Phage therapy, which uses viruses known as bacteriophages to treat bacterial infections, is a long-standing medical procedure whose mechanisms of action are still poorly understood. Scientists from the Institut Pasteur and CNRS have demonstrated in vivo in a murine model that bacteria are capable of regulating their gene expression to evade the numerous bacteriophages present in the gut environment. This research explains the difference in bacteriophage efficacy between in vitro and in vivo conditions. The findings were published in the journal Cell Host & Microbe on April 13, 2022.