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Discovery Reveals How Bacteria Distinguish Harmful Versus Helpful Viruses

Posted in News, Bacteria, Viruses, Research

Microbial frenemies: Particles of the virus ΦNM1, applied at varying concentrations, killed cells in the Staphylococcus aureus “lawn,” creating clear plaques. Researchers have shown Staph can detect whether a virus, such as ΦNM1, is destructive or potentially helpful.

When they are not busy attacking us, germs go after each other. But when viruses invade bacteria, it doesn’t always spell disaster for the infected microbes: Sometimes viruses actually carry helpful genes that a bacterium can harness to, say, expand its diet or better attack its own hosts.


“Transcription — an initial step in the process that reads genes, including those of viruses — makes the difference,” says researcher Luciano Marraffini, head of the Laboratory of Bacteriology. “The full genome of viruses in their lytic, or destructive phase, is transcribed. Meanwhile, only a few of the genes from a virus are transcribed during its lysogenic, or dormant phase.”

Viruses in their lytic phase make copies of themselves using a cell’s machinery before destroying it to liberate these new viruses. Viruses in their lysogenic phase, meanwhile, quietly integrate into a host’s genetic material. And this is where they offer their potential benefit to the bacteria, which coopt viral genes for their own ends. In fact, some disease-causing microbes, such as the bacterium responsible for diphtheria, must pick up the right virus in order to attack humans.

Scientists have only discovered this adaptive bacterial immune system relatively recently. Its function relies on CRISPRs, sections of DNA that contain repeating sequences interspersed with unique sequences called spacers (CRISPR stands for clustered regularly interspaced short palindromic repeats). The spacer sequences match the sequences in the viral genetic code, making it possible for enzymes encoded by CRISPR-associated genes (Cas) to chop out single spacer sequences from the RNA transcription of the CRISPR DNA. Other Cas enzymes then use these spacer sequences as guides to target invaders for destruction.


“Our understanding of CRISPR-Cas systems remains in the early stages, but, so far, it has generally been thought they lack a sophisticated way of discriminating their targets. In other words, once they target something, it will be chopped up,” says the study’s lead author, graduate student Gregory Goldberg. “For the first time, our work has shown that a CRISPR-Cas system, one found in Staphylococcus bacteria, can detect whether or not a virus is in its destructive phase and poses an immediate threat.”




“This discovery of a transcription requirement is likely to surprise many who work with these systems,” Marraffini says. “Although we do not yet understand the mechanism behind it, we can say that the Type-III-A system is quite different from other CRISPR-Cas systems, of which there is a mysteriously large variety. Our discovery hints at the possibility that each CRISPR type and subtype recognizes and destroys its targets in different ways, each in tune with a particular bacterium’s needs. If these different targeting mechanisms do exist, they could have important implications for biotechnology.”

Source: Rockefeller University 


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