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The Redox Regulator Fnr Is Required for Fermentative Growth and Enterotoxin Synthesis in Bacillus cereus F4430/73

Assia Zigha,¹ Eric Rosenfeld,^1, Philippe Schmitt,² and Catherine Duport^1*

Université d'Avignon-INRA, UMR A408, Sécurité et Qualité des Produits d'Origine Végétale, Avignon F-84029, France,¹ IUT Génie Biologique, Université d'Avignon-INRA, UMR A408, Sécurité et Qualité des Produits d'Origine Végétale, Avignon F-84029, France²

Received 3 November 2006/ Accepted 19 January 2007

Glucose-grown cells of Bacillus cereus respond to anaerobiosis and low extracellular oxidoreduction potentials (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 adaptive process, and we identified one gene encoding a protein predicted to have an amino acid sequence 58% identical (80% similar) to that of 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 fermentation occurred at low ORP (under highly reducing conditions). The expression of fnr was down-regulated in the presence of O₂ and nitrate which, unlike fumarate, stimulated the respiratory pathways. The 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 nonhemolytic enterotoxin (Nhe) secretion by the mutant. These data indicate 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 functions at least partially independently of the pleiotropic virulence gene regulator PlcR to regulate enterotoxin gene expression.

Published ahead of print on 26 January 2007.

Present address: Laboratoire de Biotechnologies et Chimie Bio-organique FRE 2766, CNRS, Bâtiment Marie-Curie, Pôle Sciences et Technologies, Université de La Rochelle, avenue Michel Crépeau, 17042 La Rochelle Cedex 1, France.

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