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Growth of Moraxella catarrhalis with human transferrin and lactoferrin: expression of iron-repressible proteins without siderophore production

Moraxella (Branhamella) catarrhalis, a mucosal pathogen closely related to Neisseria species, is a prominent cause of otitis media in young children and lower respiratory tract infections in adults. In this study, we investigated whether M. catarrhalis can compete for iron bound to human transferrin or human lactoferrin in a manner similar to that utilized by Neisseria meningitidis and Neisseria gonorrhoeae. Our studies demonstrated that M. catarrhalis obtains iron from these serum carrier proteins and also maintains growth with ferric nitrate in vitro. Furthermore, we report that when M. catarrhalis is grown under iron-limited conditions, the bacteria express new outer membrane proteins that are not detected in membranes of organisms cultured in an iron-rich environment. We have shown that these are iron-repressible proteins since they are not induced by other environmental stresses and the expression of these proteins is repressed when a source of iron is provided for iron-limited bacteria. The iron-repressible proteins are expressed in the absence of any detectable siderophore production. These iron-repressible proteins may be important for the acquisition and utilization of iron in vivo, which could allow M. catarrhalis to colonize and survive on human mucosal surfaces.