Epidemiology and genetic diversity of streptomycin resistance in Erwinia amylovora in Michigan

摘要

Streptomycin-resistant (Smr) strains of E. amylovora were recovered from an apple orchard in Van Buren county in southwestern Michigan, USA, in 1990 and from 22 of 89 orchards statewide during 1991-94. Smr strains were isolated from 17, 2 and 3 orchards in Van Buren, Kent (94 km north of original site) and Newaygo (132 km north of original site) counties, respectively. The Smr strains were separated into moderately resistant (MR) and highly resistant (HR) phenotypes, indicating the existence of 2 distinct genetic determinants for tolerance. HR strains were detected in Kent county, a mixture of MR and HR strains in 1 Van Buren orchard and MR strains in all remaining orchards. Resistance in HR strains was chromosomal; a single base pair mutation atcodon 43 of the rpsL gene resulted in a lysine to arginine conversion in ribosomal protein S12. Sensitivity to streptomycin was restored in HR strains with a plasmid carrying the wild-type E. amylovora rpsL gene. Resistance in MR strains was mediated byaminoglycoside-modifying enzymes encoded by genes strA and strB. In most MR strains, strA-strB genes were carried by the transferable plasmid pEa34 on transposon Tn5393, although Tn5393 had transposed to the chromosome or to universal plasmid pEA29 in other strains. Oxytetracycline is being used in those counties with Smr strains.