Sensitivity of Bacillus weihenstephanensis to acidic changes of the medium is not dependant on physiological state N. Desriac a, b, * , F. Postollec a , D. Durand b , I. Leguerinel b , D. Sohier a , L. Coroller b a ADRIA Développement, UMT 08.3 PHYSI’Opt, Z.A. de Creac’h Gwen, F-29196 Quimper Cedex, France b Université de Brest, EA3882, Laboratoire Universitaire de Biodiversité et Ecologie Microbienne, UMT 08.3 PHYSI’Opt, IFR148 ScInBioS, 6 rue de l’Université, F-29334 Quimper, France article info Article history: Received 7 February 2013 Received in revised form 19 June 2013 Accepted 24 June 2013 Available online 1 July 2013 Keywords: Bacillus weihenstephanensis Physiological state Acid resistance Predictive modelling Stresses abstract This study aims to quantify the effect of salt and acid preliminary exposure on acid resistance of vege- tative cells of Bacillus weihenstephanensis. The psychrotolerant strain KBAB4 was cultured until the mid- exponentially phase (i) in BHI, (ii) in BHI supplemented with 2.5% salt or (iii) in BHI acidified at pH 5.5 with HCl. The growing cells were subsequently inactivated in lethal acid conditions ranging from 4.45 to 4.70. Based on statistical criteria, a primary mixed-Weibull model was used to fit the acid inactivation kinetics. The acid resistance was enhanced for acid-adapted cells and decreased for salt-adapted cells. The secondary modelling of the bacterial resistance allowed the quantification of the change in pH leading to a ten folds variation of the bacterial resistance, i.e. cells sensitivity (z pH ). This sensitivity was not significantly affected whatever the preliminary mild exposure and the presence of sub-populations with different acid resistances. These results highlighted that pre-incubation conditions influence bac- terial acid resistance without affecting the sensitivity to acidic modifications, with a 10 fold reduction of Bacillus acid resistance observed for a reduction of 0.37 pH unit. Quantification of such adaptive stress response might be instrumental in quantitative risk assessment more particularly in food formulation, particularly for low-acid minimally processed foods. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Spore forming bacteria cause major problems in the food industry since they may be involved in both food poisoning and food spoilage, raising major safety and economical issues. The Bacillus cereus group consists of six closely related species: Bacillus anthracis, Bacillus thuringiensis, Bacillus mycoides, Bacillus pseudomycoides, Bacillus weihenstephanensis and B. cereus (sensu stricto). B. cereus and B. anthracis are known as human pathogens, B. thuringiensis is used as biopesticide, B. mycoides and, B. pseudomycoides are characterised by rhizoidal formations when grown on agar medium. The B. weihenstephanensis specie includes strains which can growth under chilled temperature (Lechner et al., 1998) leading major problem in ready-to-eat or ready-to-cook foods stored at chilled temperatures, also known as refrigerated processed foods of extended durability (REPFEDs). The heat resistance of Bacillus spore was extensively studied and is used for FSO establishment (Membre et al., 2006). The influence of sporulation environment of Bacillus (Baril et al., 2011a,b, 2012; Garcia et al., 2010; Gonzalez et al., 1999; Planchon et al., 2011) is known to have a great impact on spore resistance properties and spore formation abilities. Recently, Baril et al. (2012) reported that sporulation boundaries of B. weihenstephanensis were included within the range of temperatures, pH and water activities sup- porting growth. A decrease in spore heat resistance and sporulation rate was observed for spores of B. weihenstephanensis produced for non optimal growth conditions. For instance, the time to get one spore per ml was tenfold longer when sporulation occurred at 10  C rather than 30  C(Baril et al., 2011a). Since a low sporulation rate and low resistance of spores was observed under sublethal condi- tions, Bacillus vegetative cells resistance appears as key issue for food industry particularly for minimally processed food through the food process and storage. To ensure the microbial food safety and stability, the food in- dustry uses combination of hurdles, mild preservation factors (Leistner and Gorris, 1995). A commonly used hurdle is a low pH which allows the decrease in growth rate but also the inactivation of pathogens or spoilage microorganisms (Coroller et al., 2006; Greenacre et al., 2003; Ita and Hutkins, 1991). For low-acid mini- mally processed foods, the main microbiological hazard is the * Corresponding author. ADRIA Développement, UMT 08.3 PHYSI’Opt, Z.A. de Creac’h Gwen, F-29196 Quimper Cedex, France. Tel.: þ33 298 101 838. E-mail addresses: noemie.desriac@adria.tm.fr , desriacnoemie@hotmail.fr (N. Desriac). Contents lists available at SciVerse ScienceDirect Food Microbiology journal homepage: www.elsevier.com/locate/fm 0740-0020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.fm.2013.06.013 Food Microbiology 36 (2013) 440e446