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Dynamics of chromosomal and plasmid-borne copper resistance systems in Xanthomonas perforans populations

Rishi Bhandari: Auburn University

<div>Bacterial spot of tomato and pepper is a major problem around the world. Repeated use of copper bactericides has rendered pathogen populations resistant to copper. Bacterial spot xanthomonads have evolved several mechanisms to efflux copper from bacterial cells, including the <em>cop</em> (copper resistance) systems. The Cop system in bacterial spot xanthomonads collected in the 1980s was plasmid-borne, with the exception of a unique chromosomal copper resistance operon characterized in strain XVP26, isolated in Taiwan. Movement of a large copper resistance plasmid (200-300kb) has been described as a mechanism for the spread of copper resistance among pathogenic xanthomonads. We recently identified several copper-tolerant <em>X. perforans</em> (<em>Xp</em>) strains collected in Alabama and Florida between 1990-2016 that do not contain these large plasmids. Analysis of complete genomes of representative strains obtained by PacBio SMRT sequencing revealed the presence of a copper resistance operon encoding CopL, CopA, CopB and CopF on a genomic island on the chromosome. Further screening of draft genomes of additional <em>Xp</em> strains indicated that chromosomal copper resistance system is spread in certain lineages of <em>Xp</em> populations in the U.S. Our findings indicate that continuous use of copper bactericides in the field has imposed a constant selective pressure on <em>Xp</em> populations leading to a dramatic shift in the balance of chromosomal copper resistance and plasmid-encoded copper resistance.</div>