Bacterial lifestyle transitions can occur over short evolutionary distances where closely related host-associated strains may be pathogenic or beneficial. In strains of Pseudomonas fluorescens, transitions between pathogenic and mutualistic lifestyles are the result of recent horizontal gene transfer of genomic islands. The frequent gain and loss of genetic material suggest the genomes of diverse P. fluorescens must encode genetic machinery to regulate horizontally transferred virulence genes. In this undergraduate student-driven project, we used two P. fluorescens strains that are beneficial (N2E2) or pathogenic (N2C3) on the model plant Arabidopsis. We found that deletion of a highly conserved two-component system GacA/S resulted in loss of virulence in N2C3 and loss of beneficial traits in N2E2. We show that this is through transcriptional regulation of recently horizontally transferred genomic islands involved in bacterial virulence or commensalism. Because this can teach fundamental skills in molecular microbiology, and host-pathogen interactions, to dissect the downstream processes in Pseudomonas that affect bacterial lifestyles, we have begun building on this project through a CURE. Collectively our data suggest that components of the core genome may allow bacteria to acquire new genetic elements that drive adaptation to new hosts or lifestyles.
