Understanding Bacterial Antiviral Defence Systems and Phage Receptors to Better Inform Rational Phage Cocktail Design to Treat Bacterial Canker

Pseudomonas syringae is a plant pathogen complex responsible for bacterial canker in cherry. In the absence of any control measures, bacteriophages (phages) have the potential for biocontrol. However, it is crucial to first evaluate the role of bacterial antiviral defence systems (ADS) in phage infection dynamics for careful design of a phage cocktail (mixture). Investigating 250 Pseudomonas strains revealed the Ps complex possessed diverse ADS with defence profiles being influenced by phylogeny. Phage host range assays revealed five MR phages with distinct genotypes possessed strong lytic activity against several bacterial canker‐causing Ps pathovars, including syringae and morsprunorum race 1 and 2. Phage susceptibility and resistance appeared to be associated with individual ADS rather than defence profiles as a whole. Multisequence alignment of lipopolysaccharide biosynthesis genes glucose‐1‐phosphate thymidylyltransferase (gpt), glycosyltransferase family 1 (gst1) and lipopolysaccharide kinase (lpk) found these potential receptor genes to be highly conserved within Ps phylogroups. However, gpt alone appeared to influence phage infectivity. Our findings indicate that the gpt gene is a potential primary predictor of MR phage susceptibility, hypothesised to influence phage absorption, while individual ADS only have a secondary role in phage resistance. This study highlights that understanding the genetic mechanisms underlying phage‐bacterial interactions is crucial for designing more effective phage cocktails capable of targeting a broader range of pathogenic strains, but phage screening still is a powerful tool to select phages for biocontrol treatments.