Abstract :
To understand the toxicity of copper salts on Erwinia amylovora, which are used in the control of fire blight, bacterial growth and cell metabolism was assayed with copper sulphate in the presence or absence of complex-forming compounds such as various amino acids or citrate. In minimal medium without amino acids copper sulphate strongly interfered with the growth of E. amylovora. A concentration of 15 μm CuSO4 resulted in about 50% growth inhibition. In contrast to a strong effect of streptomycin, copper ions barely killed the cells when incubated in minimal medium for 1 h. The addition of 4 g asparagine per litre relieved aʹbacteriostaticʹeffect of copper ions and allowed growth of the bacteria at 2 mm CuSO4. Other amino acids had a similar effect in the protection of E. amylovora against copper ions. This was in contrast to glycine betain, which was unable to suppress growth inhibition by CuSO4. Presumably, the free ammonium groups of amino acids participated in the protective effect. The addition of citrate, exceeding the amount of copper-ions, was also protective. Bioluminescence of E. amylovora cells was expressed via a constitutive promoter from the lux-operon of Vibrio fischeri. The light emission is dependent on active cell metabolism. In a novel approach to determine the immediate response of E. amylovora after the addition of copper sulphate, the change of bioluminescence was determined. Addition of copper ions to MM3 medium strongly affected the bioluminescence, but no change in light production was noticed, when citrate or asparagine were present in addition to copper sulphate. A decrease of bioluminescence to 50% was observed for 50 μm CuSO4 in the absence of amino acids.
