Bacteria killed by silver store it in their cells, making them deadly to other bacteria. Silver nanoparticles are visible inside the bacteria as white spots.

Bacteria killed by silver store it in their cells, making them deadly to other bacteria. Silver nanoparticles are visible inside the bacteria as white spots.

Racheli Ben-Knaz Wakshlak, Rami Pedahzur, and David Avnir

Silver turns bacteria into deadly zombies

The zombie apocalypse may be more than just a horror story for some bacteria. New research shows that when exposed to a microbe-slaying silver solution, the germs can “go zombie,” wiping out their living compatriots even after death. The results may explain silver's long-lasting antibacterial power and could improve the performance of medical products that keep us safe from harmful pathogens.

The use of silver in medicine dates back thousands of years, and scientists have long known that the metal is a potent antibacterial agent. Silver ions perform their deadly work by punching holes in bacterial membranes and wreaking havoc once inside. They bind to essential cell components like DNA, preventing the bacteria from performing even their most basic functions.

But silver's "zombie effect" has gone unrecognized—until now. To uncover this grisly mechanism, scientists first killed a sample of the bacterium Pseudomonas aeruginosa using a solution of silver nitrate. Then, they carefully separated the dead bacteria from the silver solution. When they exposed living bacteria to the dead, they witnessed a microscopic massacre: Up to 99.99% of the living bacteria met their doom.

Using electron microscopy, the researchers imaged the dead bacteria and discovered what caused them to go on their killing spree. Reservoirs of silver nanoparticles had built up in their corpses, indicating that the dead bacteria act like sponges, soaking up silver as they die. The stored silver can leach out to the environment, "especially if the environment contains other sponges for that silver," says chemist David Avnir of the Hebrew University of Jerusalem, the senior author of the new study. "In our case, the other sponge is a living bacterium."

The researchers, who published their findings last week in Scientific Reports, also looked at the killing power of the solution they separated from the zombie bacteria. When they started with low concentrations of silver nitrate, the leftover solution wasn’t strong enough to completely wipe out the second round of bacteria. This indicates the bacteria are actually removing silver from the solution, researchers say. When they started out with high concentrations of silver nitrate, the solution retained its killing power through both groups of bacteria, presumably because the first round of bacteria hadn't been able to soak up all of the silver. 

“This is an important aspect of [silver] that I’ve not seen anyone talk about before,” says molecular microbiologist Simon Silver of University of Illinois, Chicago, who was not involved in the research. "This paper is a new spin on it, to me, and I think rather a good one."

The finding could lead to an enhanced ability to control the longevity of silver-based treatments. Doctors and hospitals already rely on an array of silver-infused medical products—from bandages to catheters—to prevent the proliferation of ­bacteria. The metal is commonly used on severe wounds, and coatings on door handles can cut down on germs. Consumers can even buy products to reduce unwanted microbes at home, like silver-infused socks and washing machines that disinfect clothes with silver.

"Right now, the dominant idea is, if you want a certain lifetime of antibacterial performance, you have to engineer your device to sort of give off these ions over the full course of the time you want this activity," says nanomaterials chemist Robert Hurt of Brown University, who was not involved in the research. But an understanding of the zombie effect could lead to better designs for such products, Hurt says. For example, engineers may now tailor their products to keep dead bacteria around, fortifying their antimicrobial powers and keeping germs at bay.

Zombies might not be so bad after all.