Interference of copper sulphate in growth of Erwinia amylovora

To understand the toxicity of copper salts on Erwinia amylovora, which are used in the control of fire blight, bacterial growth and cell metabolism wa s assayed with copper sulphate in the presence or absence of complex-formin g compounds such as various amino acids or citrate. In minimal medium witho ut amino acids copper sulphate strongly interfered with the growth Of E. am ylovora. A concentration of 15 mu M CuSO4 resulted in about 50% growth inhi bition. 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 a 'bacteriostatic' effect of copper ions and allowed growth of the bacteria at 2 mM CuSO4. Other amino acids had a s imilar effect in the protection of E, amylovora against copper ions. This w as in contrast to glycine betain, which was unable to suppress growth inhib ition by CuSO4. Presumably, the free ammonium groups of amino acids partici pated in the protective effect. The addition of citrate, exceeding the amou nt of copper-ions, was also protective. Bioluminescence of E. amylovora cel ls was expressed via a constitutive promoter from the lux-operon of Vibrio fischeri. The light emission is dependent on active cell metabolism. In a n ovel 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 bioluminescenc e. but no change in light production was noticed, when citrate or asparagin e were present in addition to copper sulphate. A decrease of bioluminescenc e to 50% was observed for 50 mu M CuSO4 in the absence of amino acids.