Bacteria Can Play Dead to Avoid Bacteriophage Infection

Like opossums that roll over and play dead when approached by a predator, bacteria can put themselves into a temporary sleep to prevent infection with bacteriophages. These new findings were published this week in Nature by scientists at Rockefeller University.

Bacteria have evolved a multitude of defenses against invading bacteriophages, including the immune mechanisms known as CRISPRs, which detect and destroy foreign genetic material with Cas enzymes.

While most of the Cas enzymes cleave viral DNA, one of the CRISPR systems contains an enzyme called Cas13 that works by cleaving RNA. It was previously thought that Cas13 might impede phages with RNA genomes. However, nothing that RNA phages are exceedingly rare, the Rockefeller researchers set out to determine if this system might have another purpose.

What they found is that activation of Cas13 actually protects against phages with DNA genomes. Through a series of experiments, they were able to demonstrate that the RNA cleaving Cas13 helps bacteria by hindering their growth. The enzyme cleaves host RNA, sending the bacteria into a dormant state. Since the bacteriophages rely on host RNA to replicate, this strategy is effective at hindering viral spread.

One benefit of such a system is that it allows for a more general defense strategy against viruses. Most CRISPR-Cas systems are highly specific, only cutting bits of DNA that match a precise genetic sequence. If the invading virus mutates just a bit, the CRISPR system can no longer identify it and the phage will be free to infect.

The researchers note that the real benefit of Cas13 lies in its effects at the population level. While a life of dormancy is not ideal for an individual bacteria, going dormant will stop the phage from propagating and infecting even more of the colony.