary for the functioning of ribosomes — a cellular machine that translates genetic material into proteins (the proteins are the molecules that carry out DNA's instructions). These proteins aren't typically found in viruses, but they are found in bacteria and archaea, according to the statement. 

Some of these newfound phages may also use the ribosomes in their bacteria host to make more copies of their own proteins, according to the statement. 

"Typically, what separates life from nonlife is to have ribosomes and the ability to do translation; that is one of the major defining features that separate viruses and bacteria, nonlife and life," co-lead author Rohan Sachdeva, a research associate at UC Berkeley, said in the statement. "Some large phages have a lot of this translational machinery, so they are blurring the line a bit."

Originally published on Live Science.

  • William Pennat
    It's absurd not to include viruses in the category of living organisms. The main criterion for the definition of "life" should be goal-directed behavior and viruses certainly exhibit that. Just another example of the limitations of "science" as currently defined and practiced. In his book Mind and Cosmos philosopher Thomas Nagel argues for the inclusion of teleology or purposeful action as one of the aspects of the world that can be studied by science. Doing so would definitely place viruses where they belong, in the category of living things.
  • abrogard
    It is not sensible to ascribe a virus with purpose. It is merely a chemical process. It doesn't choose to do what it does.

    In fact all of life can be just about shown to have originated as mindless chemical process that evolved to where we are today.

    Which would appear to make us 'lifeless' or make everything 'life', whichever way we choose to look at it.

    There's a large body argues, or stipulates, that all is alive - all is 'life' - that life is the one-ness of reality and that seen that way the whole of reality is alive.

    Pantheism either says this or something close to it.

    Such speculations seem somewhat removed from where we commonly want to operate. Down on our colloquial level perhaps a definition of 'life', 'alive' might be better if it referenced 'choice to live'.

    That is: truly living things seem to share 'emotion' to the extent that they can and do sometimes suffer emotional feelings to such an extent they'll surrender all desire to live and will die.

    Mindless, lifeless, chemical reactions don't do that: while ever the inputs are there the processes will continue.

    This would make plants 'dead' and that's not likely to be widely accepted, I know. However I do think it a more workable notion than any other.