Adaptation of the food-borne pathogen Bacillus cereus to carvacrol
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O R I G I N A L PA P E R
Annemieke Ultee · Edwin P. W. Kets · Mark Alberda · Folkert A. Hoekstra · Eddy J. Smid
Adaptation of the food-borne pathogen Bacillus cereus to carvacrol
Received: 21 February 2000 / Revised: 23 June 2000 / Accepted: 17 July 2000 / Published online: 1 September 2000 © Springer-Verlag 2000
Abstract Carvacrol, a natural antimicrobial compound present in the essential oil fraction of oregano and thyme, is bactericidal towards Bacillus cereus. A decrease of the sensitivity of B. cereus towards carvacrol was observed after growth in the presence of non-lethal carvacrol concentrations. A decrease of the melting temperature (Tm) of membranes from 20.5 °C to 12.6 °C was the immediate effect of the addition of carvacrol. Cells adapted to 0.4 mM carvacrol showed a lower membrane fluidity than nonadapted cells. Adaptation of 0.4 mM carvacrol increased the Tm from 20.5 °C to 28.3 °C. The addition of carvacrol to cell suspensions of adapted B. cereus cells decreased Tm again to 19.5 °C, approximately the same value as for the non-adapted cells in the absence of carvacrol. During adaptation, changes in the fatty acid composition were observed. The relative amount of iso-C13:0, C14:0 and isoC15:0 increased and cis-C16:1 and C18:0 decreased. The head-group composition also changed, two additional phospholipids were formed and one phospholipid was lacking in the adapted cells. It could be concluded that B. cereus adapts to carvacrol when present at non-lethal concentrations in the growth medium by lowering its membrane fluidity by changing the fatty acid and headgroup composition. Keywords Bacillus cereus · Carvacrol · Adaptation · Membranes · Gel-to-liquid-crystalline transition temperature · Fatty acid composition · Head-group composition · Membrane fluidity
A. Ultee (✉) · E. P. W. Kets · E. J. Smid Agrotechnological Research Institute (ATO-DLO), PO Box 17, 6700 AA Wageningen, The Netherlands Tel.: +31-317-475171, Fax: +31-317–475347 A. Ultee · E. P. W. Kets Wageningen Centre for Food Sciences, PO Box 557, 6700 AN Wageningen, The Netherlands M. Alberda · F. A. Hoekstra Wageningen University, Laboratory of Plant Physiology, Arboretumlaan 4, 6703 BD Wageningen, The Netherlands
Introduction The use of natural antimicrobial systems for the preservation of foods is increasing. An example of a natural preservative agent is carvacrol, which is present in the essential oil fraction of oregano (60–74% carvacrol) and thyme (45% carvacrol) (Lagouri et al. 1993; Arrebola et al. 1994). Several studies have shown the inhibition of growth of different micro-organisms by carvacrol (Knobloch et al. 1986; Thompson 1990, 1996; Conner 1993; Juven et al. 1994; Kim et al. 1995a , b; Sivropoulou et al. 1996). Addition of carvacrol to vegetative Bacillus cereus cells results in a dose-related extension of the lag phase and a lower final population density (Ultee et al. 1998). Above approximately 1 mM (dependent on the temperature), carvacrol decreases the viability exponentially. Exposure of B. cereus to carvacrol leads to a
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