Photo (colorized) of scanning electron microscopy of a bacterium lyzed by the phages (© L. Debarbieux, Institut Pasteur; M. and C. Rohde, Helmholtz Centre for Infection Research).

Antibiotic resistance represents a major public health challenge, associated with a high mortality rate. While bacteriophages – viruses that kill bacteria – could be a solution for fighting antibiotic-resistant pathogens, various obstacles stand in the way of their clinical development. To overcome them, researchers from Inserm, Université Sorbonne Paris Nord and Université Paris-Cité at the IAME Laboratory, in close collaboration with their counterparts at Institut Pasteur and the Paris Public Hospitals Group (AP-HP), have developed a model to better predict the efficacy of phage therapy and possibly develop more robust clinical trials. Their findings have been published in Cell Reports.

The discovery of antibiotics had revolutionized the history of medicine in the 20th century, allowing us to effectively fight bacteria for the first time. However, antibiotic resistance – a phenomenon during which bacteria become resistant following mass, repeated use – has become a major public health issue in recent decades. Each year, these resistant bacteria are estimated to be responsible for 700,000 deaths worldwide. Yet the discovery of new antibacterial agents has been stagnating for several years.

In this context, phage therapy has recently generated renewed interest. This therapeutic approach involves the use of bacteriophages that target and destroy pathogenic bacteria whilst being unable to infect humans. While the concept has been in existence for a long time, its clinical development has been hampered by various limitations. Unlike “conventional” medicines, bacteriophages are complex biologics, whose action in the body, optimal dose, and most effective route of administration are difficult to study and anticipate.

In order to remove some of these obstacles, Jérémie Guedj’s research team at Inserm, in collaboration with Laurent Debarbieux’s team at Institut Pasteur, has developed a new mathematical model to better define the interactions between bacteriophages and pathogenic Escherichia coli bacteria in animals and to identify the key parameters that influence the efficacy of phage therapy.

Supporting clinical development

Various data from in vitro and in vivo experiments were used to construct this model. In particular, the researchers used the bacteriophages’ infection parameters determined in the laboratory (for example, the duration of the infectious cycle of the bacteria, the number of viruses released when a bacterium is destroyed…) and information collected during experiments using a mouse model of lung infection.

Some of the animals were infected with a bioluminescent strain of E. Coli (in order to best monitor it within the body). Among them, some were treated with bacteriophages at different doses and using different routes of administration. The quantities of bacteria and bacteriophages thus measured over time helped to feed the mathematical model and test which were the most important parameters for effective phage therapy. 

Using their model, the scientists show that the route of administration is an important parameter to consider when it comes to improving the animals’ survival: the more rapidly it brings the bacteriophages into contact with the bacteria, the more it is effective. In the animal model, the phage therapy administered intravenously was therefore less effective in comparison with the intratracheal route because fewer bacteriophages were reaching the lungs. On the other hand, when administered by intratracheal route, the model suggests that the dose of the medication given has little effect on the efficacy of the therapy.

Another important point is that this model incorporates data on the animals’ immune response in the context of phage therapy. The model confirms and extends the principle that bacteriophages act in synergy with the immune system of infected animals, enabling more effective elimination of pathogenic bacteria.

“In this study, we propose a new approach to streamline the clinical development of phage therapy, which otherwise continues to have its limitations. Our model could be reused to predict the efficacy of any bacteriophage against the bacteria it targets, once a limited number of in vitro and in vivo data are available on its action. Beyond phage therapy, the model could also be used to test anti-infective therapies based on the association between bacteriophages and antibiotics,” concludes Guedj.

Human respiratory epithelial cells (in red) infected with an influenza virus (in green). © A. Cezard, D. Diakite, A. Guillon, M. Si-Tahar, PST ASB-Microscopy Department.

Seasonal influenza is a major public health issue because it continues to remain associated with considerable mortality, particularly among people who are elderly, immunocompromised, or both. It also has a significant socioeconomic cost. With vaccination and current treatments still being of limited efficacy, research teams are trying to develop new therapeutic approaches. At the Research Center for Respiratory Diseases in Tours, scientists from Inserm, Université de Tours and Tours Regional University Hospital have shown that in a context of influenza infection, a metabolite[1] called succinate, which is naturally present in the body, has an antiviral and anti-inflammatory action. These findings open up new therapeutic prospects based on the use of succinate derivatives. The study has been published in EMBO Journal.

Often considered a mild disease, influenza continues to cause the deaths of 10,000 to 15,000 people each year in France. The socioeconomic cost of the disease is also significant because it is associated with high levels of absenteeism and a major burden on hospitals.

Seasonal influenza vaccination is a central pillar of the preventive strategies deployed to reduce the number of cases and fight the disease. However, its efficacy can vary from year to year, depending on the influenza viruses in circulation and the suitability of the vaccine to them. Drugs that directly target the influenza virus are available for severe cases, but the window of time for effective action with these treatments is very short. What is more, influenza viruses have become resistant to their action.

In this context, the development of innovative therapies is a priority. While current treatments work by targeting certain components of the virus, Inserm Research Director Mustapha Si-Tahar and his colleagues at the Research Center for Respiratory Diseases are trying to better understand the host’s cellular and molecular responses to viral infection, with the long-term aim of developing novel therapeutic strategies aimed at strengthening these responses.

The role of metabolites in immune response

While metabolism¹ has long been considered to be a purely energetic mechanism essential for cell function, recent research has shown that some metabolites can also regulate the immune response.

Based on these data, Si-Tahar’s team wondered whether an influenza infection could cause the reprogramming of the metabolism of the target cells of the virus and whether specific metabolites play an especially active role in the immune response.

In mice infected with influenza, the researchers observed that a metabolite called succinate accumulates in the lungs. A phenomenon which was then confirmed in humans by comparing respiratory fluids from intensive care patients with and without influenza pneumonia. The presence of succinate was found to be significantly higher in the influenza patients.

They then exposed cells from the pulmonary epithelium to succinate, thereby demonstrating that the molecule has antiviral activity by blocking multiplication of the virus. Succinate also helps to reduce the strong inflammatory response that is triggered in the lungs following infection with influenza.

The researchers also found that mice exposed to the virus receiving intranasal succinate are better protected against infection and have a higher survival rate than those that do not receive it.

In search of molecular mechanisms

In order to better understand these different phenomena, the scientists sought to decipher the molecular mechanisms behind the antiviral action of succinate.

This involved analyzing the impact of succinate on the various stages of the viral replication cycle and demonstrating that, while there is no influence on the early stages of the cycle (entry, transcription, and translation), there is an influence on a later stage. The findings show that this metabolite prevents a major structural protein of the virus, the “nucleoprotein”, from exiting the nucleus of infected cells, thereby preventing the assembly of the final viral particle and interrupting the multiplication cycle of the virus.

These data all point to the key role of succinate in controlling influenza infection, as well as its therapeutic value.

Additional studies are needed in order to test the therapeutic potential of succinate and identify other metabolites of interest.

[1] Metabolism refers to all of the chemical reactions that take place inside the body’s cells. A metabolite is an organic substance derived from metabolism.

[2] In keeping with this research, Si-Tahar has since early 2021 coordinated a French National Research Agency (ANR) program entitled “Development of succinate-based formulations and analogues against SARS-CoV-2-induced respiratory infections and influenza viruses.” His team is also the beneficiary of an “ERS-RESPIRE4 Marie Skłodowska-Curie fellowship” to develop a project entitled “Succinate-Producing Probiotics as an Innovative Therapy for Viral Respiratory Infections: a proof-of-concept study”.