Abstract
The bacterial pathogen Legionella pneumophila alternates between intracellular replication within host eukaryotes and extracellular residence in multi-species biofilms. To persist in the extracellular state, L. pneumophila must withstand competition from neighboring bacteria, including other Legionella. Here, we find that L. pneumophila can exclude other Legionella species from its local environment using a secreted inhibitor: HGA (homogentisic acid), the unstable, redox-active precursor molecule to L. pneumophila’s brown-black pigment. Unexpectedly, we find that L. pneumophila is itself inhibited by HGA secreted from neighboring isogenic strains. This HGA sensitivity is density-dependent and HGA secretion is linked to growth phase, suggesting that production of – and resistance to – this inhibitor are functionally linked. Our genetic approaches further identify lpg1681 as a gene that modulates Legionella susceptibility to HGA. We propose that HGA behaves as an unusual public good, allowing established Legionella communities to locally protect against invasion by low-density competitors.
