Removing Legionella pneumophila and biofilms from water supply systems using plant essential oils

Antibacterial and antibiofilm activities of sage (Salvia officinalis), thyme (Thymus vulgaris), peppermint (Mentha x piperita) and lemon (Citrus limonum) essential oils (EOs), supplied as commercial preparations, were investigated on Legionella pneumophila. Results showed that all EOs had antibacterial activity. Lemon oil demonstrated the best activity, with minimum inhibition concentration (MIC) and minimum bactericidal concentration values detected as 0.83–1.25 and 1.25–3.75 μL/mL, respectively. Antibiofilm activities of EOs were studied in two ways: (1) inhibition of biofilm formation at subMIC concentrations; and (2) eradication of preformed biofilms at MIC and subMIC concentrations. The highest biofilm inhibition values were 63.3, 45.5, 35.6 and 23%, and the highest eradication values were 34.2, 16.7, 39.4 and 31.9% for lemon oil, peppermint oil, sage oil and thyme oil, respectively. Lemon oil was detected to have the highest antibiofilm activity, as with antibacterial activity. The minimum antibiofilm activity was found for thyme oil. For all the EOs, the inhibition of biofilm formation was higher than the eradication of preformed biofilms at the same concentrations. The results suggested that EOs of lemon, peppermint, sage and thyme had the potential for being used in the removal of L. pneumophila from potable water systems, due to their antibacterial and antibiofilm activities.