Antibiotics that affect translation can antagonize phage infectivity by interfering with the deployment of counter-defences.

It is becoming increasingly clear that antibiotics can both positively and negatively impact the infectivity of bacteriophages (phage), but the underlying mechanisms often remain unclear. Here we demonstrate that antibiotics that target the protein translation machinery can fundamentally alter the outcome of bacteria-phage interactions by interfering with the production of phage-encoded counter-defence proteins. Specifically, using Pseudomonas aeruginosa PA14 and phage DMS3vir as a model, we show that bacteria with CRISPR-Cas immune systems have elevated levels of immunity against phage that encode anti-CRISPR (acr) genes when translation inhibitors are present in the environment. CRISPR-Cas are highly prevalent defence systems that enable bacteria to detect and destroy phage genomes in a sequence-specific manner. In response, many phages encode acr genes that are expressed immediately following infection to inhibit key steps of the CRISPR-Cas immune response. Our data show that while phage carrying acr genes can amplify efficiently on bacteria with CRISPR-Cas immune systems in the absence of antibiotics, the presence of antibiotics that act on protein translation prevents phage amplification, while protecting bacteria from lysis. These results help to understand how antibiotics-phage synergy and antagonism depend on the molecular interactions that define phage infectivity and host immunity.