Virus that Infects Bacteria Turns Immune Response Away from P. aeruginosa, Study Shows

Pseudomonas aeruginosa, the most common bacteria found in cystic fibrosis (CF) patients, uses a virus to go unnoticed by the host immune system, a new study shows.

Treatment with a vaccine designed to help the immune system detect the virus was found to significantly reduce P. aeruginosa’s ability to infect mice, suggesting it might represent a new way to treat CF patients.

The study, “Bacteriophage trigger antiviral immunity and prevent clearance of bacterial infection,” was published in the journal Science.

P. aeruginosa, dangerous for CF patients, is also common in diabetic ulcers, bedsores, and burn wounds. At the moment there is no approved vaccine against these bacteria, and they are increasingly becoming resistant to antibiotics.

The World Health Organization categorized P. aeruginosa as one of the most dangerous pathogens to human health.

“I see this every day in my clinical practice,” Paul Bollyky, MD, PhD, assistant professor of infectious diseases, microbiology, and immunology at Stanford University School of Medicine, and the study’s senior author, said in a university news release written by Bruce Goldman.

“What starts off as a little cut can’t heal as a result of a persistent, drug-resistant bacterial infection. The toll in terms of sickness, death, and dollars is enormous,” he said. For example, infected diabetic foot ulcers that are resistant to antibiotics are the main reason for amputations, Bollyky said.

P. aeruginosa is itself often naturally infected by a virus, known as bacteriophage. This phage lives and multiplies inside bacteria, and is eventually released into the bacteria’s surrounding environment.

“We’ve long known that you’ve got up to 10 quadrillion phages in your body, but we just figured whatever they were doing was strictly between them and your commensal bacteria [the natural community of bacteria that lives within the body and is vital for good health],” Bollyky said. “Now we know that phages can get inside your cells, too, and make you sick.”

The team analyzed the bacterial content of samples collected from 111 patients with infected, non-healing wounds, and found that in 37 cases the wounds were infected with P. aeruginosa. Twenty- five of the 37 infected wounds were found to be positive for a phage. The longer the wound lasted, the higher the percentage of phage-infected bacteria became.

Researchers reasoned that phages not only coexist with P. aeruginosa bacterium, but they also help it infect the host.

To prove the hypothesis, the team infected small wounds in the skin of mice with P. aeruginosa that were either infected with phages or not. The team saw that when P. aeruginosa was alone, without phages, there needed to be about 50 times more bacteria present to cause an infection compared with phage-infected P. aeruginosa.

The team believes this partnership works because the immune response is activated against the virus, not the bacteria.

The immune cells responsible for bacteria hunting are called phagocytes. These cells are able to “eat” bacteria, and call for the help of other protective immune cells if necessary. But if phagocytes sense a virus, they will pause the “eating” process, and will no longer protect the body from it, as if they get the virus inside them they will also become infected.

In lab tests, researchers saw that the presence of the phage in P. aeruginosa reduced the “eating” process by phagocytes 10-fold, preventing bacteria clearance from skin wounds. “The phagocytes lost their appetite,” Bollyky said.

To fight the virus, the body naturally reacts by producing antibodies, which will bind to cells carrying the virus and promote the activation of other immune cells to destroy them. In this case, P. aeruginosa goes unnoticed by the immune system because the virus induces the production of signaling molecules inside phagocytes that will prevent them from signaling the bacterial threat.

To some extent, what the phage does inside phagocytes can be compared to pulling a fire alarm when someone should have called the police instead, Bollyky said. “If 20 fire engines pull up to the scene of the crime, it makes it easier for the thief to get away,” he said.

In order to overcome this inhibitory effect of phages, the team designed a vaccine targeting a part of the phage’s coat protein, and tested it in mice infected with P. aeruginosa.

The vaccine was able to reduce bacteria-infected wounds by half. Likewise, administration of antibodies against the same phage protein could reduce P. aeruginosa with the same efficacy.

Overall, the findings show a direct role of phages in promoting bacterial infections, a phenomenon not restricted to P. aeruginosa as other bacteria also have the ability to harbor similar phages. Bollyky said E. coli and Klebsiella pneumoniae bacteria can also carry phages, and they tend to co-infect wounds already infected with P. aeruginosa.

He believes that vaccinating people against phages after a diagnosis of CF or diabetes may protect them against P. aeruginosa infections. Vaccination should also be extended to people in nursing homes and hospitals, because they are at higher risk of developing P. aeruginosa-positive bedsores.

If in need of faster responses, another strategy includes the use of phage-targeting antibodies, because they may trigger the immune system faster than a vaccine.

The researchers are currently applying for a patent on the vaccine intellectual property. Their plans include tests on larger animals, and eventually clinical trials.