'Add, stir and reduce': Yersinia spp. as model bacteria for pathogen evolution

Key Points The evolution of mammalian pathogenesis in the Yersinia genus has occurred in different lineages in parallel through a balanced mixture of gene gain and gene loss events. Only by sequencing pathogenic and non-pathogenic representatives from an entire bacterial genus can such observations be made. The parallel evolution of pathogenesis is even shared with enteric pathogens outside of the Yersinia genus, notably in the Salmonella genus. Gene loss events lead to niche restriction owing to a reduction in metabolic flexibility, which is often seen in lineages that evolve a more acutely pathogenic phenotype. The potential of loss of fitness from the expression of genes acquired in gene gain events is mediated by the transcriptional silencing of, or fine control of, these acquired elements by ancestral regulons that are regulated by factors such as RovA and H-NS. In the genomics era, Yersinia has proven to be a model genus for studying the emergence of pathogenesis. Focusing on this model, McNally and colleagues highlight the events in genome evolution that underlie pathogenesis and argue for an 'eco–evo' perspective of pathogen evolution. Pathogenic species in the Yersinia genus have historically been targets for research aimed at understanding how bacteria evolve into mammalian pathogens. The advent of large-scale population genomic studies has greatly accelerated the progress in this field, and Yersinia pestis , Yersinia pseudotuberculosis and Yersinia enterocolitica have once again acted as model organisms to help shape our understanding of the evolutionary processes involved in pathogenesis. In this Review, we highlight the gene gain, gene loss and genome rearrangement events that have been identified by genomic studies in pathogenic Yersinia species, and we discuss how these findings are changing our understanding of pathogen evolution. Finally, as these traits are also found in the genomes of other species in the Enterobacteriaceae, we suggest that they provide a blueprint for the evolution of enteropathogenic bacteria.