Occurrence and Properties of Copper-resistance in Plant Pathogenic Bacteria

Abstract

Copper-resistance was analyzed with 189 strains of plant pathogenic bacteria that belonged to the genera Pseudomonas, Xanthomonas, Erwinia, Agrobacterium, Clavibacter and Curtobacterium. For the assay, CuSO₄ or copper compounds (copper fungicides) were incorporated into Bacto potato-dextrose agar (PDA). Bacteria that grew on PDA containing CuSO₄ at concentrations of 1.25mM or more were arbitrarily categorized as copperresistant. The minimum inhibitory concentrations (MIC) of CuSO₄ on the copper-PDA medium were one to two times greater than with casitone-yeast extract-glycerol agar medium, and 100 times greater than with aqueous solution. Copper-resistant strains were detected in many of the plant pathogenic bacteria, particularly in pseudomonads. The high MIC of CuSO₄ were specifically found in members of the rRNA II subgroup of Pseudomonas such as P. cepacia and P. gladioli. Resistance to CuSO₄ did not necessarily correlate with resistance to copper fungicides. In P. cepacia, P. gladioli, P. syringae pv. actinidiae, A. radiobacter and A. tumefaciens, copper- and streptomycin-resistances were concurrently found in the same strains. Copper-resistant strains of A. tumefaciens belonged to biovar 1 (all of 3 strains), biovar 2 (1 of 8 strains) and biovar undetermined (1 of 2 strains). A copper-resistant strain of A. radiobacter also belonged to biovar 1. Six strains of A. vitis tested were all copper-sensitive.