A Novel IncX Plasmid Mediates High-Level Oxytetracycline and Streptomycin Resistance in from Commercial Pear Orchards in California.

Isolates of the fire blight pathogen with high-level resistance to oxytetracycline (minimal inhibitory concentration [MIC] > 100 μg/ml) and to streptomycin (MIC > 100 μg/ml) were recovered from four commercial pear orchards in California between 2018 and 2020. The two representative oxytetracycline- and streptomycin-resistant (OxyTc-Sm) strains 32-10 and 33-1 were as virulent as the antibiotic susceptible strain 13-1 in causing blossom blight of pear and were recovered more than 50% of the time 7 days after co-inoculation to pear flowers with strain 13-1. In the field, inoculation of strain 32-10 to pear flowers that were pretreated with oxytetracycline at 200 μg/ml did not reduce disease compared with an untreated control. Four OxyTc-Sm strains were subjected to draft genome sequencing to identify the genetic determinants of antibiotic resistance and their location. A 43.6-kb IncX plasmid, designated pX11-7, was detected in each of the four strains, and this plasmid encoded the tetracycline-resistance gene and the streptomycin-resistance gene pair within a large putatively mobile genetic element consisting of the transposon Tn that had inserted within the streptomycin-resistance transposon Tn. We also determined that pX11-7 was conjugative and was transferred at a rate that was 10 to 10 higher into an strain isolated in California compared with an strain that was isolated in Michigan. The occurrence of high levels of resistance to both oxytetracycline and streptomycin in strains from commercial pear orchards in California significantly limits the options for blossom blight management in these locations.