Legionella spp. in UK composts – a potential public health issue?

Over the past five years, a number of cases of Legionellosis in Scotland have been associated with compost use; however studies investigating sources of infection other than water systems remain limited. This study delivers the first comprehensive survey of composts commonly available in the UK for the presence of Legionella species. Twenty-two store-bought composts, one green-waste compost and one homemade compost were tested for Legionella by culture methods on BCYE-α media and confirmed by macrophage infectivity potentiator (mip) speciation. Twenty-two of the samples were re-tested after an enrichment period of eight weeks. In total, 15 of 24 composts tested positive for Legionella spp: a higher level of contamination than previously seen in Europe. Two isolates of L. pneumophila were identified, while L. longbeachae Sg 1 was found to be one of the most commonly isolated species. L. longbeachae infection would not be detected by routine Legionella urinary antigen assay, therefore such testing should not be used as the sole diagnostic technique in atypical pneumonia cases, particularly where there is an association with compost use. The occurrence of Legionella in over half of the samples tested indicates compost could pose a public health risk. The addition of general hygiene warnings to compost package may be beneficial in protecting public health.