JB Accepts, published online ahead of print on 11 May 2007

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J. Bacteriol. doi:10.1128/JB.00239-07
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Low concentrations of bile salts induce stress responses and reduce motility in the Bacillus cereus type strain

Simen M. Kristoffersen, Solveig Ravnum, Nicolas J. Tourasse, Ole Andreas kstad, Anne-Brit Kolstø, and William Davies^*

Department of Molecular Biosciences, University of Oslo, PB 1041 Blindern, 0316 Oslo, Norway; Department of Pharmaceutical Biosciences, School of Pharmacy, University of Oslo, PB 1068 Blindern, 0316 Oslo, Norway

^* To whom correspondence should be addressed. Email: william.davies{at}imbv.uio.no.

Tolerance to bile salts was investigated in forty Bacillus cereus strains, including 17 environmental isolates, 11 dairy isolates, 3 isolates from food poisoning outbreaks and 9 other clinical isolates. Growth of all strains was observed at low bile salt concentrations but no growth was observed on LB-agar plates containing over 0.005% bile salts. Pre-incubation of the B. cereus type strain, ATCC 14579, in low levels of bile salts did not increase tolerance levels.

B. cereus ATCC 14579 was grown to mid-exponential growth phase and shifted to medium containing bile salts (0.005%). Global expression patterns were determined by hybridization of total cDNA to a 70-mer oligonucleotide microarray.

A general stress response and a specific response to bile salts was observed. The general response was similar to that observed in cultures grown in the absence of bile salts but at a higher (2x) cell density. Up-regulation of several putative multidrug exporters and transcriptional regulators, and down-regulation of most motility genes were observed as part of the specific response. Motility experiments in soft agar showed that motility decreased following bile salts exposure, in accordance with the transcriptional data. Genes encoding putative virulence factors were either unaffected or down-regulated.

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