TheBordetellaBfe System: Growth and Transcriptional Response to Siderophores, Catechols, and Neuroendocrine Catecholamines

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Abstract
Ferric enterobactin utilization by Bordetella bronchiseptica and Bordetella pertussis requires the BfeA outer membrane receptor. Under iron-depleted growth conditions, transcription of bfeA is activated by the BfeR regulator by a mechanism requiring the siderophore enterobactin. In this study, enterobactin-inducible bfeA transcription was shown to be TonB independent. To determine whether other siderophores or nonsiderophore catechols could be utilized by the Bfe system, various compounds were tested for the abilities to promote the growth of iron-starved B. bronchiseptica and induce bfeA transcription. The BfeA receptor transported ferric salmochelin, corynebactin, and the synthetic siderophores TRENCAM and MECAM. Salmochelin and MECAM induced bfeA transcription in iron-starved Bordetella cells, but induction by corynebactin and TRENCAM was minimal. The neuroendocrine catecholamines epinephrine, norepinephrine, and dopamine exhibited a remarkable capacity to induce transcription of bfeA. Norepinephrine treatment of B. bronchiseptica resulted in BfeR-dependent bfeA transcription, elevated BfeA receptor production, and growth stimulation. Pyrocatechol, carbidopa, and isoproterenol were similarly strong inducers of bfeA transcription, whereas tyramine and 3,4-dihydroxymandelic acid demonstrated low inducing activity. The results indicate that the inducer structure requires a catechol group for function and that the ability to induce bfeA transcription does not necessarily correlate with the ability to stimulate bacterial growth. The expanded range of catechol siderophores transported by the BfeA receptor demonstrates the potential versatility of the Bordetella Bfe iron retrieval system. The finding that catecholamine neurotransmitters activate bfeA transcription and promote growth suggests that Bordetella cells can perceive and may benefit from neuroendocrine catecholamines on the respiratory epithelium.

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