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J. Bacteriol. doi:10.1128/JB.01701-06
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

The redox regulator Fnr is required for fermentative growth and enterotoxin synthesis in Bacillus cereus F4430/73

Université d'Avignon, UMR A408, Sécurité et Qualité des Produits d'Origine Végétale, F-84029, France; INRA, Avignon, F-84914, France; IUT Génie Biologique, Université d'Avignon, UMR A408, Sécurité et Qualité des Produits d'Origine Végétale, F-84029, France; INRA, Avignon, F-84914, France

^* To whom correspondence should be addressed. Email: catherine.duport{at}univ-avignon.fr.

	Abstract

Glucose-grown cells of Bacillus cereus respond to anaerobiosis and low extracellular oxidoreduction potential (ORP), notably by enhancing enterotoxin production. This response involves the ResDE two-component system. We searched the B. cereus genome for other redox response regulators potentially involved in this adaptative process, and we identified one gene encoding a protein predicted as having an amino acid sequence 58% identical (80% similar) to the Bacillus subtilis Fnr redox regulator. The fnr gene of the food-borne pathogen B. cereus F4430/73 has been cloned and partially characterized. We showed that fnr was up-regulated during anaerobic fermentation, especially when performed at low ORP (highly reducing conditions). The expression of fnr was down-regulated in the presence of dioxygen and nitrate which, unlike fumarate, stimulated the respiratory pathways. Inactivation of B. cereus fnr abolished fermentative growth but only moderately affected aerobic and anaerobic nitrate respiratory growth. Analyses of glucose by-products and the transcription profiles of key catabolic genes confirmed the strong regulatory impact of Fnr on B. cereus fermentative pathways. More importantly, the fnr mutation strongly decreased the expression of PlcR-dependent hbl and nhe genes, leading to the absence of Hemolysin BL (Hbl) and Non-Hemolytic enterotoxin (Nhe) secretion by the mutant. These data indicated that fnr is essential for both fermentation and toxinogenesis. The results also suggest that both Fnr and the ResDE two-component system belong to a redox regulatory pathway that at least partially functions independently of the pleiotropic virulence gene regulator PlcR to regulate enterotoxin gene expression.

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