They are called phages: because they eat bacteria:
Scientists have discovered hundreds of unusually large, bacteria-killing viruses with capabilities normally associated with living organisms, blurring the line between living microbes and viral machines.
These phages — short for bacteriophages, so-called because they “eat” bacteria — are of a size and complexity considered typical of life, carry numerous genes normally found in bacteria and use these genes against their bacterial hosts.
University of California, Berkeley, researchers and their collaborators found these huge phages by scouring a large database of DNA that they generated from nearly 30 different Earth environments, ranging from the guts of premature infants and pregnant women to a Tibetan hot spring, a South African bioreactor, hospital rooms, oceans, lakes and deep underground.
Altogether they identified 351 different huge phages, all with genomes four or more times larger than the average genomes of viruses that prey on single-celled bacteria.
Among these is the largest bacteriophage discovered to date: Its genome, 735,000 base-pairs long, is nearly 15 times larger than the average phage. This largest known phage genome is much larger than the genomes of many bacteria…
“Typically, what separates life from non-life is to have ribosomes and the ability to do translation; that is one of the major defining features that separate viruses and bacteria, non-life and life,” Sachdeva said. “Some large phages have a lot of this translational machinery, so they are blurring the line a bit.”University of California – Berkeley, “Huge bacteria-eating viruses close gap between life and non-life” at ScienceDaily
Paper. (open access)
It’s not clear why phages are not just assumed to be life forms. If they turn out to communicate and learn from experience, as bacteria do, that would strengthen the case.
See also: In What Ways Are Bacteria Intelligent?
As antibiotic resistance grows, researchers are discovering that these microbes are not just single, simple cells. We must understand the surprisingly complex ways bacteria “think” in order to keep them in check.