Viruses are usually associated with causing infections, but some can actually help fight them. Scientists are now working to turn these bacteria-killing viruses into treatments for dangerous infections that no longer respond to conventional antibiotics.
An international team of experts has taken an important step toward making this approach safer and more practical for medical use. Led by the German Society for Infectious Diseases (DGI), the group has developed the first internationally coordinated clinical guidelines for personalized phage therapy, a treatment that uses carefully selected viruses to attack harmful bacteria.
The effort involved 20 professional organizations, 18 international experts, including representatives from regulatory agencies, and two patient advocacy organizations. Together, they established more than 60 recommendations covering how phages should be selected, tested, manufactured, administered, and monitored. The guidelines also address laboratory infrastructure, quality control, treatment documentation, and priorities for future research.
The consensus statement was published in the journal Nature Medicine. Until now, doctors and researchers have lacked a unified international framework for many of the essential procedures and quality standards involved in using phages as medical treatments.
How Bacteria-Killing Viruses Could Fight Antibiotic Resistance
Antibiotic resistance is making bacterial infections increasingly difficult to treat around the world. As some bacteria develop defenses against commonly used medications, doctors may find themselves with few effective options.
One possible alternative involves bacteriophages, commonly called phages. These are viruses that naturally infect bacteria and can destroy them. Unlike many antibiotics, which can act against a broad range of bacteria, phages typically target particular bacterial species or even individual strains.
That precision makes phage therapy especially interesting, but it also creates a challenge. A virus capable of killing one strain of bacteria may have little or no effect on another. Before treatment begins, specialists must therefore determine whether a particular phage can successfully attack the bacteria responsible for a patient’s infection.
Personalized phage therapy may be considered when established treatments have failed, are unavailable, or cannot be used safely. Research so far suggests that properly manufactured and administered phages generally have a favorable safety profile. However, controlled clinical trials have not yet provided sufficient evidence to establish how reliably the treatment works.
For that reason, the new guidelines do not recommend phage therapy as a routine treatment for bacterial infections. Instead, they emphasize carefully evaluating individual patients and using the approach only when the circumstances justify it.
Early Results Show Promise Against Difficult Infections
Although major questions remain, early clinical experiences suggest that phage therapy could benefit some patients with difficult infections.
In a Belgian case series involving 100 people who received phage treatment, 77 percent experienced an improvement in their clinical condition. In 61 percent of cases, tests could no longer detect the bacteria that the treatment had targeted.
These findings are encouraging, particularly for infections that are difficult to control with existing medications. However, observational studies cannot establish whether phages were responsible for the improvements or determine how well the treatment would perform in a controlled clinical trial.
Larger, carefully designed studies will be needed to clarify the effectiveness of phage therapy and identify which patients are most likely to benefit.
Finding the Right Virus for Each Bacterial Infection
A central challenge in personalized phage therapy is finding the correct virus for each patient.
Because phages are highly selective, laboratories must test whether a candidate virus can kill the specific bacterial strain involved in an infection. This process, known as a “phage match,” should be completed as quickly as possible before treatment begins.
Timing matters because bacteria can change their susceptibility to phages during an infection. If multiple bacterial pathogens are present, specialists should test all of the relevant organisms rather than focusing on just one.
The guidelines recommend that treatment decisions involve a team of specialists, including the patient’s physicians, clinical microbiologists, and recognized phage experts. This collaborative approach is intended to help clinicians select appropriate phages and determine whether treatment is justified.
The recommendations also establish expectations for cooperation among laboratories, phage manufacturing facilities, pharmacies, and clinical teams. Consistent testing methods and regular proficiency assessments should help ensure that results from different facilities are reliable and comparable.
Another priority is collecting detailed information about every treatment. The guidelines call for cases to be systematically documented and entered into registries, allowing researchers to compare clinical experiences, identify problems, and investigate unanswered questions.
“The standardization of quality criteria for the production of phages for personalized use, as well as the identification of infrastructural requirements, are essential components of the guideline and important prerequisites for reliable treatments,” says Dr. Annika Claßen.
Claßen, a researcher with the German Center for Infection Research (DZIF), played a leading role in developing the recommendations. She also directs the “Translational Bacteriophage Research Group” at University Hospital Cologne.
Other major contributors include PD Dr. Silvia Würstle, Prof. Maria Vehreschild, and Dr. Simone Lieberknecht-Jouy of Goethe University Frankfurt. Vehreschild serves as deputy coordinator of the DZIF research area “Healthcare-Associated Infections,” where Claßen, Würstle, and Lieberknecht-Jouy are also involved in research.
German Hospital Begins Using Personalized Phage Therapy
The potential of this approach is already being explored in clinical practice.
“The guideline provides physicians, pharmacists, and regulatory authorities with a common framework for safely and responsibly incorporating phage therapy into clinical practice,” says PD Dr. Würstle, the corresponding author of the guideline.
At Frankfurt University Medical Center, Würstle is overseeing the development of the facilities and procedures needed to provide phage therapy in accordance with the new recommendations.
The medical center reports that ten patients have been successfully treated through its guideline-based phage therapy program since February 2026. The treatments use viruses individually selected to target the bacteria causing each patient’s infection.
Among the pathogens treated are Pseudomonas aeruginosa and Staphylococcus aureus, two bacterial species associated with infections that can be particularly challenging to manage, especially when antibiotic resistance is involved.
The method of delivering the phages depends on the infection and the patient’s medical needs. Treatment may involve intravenous administration, inhalation, a nasal spray, or application through a sterile hydrogel.
These different delivery methods illustrate how personalized phage therapy can be adapted to specific clinical situations rather than relying on a single treatment approach.
New International Standards Could Shape the Future of Phage Therapy
Building a reliable system for phage treatment requires coordination across numerous medical and scientific specialties.
The guidelines highlight PhageNET DZIF, a network established and coordinated by Claßen, as an important platform for exchanging scientific knowledge and clinical experience.
By bringing together expertise in microbiology, clinical medicine, pharmacy, manufacturing, and regulation, the new framework aims to help specialists understand both the possibilities and the limitations of personalized phage therapy.
The recommendations were developed through a structured consensus process under the AWMF (Association of the Scientific Medical Societies in Germany). They are valid from January 5, 2026, through June 29, 2030, with the responsible committee reviewing them annually to determine whether updates are needed.
While phage therapy still requires stronger evidence from controlled clinical trials, these international standards establish a clearer path for its responsible development. For patients facing infections that conventional antibiotics cannot adequately treat, bacteria-killing viruses may eventually provide another valuable option.