Discovery of a nonclassical siderophore, legiobactin, produced by strains of Legionella pneumophila

The mechanisms by which Legionella pneumophila, a facultative intracellular parasite and the agent of Legionnaires' disease, acquires iron are largely unexplained. Several earlier studies indicated that L. pneumophila does no t elaborate siderophores. However, we now present evidence that supernatant s from L. pneumophila cultures can contain a nonproteinaceous, high-affinit y iron chelator. More specifically, when aerobically grown in a low-iron, c hemically defined medium (CDM), L. pneumophila secretes a substance that is reactive in the chrome azurol S (CAS) assay. Importantly, the siderophore- like activity was only observed when the CDM cultures were inoculated to re latively high density with bacteria that had been grown overnight to log or early stationary phase in CDM or buffered yeast extract. Inocula derived f rom late-stationary-phase cultures, despite ultimately growing, consistentl y failed to result in the elaboration of siderophore-like activity. The Leg ionella CAS reactivity was detected in the culture supernatants of the sero group 1 strains 130b and Philadelphia-1, as well as those from representati ves of other serogroups and other Legionella species. The GAS-reactive subs tance was resistant to boiling and protease treatment and was associated wi th the <1-kDa supernatant fraction. As would also be expected for a siderop hore, the addition of 0.5 or 2.0 mu M iron to the cultures repressed the ex pression of the GAS-reactive substance. Interestingly, the supernatants wer e negative in the Arnow, Csaky, and Rioux assays, indicating that the Legio nella siderophore was not a classic catecholate or hydroxamate and, hence, might have a novel structure. We have designated the L. pneumophila siderop hore legiobactin.