The inhibitory effect of copper, zinc and manganese on Legionella longbeachae in potting mix leachate

Abstract

Legionella longbeachae is the leading cause of Legionnaires’ disease (LD) in Australasia and has been linked to exposure to compost and potting soils. Adding antimicrobial metal ions such as copper (Cu²⁺), zinc (Zn²⁺) and manganese (Mn²⁺) to potting soils may reduce the load of L. longbeachae bacteria and infection risk. Baseline concentrations of metal ions in leachate from peat, bark dust, bagging base and an all-purpose potting soil were: iron 0.40 µg/mL to 0.99 µg/mL, Cu of 0.003 µg/mL to 0.03 µg/mL, Zn 0.01 µg/mL to 0.06 µg/mL and Mn 0.11 µg/mL to 0.29 µg/mL. Addition of Cu²⁺ ions to leachate reduced L. longbeachae viability in a concentration dependent manner. A similar effect was seen in potting soil with Zn²⁺ and Mn²⁺ but ten-fold higher concentrations were needed. These metal ions have potential to reduce the load of L. longbeachae in potting soils but toxicity in plants needs to be determined.