Abstract
Legionnaires’ disease (LD) is a community acquired pneumonia caused by Legionella bacteria namely Legionella pneumophila and Legionella longbeachae. In New Zealand it is L. longbeachae that is the main causative agent of LD and is associated with the use of commercial potting soil and compost. A personal risk factor for developing LD is old age and since New Zealand has an aging population understanding the spatial and temporal distribution of Legionella bacteria in commercial potting soil is important. By understanding the distribution of these bacteria within potting soil the development of a safer potting soil can be developed to reduce the risk of contracting LD from its usage.
A systematic analysis of commercial potting soil revealed it serves as a significant reservoir for Legionella spp. including L. longbeachae showing a sporadic and focal distribution pattern. Legionella spp. and L. longbeachae were more prevalent during spring and summer seasons, aligning with epidemiological trends of LD. Testing a range of agricultural compounds in vitro demonstrated that copper (Cu2+), zinc (Zn2+) and manganese (Mn2+) sulphates effectively inhibited L. longbeachae growth, with Cu2+ showing the most rapid bactericidal activity. Further, reducing bioavailable iron (Fe2+) concentrations enhanced the antimicrobial efficacy of these compounds, suggesting that iron plays a critical role in L. longbeachae survival and proliferation.
The addition of these metal ions to potting soil showed that Cu2+ remained the most effective antimicrobial agent, successfully, inhibiting L. longbeachae in potting soil. However, the binding of copper ions to organic matter in the soil necessitated a ten-fold increase in concentration (0.5mg/mL) to achieve effective inhibition. While this Cu2+ concentration was effective against the bacterium it significantly reduced seed germination and inhibited the growth of viola and cabbage plants. Adverse effects included reduced plant height, biomass, chlorophyll content, and overall health. Inductively coupled plasma analysis further confirmed Cu2+accumulation in plant tissues, raising concerns about its ingestion.
These findings highlight the challenges of controlling Legionella spp. including L. longbeachae in potting soil while maintaining plant growth and soil health and exploring alternative strategies to mitigate Cu2+ toxicity is crucial.