Introduction: Bacteria of the genus Legionella are the cause of diseases collectively called legionellosis. Infection with Legionella bacteria can occur wherever they are present in sufficiently large numbers, and where water spray and generation of aerosols occur. The risk is greater in the water supply systems of buildings where the water temperature is between 20 and 50 °C. Physical and chemical approaches are used to prevent the reproduction of Legionella bacteria or to eliminate it, but these are often ineffective or may cause damage to plumbing and equipment due to the frequent use of aggressive disinfectants. Purpose: The aim of the research was to determine the effect of natural antimicrobial agents of tea tree and lemon eucalyptus essential oils on Legionella pneumophila bacteria. Methods: To determine the reproduction intensity of Legionella pneumophila bacteria at different concentrations, the antibacterial potential of selected natural extracts was analysed using the microdilution method. Results: The results showed that tea tree essential oil was effective in preventing biofilm formation in all three bacterial strains of Legionella pneumophila. It was most effective against Legionella pneumophila strain 130b bacterium, isolated from its natural environment. We achieved a 50% efficacy in preventing biofilm formation at MIC 1, a 53% efficacy at MIC 2, and a 56% efficacy at MIC 3. The lowest efficacy of essential oils in preventing biofilm formation was demonstrated in Legionella pneumophila subsp. ATCC 33152 bacterium, which suggests that this particular bacterium is more resistant to the effects of essential oils than the other two. Discussion and conclusion: During the research, we found that tea tree essential oil has the highest efficacy against biofilm formation and has antibacterial properties. Its active substances have been shown to have a positive effect on the prevention of biofilm formation. Development in the field of green chemistry represents a promising alternative to conventional methods of inhibiting the growth and development of biofilms, as further in-depth research could help reduce the use of conventional methods and, consequently, reduce microbial resistance and excessive use of conventional antimicrobial agents. Further research in this field would be necessary to determine the efficacy of the active components on different bacterial strains, together with the potential for synergistic activity of different essential oils.
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