One variety of the bacterial immune system known as the CRISPR-Cas system can distinguish viral friend from foe by watching for one particular cue called transcription, the findings showed.

"Transcription - an initial step in the process that reads genes, including those of viruses - makes the difference," said researcher Luciano Marraffini from the Rockefeller University in US.

"The full genome of viruses in their lytic, or destructive phase, is transcribed. Meanwhile, a few of the genes from a virus are transcribed during its lysogenic, or dormant phase," Marraffini explained.

Viruses in their lytic phase make copies of themselves using a cell's machinery before destroying it to liberate these new viruses. Meanwhile they quietly integrate into a host's genetic material. And this is where they offer their potential benefit to the bacteria, which co-opt viral genes for their own ends.

In fact, the bacterium responsible for diphtheria must pick up the right virus in order to attack humans. Scientists have discovered this adaptive bacterial immune system recently.

The study appeared in the journal Nature.

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