Commercial potting soils as an alternative infection source of Legionella pneumophila and other Legionella species in Switzerland

Legionella spp. are pathogens that can cause Legionnaires’ disease in humans through inhalation of contaminated aerosols. The principal reservoir for these microorganisms is water, but Legionella spp. have been isolated from composted vegetable and plant material, and from many potting mixes as well. In Australia, there have been several cases of Legionnaires’ disease in which Legionella longbeachae has been isolated from potting soils. In Switzerland, the source of infection cannot always be identified as water or cooling towers: therefore, we have investigated 46 commercially available potting soils in Switzerland to determine the presence of Legionella spp. We were able to detect Legionella spp. in 45.7% (21/46) of the potting soil samples analysed by culture. Legionella pneumophila was present in 19.6% (9/46) of the samples and L. pneumophila serogroup 1 in 6.5% (3/46). Quantification by both culture and quantitative real-time PCR revealed high concentrations of legionellae in potting soils, ranging between 103 CFU/g and 105 CFU/g and 104 genomic units (GU)/g and 106 GU/g, respectively. Thus, potting soils may represent an alternative reservoir for Legionella spp. in Switzerland.