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-defense proteins. Specifically, using Pseudomonas aeruginosa PA14 and phage DMS3vir as a model, we show that bacteria with Clustered Regularly Interspaced Short Palindromic Repeat, CRISPR associated (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 defense 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 the 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.

Bacteria have a broad array of defence mechanisms to fight bacteria-specific viruses (bacteriophages, phages) and other invading mobile genetic elements. Among those mechanisms, the 'CRISPR-Cas' (Clustered Regularly Interspaced Short Palindromic Repeats - CRISPR-associated) system keeps record of previous infections to prevent re-infection and thus provides acquired immunity. However, phages are not defenceless against CRISPR-based bacterial immunity. Indeed, they can escape CRISPR systems by encoding one or several anti-CRISPR (Acr) proteins. Acr proteins are among the earliest proteins produced upon phage infection, as they need to quickly inhibit CRISPR-Cas system before it can destroy phage genetic material. As a result, Acrs do not perfectly protect phage from the CRISPR-Cas system, and infection often fails. However, even if the infection fails, Acr can induce a lasting inactivation of the CRISPR-Cas system. The method presented here aims to assess the lasting CRISPR-Cas inhibition in Pseudomonas aeruginosa induced by Acr proteins by:•Infecting the P. aeruginosa strain with a phage carrying an acr gene.•Making the cell electrocompetent while eliminating the phage•Transforming the cells with a plasmid targeted by the CRISPR-Cas system and a non-targeted one to measure the relative transformation efficiency of the plasmids. This method can be adapted to measure which parameters influence Acr-induced immunosuppression in different culture conditions.

Small GTPases are highly regulated proteins that control essential signaling pathways through the activity of their effector proteins. Among the RHOA subfamily, RHOB regulates peculiar functions that could be associated with the control of the endocytic trafficking of signaling proteins. Here, we used an optimized assay based on tripartite split-GFP complementation to localize GTPase-effector complexes with high-resolution. The detection of RHOB interaction with the Rhotekin Rho binding domain (RBD) that specifically recognizes the active GTP-bound GTPase, is performed in vitro by the concomitant addition of recombinant GFP1–9 and a GFP nanobody. Analysis of RHOB-RBD complexes localization profiles combined with immunostaining and live cell imaging indicated a serum-dependent reorganization of the endosomal and membrane pool of active RHOB. We further applied this technology to the detection of RHO-effector complexes that highlighted their subcellular localization with high resolution among the different cellular compartments.