Experimental Evolution of Leads to Loss of Motility, N (σ54) Deletion and Genome Reduction.

Evolution experiments in the laboratory have focused heavily on model organisms, often to the exclusion of clinically relevant pathogens. The foodborne bacterial pathogen belongs to a genus whose genomes are small compared to those of its closest genomic relative, the free-living genus , suggesting genome reduction during the course of evolution to host association. In an experiment, serially passaged in rich medium in the laboratory exhibited loss of flagellar motility-an essential function for host colonization. At early time points the motility defect was often reversible, but after 35 days of serial culture, motility was irreversibly lost in most cells in 5 independently evolved populations. Population re-sequencing revealed disruptive mutations to genes in the flagellar transcriptional cascade, N (σ54)-therefore disrupting the expression of the genes σ54 regulates-coupled with deletion of N in all evolved lines. Additional mutations were detected in virulence-related loci. In separate experiments, we demonstrate that a phase variable (reversible) motility mutant carrying an adenine deletion within a homopolymeric tract resulting in truncation of the flagellar biosynthesis gene R was deficient for colonization in a C57BL/6 IL-10 mouse disease model. Re-insertion of an adenine residue partially restored motility and ability to colonize mice. Thus, a pathogenic strain was rapidly attenuated by experimental laboratory evolution and demonstrated genomic instability during this evolutionary process. The changes observed suggest is able to evolve in a novel environment through genome reduction as well as transition, transversion, and slip-strand mutations.