Intra-strain genomic microevolution and phage resistance in Pseudomonas aeruginosa PAO1 laboratory isolates.

Pseudomonas aeruginosa is a major opportunistic pathogen, and its laboratory reference strain, PAO1, is widely used in microbiological and genetic studies. However, PAO1 often exhibits phenotypic variability that can affect experimental reproducibility. Our PAO1 stock, obtained from a public biobank, is resistant to PP7, a pilus-dependent single-stranded RNA phage known to infect PAO1. This suggests the presence of genetic variants in the stock. To check this possibility, we isolated six phenotypically distinct variants (GU1-GU6) and performed genomic and phenotypic analyses. Notable differences were observed among the isolates in terms of motility, pyocyanin production, and susceptibility to PP7. Whole-genome sequencing revealed that four of the six variants harbored mutations in pilus-associated genes. Among these, GU3 carried a mutation in pilT, which encodes a motor protein essential for type IV pilus retraction, and the loss of retraction led to the PP7 resistance. GU2, GU4, and GU6 shared a nonsense mutation in pilJ, a gene involved in chemotaxis and pilus extension, resulting in reduced twitching motility and lower PP7 infection efficiency. Additionally, we found that a mutation in lasR, a master regulator of quorum sensing, promoted the replication of prophage Pf6, which was integrated into the PAO1 genome. Pf6 replication interferes with PP7 infection, providing an alternative mechanism of resistance. These findings offer new insights into the complexity of phage-host interactions and emphasize the importance of routine verification and careful handling of P. aeruginosa sublines used in bacteriological and phage research.IMPORTANCEPhenotypic and genotypic variability in Pseudomonas aeruginosa PAO1 has been widely reported, raising concerns regarding the reproducibility of laboratory studies that rely on this reference strain. In this study, we isolated six PAO1 variants from a single laboratory stock and demonstrated that they differed markedly in motility, pyocyanin production, and susceptibility to the ssRNA phage PP7. Whole-genome sequencing has revealed that even a single mutation in a pilus-associated gene can profoundly affect bacterial motility and phage susceptibility. Furthermore, we showed that a mutation in lasR, a key regulator of the quorum-sensing system, triggered replication of the Pf6 prophage, which in turn hindered PP7 infection. These findings underscore the dynamic nature of laboratory strains and highlight the need for caution when interpreting results from phage-host interaction studies using reference strains. Our results provide a new understanding of how subtle genetic changes in model strains influence experimental outcomes in microbiology.