Screening for copper and antibiotic resistance in Erwinia amylovora population from Serbia

Continuous use of copper-based treatments and antibiotics in pear and apple protection programs may result in appearance of copper or antibiotic resistant strains of Erwinia amylovora. In order to determine the potential occurrence of copper or/and antibiotic resistance in Serbia we studied in vitro effect of different concentrations of copper sulfate, streptomycin and kasugamycin on development and growth of 40 E. amylovora strains originating from this region. Filter sterilized solution of the bactericides was added to the NA medium after sterilization to a final concentration of 100 and 200 ppm, respectively, followed by spot inoculation of each strain on the medium. None of the strains developed on NA amended with 100 or 200 ppm of streptomycin or kasugamycin. Our study showed that, despite the potential exposure to the antibiotics, E. amylovora strains from Serbia did not develop resistance to streptomycin or kasugamycin. On the other hand, all strains had normal growth on NA amended with 100 ppm of copper sulfate indicating certain level of resistance to copper ions. At 200 ppm, 28 strains formed colonies of normal size and appearance, while 12 strains had smaller colonies with reduced growth. This might be due to the extensive and frequent use of copper-based compounds to control fire blight in Serbia.