Nav: Home

To curb infection, bacteria direct their defenses against themselves

May 30, 2019

Sometimes, the best defense against hostile invaders is a good, long nap. Or at least, that strategy seems to work for bacteria.

In a new study, described in Nature, Rockefeller scientists showed that microbes under viral attack turn their defenses not only on their enemies, but also on themselves. This drastic measure, the researchers found, doesn't kill the bacteria, but rather sends them into a dormant state that prevents the infection from spreading.

Vicious viruses

Among bacteria, viruses called bacteriophages are public enemy number one.

These pathogens propagate by injecting their genome into unsuspecting microbes, eventually causing their host cell to rupture, at which point progeny phage is released to infect other members of a bacterial colony.

To mitigate these attacks, bacteria have evolved immune mechanisms known as CRISPRs, or clustered regularly interspaced short palindromic repeats, which, with the help of associated Cas enzymes, detect and destroy foreign genetic material.

Microbes have at their disposal many different CRISPR systems, one of which caught the attention of Luciano Marraffini due to its unique strategy for fending off intruders. Whereas most Cas enzymes destroy viral DNA, this particular enzyme, Cas13, works by cleaving RNA.

"Since Cas13 targets RNA, it was initially thought to have evolved to impede phages with RNA genomes. The problem is, RNA phages exceedingly rare," he says. "So we wanted to see whether it might have evolved to serve a different function."

Working with Helen Hay Whitney postdoctoral associate Alexander Meeske, Marraffini showed that activation of Cas13 actually protects bacteria from phages with DNA genomes, which are far more common. But how, they wondered, could an RNA-cutting enzyme actually defend microbes from this kind of virus?

Through a series of experiments, the researchers found that Cas13 helps bacteria, ironically, by hindering them. That is, the enzyme cuts up bits of host RNA, sending the bacteria into a dormant state--a kind resting phase in which the microbes remain alive but don't grow. This strategy works, says Meeske, because viruses need host RNA to replicate.

"Phages are parasites: They don't have all the elements needed for their propagation, so they rely on the host," he explains. "And if the host cell isn't making those elements, the phage cannot propagate."

No escape

The researchers also found that Cas13 kills viruses more thoroughly than other Cas enzymes. Standard CRISPR-Cas systems are highly specific, cutting up bits of DNA that match a precise genetic sequence. And while this specificity can be an asset, it also comes with a big drawback: If a virus mutates, CRISPR cannot recognize the invader, and the phage escapes scot-free.

"If a phage has a single point mutation in its target sequence, then usually the virus is invisible to Cas and the infection will succeed," says Marraffini. "But with Cas13 we didn't see any escaper mutants."

The researchers attribute this superb virus-fighting power to the fact that cell dormancy does not target one particular virus, but rather makes it impossible for any phage--including mutants--to propagate. And while an indefinite nap may not seem like much of a life for a microbe, Meeske notes that the real benefit of Cas13 lies not at the level of the individual, but of the bacterial community as a whole.

"The phage has one shot to deliver its genetic payload and replicate," he says. "So if they inject their genome into a host that turns out to be inhospitable, the infection stops there. The phage loses, and the bacterial colony wins."
-end-


Rockefeller University

Related Bacteria Articles:

Conducting shell for bacteria
Under anaerobic conditions, certain bacteria can produce electricity. This behavior can be exploited in microbial fuel cells, with a special focus on wastewater treatment schemes.
Controlling bacteria's necessary evil
Until now, scientists have only had a murky understanding of how these relationships arise.
Bacteria take a deadly risk to survive
Bacteria need mutations -- changes in their DNA code -- to survive under difficult circumstances.
How bacteria hunt other bacteria
A bacterial species that hunts other bacteria has attracted interest as a potential antibiotic, but exactly how this predator tracks down its prey has not been clear.
Chlamydia: How bacteria take over control
To survive in human cells, chlamydiae have a lot of tricks in store.
Stress may protect -- at least in bacteria
Antibiotics harm bacteria and stress them. Trimethoprim, an antibiotic, inhibits the growth of the bacterium Escherichia coli and induces a stress response.
'Pulling' bacteria out of blood
Magnets instead of antibiotics could provide a possible new treatment method for blood infection.
New findings detail how beneficial bacteria in the nose suppress pathogenic bacteria
Staphylococcus aureus is a common colonizer of the human body.
Understanding your bacteria
New insight into bacterial cell division could lead to advancements in the fight against harmful bacteria.
Bacteria are individualists
Cells respond differently to lack of nutrients.

Related Bacteria Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Anthropomorphic
Do animals grieve? Do they have language or consciousness? For a long time, scientists resisted the urge to look for human qualities in animals. This hour, TED speakers explore how that is changing. Guests include biological anthropologist Barbara King, dolphin researcher Denise Herzing, primatologist Frans de Waal, and ecologist Carl Safina.
Now Playing: Science for the People

#SB2 2019 Science Birthday Minisode: Mary Golda Ross
Our second annual Science Birthday is here, and this year we celebrate the wonderful Mary Golda Ross, born 9 August 1908. She died in 2008 at age 99, but left a lasting mark on the science of rocketry and space exploration as an early woman in engineering, and one of the first Native Americans in engineering. Join Rachelle and Bethany for this very special birthday minisode celebrating Mary and her achievements. Thanks to our Patreons who make this show possible! Read more about Mary G. Ross: Interview with Mary Ross on Lash Publications International, by Laurel Sheppard Meet Mary Golda...