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Journal of Bacteriology, February 2000, p. 749-757, Vol. 182, No. 3
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Discovery of a Nonclassical Siderophore, Legiobactin, Produced by Strains of Legionella pneumophila

Mark R. Liles, Tracy Aber Scheel, and Nicholas P. Cianciotto*

Department of Microbiology and Immunology, Northwestern University Medical School, Chicago, Illinois 60611

Received 3 September 1999/Accepted 6 November 1999

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 not elaborate siderophores. However, we now present evidence that supernatants from L. pneumophila cultures can contain a nonproteinaceous, high-affinity iron chelator. More specifically, when aerobically grown in a low-iron, chemically 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 relatively high density with bacteria that had been grown overnight to log or early stationary phase in CDM or buffered yeast extract. Inocula derived from late-stationary-phase cultures, despite ultimately growing, consistently failed to result in the elaboration of siderophore-like activity. The Legionella CAS reactivity was detected in the culture supernatants of the serogroup 1 strains 130b and Philadelphia-1, as well as those from representatives of other serogroups and other Legionella species. The CAS-reactive substance was resistant to boiling and protease treatment and was associated with the <1-kDa supernatant fraction. As would also be expected for a siderophore, the addition of 0.5 or 2.0 µM iron to the cultures repressed the expression of the CAS-reactive substance. Interestingly, the supernatants were negative in the Arnow, Csáky, and Rioux assays, indicating that the Legionella siderophore was not a classic catecholate or hydroxamate and, hence, might have a novel structure. We have designated the L. pneumophila siderophore legiobactin.

* Corresponding author. Mailing address: Department of Microbiology and Immunology, Northwestern University Medical School, 320 East Superior St., Chicago, IL 60611-3010. Phone: (312) 503-0385. Fax: (312) 503-1339. E-mail: n-cianciotto{at}nwu.edu.

Journal of Bacteriology, February 2000, p. 749-757, Vol. 182, No. 3
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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