Siderophore-mediated inhibition of Legionella pneumophila by environmental Pseudomonas isolates

The genus Legionella comprises opportunistic pathogenic bacteria commonly found in natural and engineered water systems, where they interact with environmental microbes and protozoa, primarily in biofilms. Legionella pneumophila is the main causative agent of Legionnaires disease and is transmitted through inhalation of contaminated aerosols. Iron availability is a critical factor for L. pneumophila growth, persistence, and virulence, yet iron is often limited in aquatic environments. To overcome iron scarcity, many bacteria produce siderophores, secondary metabolites that scavenge ferric iron. Because siderophores are chemically diverse and species specific, they play a key role in inter-species competition and can withhold iron from competitors. Here, we investigated the effects of iron depletion and siderophore-mediated competition on L. pneumophila using commercial pyoverdines and extracellular metabolites from environmental Pseudomonas strains. Growth assays showed that L. pneumophila can grow under iron-limited conditions but with lag phases extended by more than 20 hours. Pyoverdines inhibited growth in a concentration-dependent manner, primarily increasing the time to mid-log phase (t_mid). Supernatants and crude pyoverdine extracts from siderophore-producing Pseudomonas strains caused the strongest inhibition, including lag-phase extensions of up to 55 hours or complete growth arrest. These results demonstrate that siderophore-producing bacteria can suppress L. pneumophila by limiting iron availability.