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HGT drives evolutionary transitions in Rhodococcus and establishes new pathogenic lineages

Jeff Chang: Oregon State University, Botany and Plant Pathology

<div><em>Rhodococcus</em> are cosmopolitan Gram-positive bacteria with members of specific clades adapted to plants. These bacteria are repeatedly recovered from tissues of various plant species, and are implicated in providing benefits to their hosts. In contrast, <em>Rhodococcus</em> with a virulence plasmid are broad host-range pathogens that cause growth deformations to over 100 genera of plants, leading to significant losses to the horticultural industry. We used a genomic epidemiological approach to trace the transmission of <em>Rhodococcus</em> in US nurseries. Analysis of chromosomal single nucleotide polymorphisms showed multiple lineages of <em>Rhodococcus</em> can persist on plants. In nurseries, there is evidence for multiple introductions, reservoir populations, and point source outbreaks of the pathogen. Characterization of key isolates confirmed the necessity and sufficiency of the virulence plasmid for pathogenicity and revealed that isolates lacking the plasmid cause beneficial growth to plants. Additionally, results from analyzing the diversity and distribution of virulence plasmids in pathogenic lineages were inconsistent with predicted patterns of pathogen transmission. Evidence suggests that the horizontal acquisition of the virulence plasmid establishes new lineages of pathogens, and can drive the rapid evolutionary transition of the bacteria from being beneficial to being pathogenic.</div>