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Understanding the role of Type VI Secretion Systems for intra-specific competition and pathogenicity in Erwinia tracheiphila

Claudio Vrisman: Department of Plant Pathology, The Ohio State University

<div>In many pathogenic bacteria, Type VI Secretion Systems (T6SS) are important for intra-specific bacterial competition. Genetically distinct populations have recently been observed within the phytopathogen <em>Erwinia</em> <em>tracheiphila </em>(<em>Et</em>). To assess whether genetically distinct <em>Et</em> strains interact <em>in planta</em>, co-inoculation assays using TedCu10 (TC) and BHKY (BH), strains isolated from cucumber and squash, respectively, were performed <em>in vitro</em> and <em>in planta </em>using squash. In single inoculations BH, but not TC, is pathogenic to squash. All BH inoculated plants died within 26 days post inoculations. For co-inoculations, the second true leaves of squash plants were wound inoculated with TC at concentrations of 10<sup>8</sup>, 10<sup>7</sup>, 10<sup>6</sup>, 10<sup>5</sup> or 0 colony forming units (CFU)/ml and then immediately inoculated with BH at 10<sup>8</sup> CFU/ml in the same sites. Plants inoculated with TC at 10<sup>8</sup> CFU/ml followed by BH did not die. Approximately 70% of plants inoculated with TC at 10<sup>5</sup> CFU/ml followed by BH died. A decrease in BH population was observed <em>in vitro</em> when BH was mixed at a ratio of 1:1 with TC on solid but not in liquid medium, suggesting that the interaction effect observed <em>in planta </em>could be contact dependent T6SS mediated. Whole genome sequencing showed that TC has three T6SS loci while BH contains two. Deletion mutants of the different T6SS loci in TC and BH are being generated to evaluate the effect of these loci on pathogenicity, disease suppression and within host bacterial competition.</div>