Antibacterial Activity of Cyanobacterial Extracts against Legionella spp. ^†

Cyanobacteria are recognized sources of natural compounds with a pharmaceutical interest, namely for their antimicrobial activity. Several studies have shown the inhibitory effect of cyanobacteria against the most common bacterial pathogens. However, the bioactivity against Legionella pneumophila was never reported. L. pneumophila is ubiquitous in water environments and causes respiratory infections through water–air transmission. A rise in Legionella outbreaks might be expected, considering that climate changes will exacerbate water-borne diseases. In this work, we evaluated the antibacterial potential of 25 freshwater cyanobacteria from ESSACC collection, against seven Legionella strains (two reference, two clinical and three environmental). Cyanobacterial biomass was extracted with n-hexane, dicloromethane:methanol (2:1), methanol 70%, and water and extracts were dried and dissolved in DMSO (25%). The disk diffusion method was adapted to Legionella growth using 1 McFarland suspension in BCYE plates. Levofloxacin (10 μg) and DMSO (25%) were used as positive/negative controls, respectively. Methanolic extracts from Dolichospermum flos-aquae (LMECYA 165), Limnothrix redekei (LMECYA 145), Microcystis aeruginosa (LMECYA 127) and Planktothrix agardhii (LMECYA 257) induced inhibition zones ≥10 mm, demonstrating their antibacterial activity against L. pneumophila. These results encourage us to further investigate the potentiality of cyanobacteria as natural sources of antibiotics and/or water disinfectants, to overcome the occurrence of pathogenic Legionella in water environments.