P-030 - Examining high level of antibiotic resistance in Pseudomonas spp. isolated from Washington apple orchards

Saprophytic bacteria can function as reservoirs for antibiotic resistance (AR) genes transferable to pathogens like Erwinia amylovora, the causal agent of fire blight. In this work, streptomycin (Sm)-, kasugamycin (Ksg)- and/or oxytetracycline (OTc)-resistant bacteria were co-isolated with E. amylovora from symptomatic apple shoots. Among the bacteria recovered, Pseudomonas parakoreensis strain 2180 (Pp2180) exhibited the highest AR, with minimum inhibitory concentrations (MIC) for Sm, Ksg and OTc above 2.5, 3.2 and 1 mg/mL, respectively. P. syringae ABR5 (PsABR5) also exhibited high Sm resistance (MIC > 2.5 mg/mL). Genome sequencing results showed that PsABR5 and Pp2180 carried one plasmid of 73 kb and 57.7 kb, respectively. The plasmid of PsABR5, similar to other plasmids in Pseudomonas, contained resistance genes, encoding Sm (strAB), heavy metals (copA, arsB) and glufosinate (pat) resistance, as well as efflux pump components (tolC, mdcB). The plasmid of Pp2180 was similar to plasmids typically found in enterobacteria and some pseudomonads and harbored Sm (strAB), OTc (tetAR) and multidrug resistance (eamA) genes. In addition, Pp2180 also had a chromosomal Sm resistance gene aadA16. Plasmid curing and conjugation assays confirmed that both plasmids can be conjugated into other related Pseudomonas species and AR is mainly plasmid-mediated. Continuous AR monitoring in orchard environments is a useful tool to identify AR sources in apple orchards, detect emergence of resistant pathogenic strains and safeguard the efficacy of current disease management strategies.