Screening for Growth-Inhibitory ORFans in Pseudomonas aeruginosa-Infecting Bacteriophages

Like all viruses, bacteriophages heavily depend on their host's physiology for reproduction. Therefore, phages have evolved numerous proteins that influence the host metabolism to facilitate the infection process. Some of these proteins strongly perturb the host cell, ultimately leading to cell death. These growth-inhibitory phage proteins presumably target key metabolic processes, which may provide a basis for innovative phage-derived antibacterials. Unfortunately, most of these proteins are the so-called ORFans, since they have no known function or sequence homology to any other gene. We here describe a screening method for the identification of growth-inhibitory ORFans of bacteriophages infecting gram-negative bacteria (e.g., Pseudomonas aeruginosa), using the pUC18-mini-Tn7T-Lac vector system, which allows for stable single-copy integration of the phage ORFans in the Pseudomonas genome under the control of an IPTG-inducible promoter. Furthermore, we describe a method to examine the effect of the phage proteins in different hosts, using different vector copy numbers. Finally, we explain how to investigate the effect of ORFan expression on the host morphology using time-lapse microscopy.