Influence of pH on bioactivity of cinnamon oil against Legionella pneumophila and its disinfection efficacy in hot springs

Cinnamon oil extracted from leaves of Cinnamomum osmophloeum has recently been proved as a promising antibacterial agent against Legionella pneumophila, an etiological agent of human pneumonia known as Legionnaires' disease. However, the pH effects on the efficacy of cinnamon oil against L. pneumophila and its applicability to recreational spring water remain unknown. We therefore determined the bactericidal activity of cinnamon oil at pH 3–10 in phosphate-buffered saline (PBS) and in four kinds of springs with various conductivity (259–5595 μs cm−1) and pH (2.1–7.7) levels. Results show L. pneumophila cells were more susceptible to cinnamon oil at pH 8–10 than at pH 4–6 in PBS, which became more evident as increasing contact time from 10 to 60 min. An increase in concentration of cinnamon oil and contact time significantly increased the anti-L. pneumophila activity (P ≤ 0.001), indicating a consistent biocidal effect regardless of pH. Interestingly, this dose–response biocidal effect was also observed in spring waters. Moreover, L. pneumophila of 4 log CFU ml−1 in spring waters was completely inactivated within 60 min by cinnamon oil at 300–750 μg ml−1, with the highest inactivation in alkaline hydrogen carbonate spring. The great bioactivity of cinnamon oil demonstrates its potential to be used to control Legionella growth in recreational spring water and possibly other niches generally at basic pH, e.g., cooling towers.