Abstract
Fatty acids of two mesophilic and one psychrotrophic strains of the foodborne pathogen Bacillus cereus were analyzed by gas chromatography coupled to mass spectrometry during growth at cold (10 and 12°C) vs. optimal (30°C) temperatures and during the whole growth process (6–7 sampling times) from lag to stationary phase. In all these strains, a sequential change of fatty acids during cold growth was observed. Fatty acids were modified as soon as the end of lag, with an increase of the short-chain fatty acids (less than 15 carbons), particularly i13. These short-chain fatty acids then reached a maximum at the beginning of growth and eventually decreased to their initial level, suggesting their importance as a rapid cold adaptation mechanism for B. cereus. In a second step, an increase in Δ5,10 di-saturated fatty acids and in monounsaturated fatty acids in Δ5 position, at the expense of unsaturation in Δ10, started during exponential phase and continued until the end of stationary phase, suggesting a role in growth consolidation and survival at cold temperatures. Among these unsaturated fatty acids, those produced by unsaturation of n16 increased in the three strains, whereas other unsaturated fatty acids increased in some strains only. This study highlights the importance of kinetic analysis of fatty acids during cold adaptation.
Highlights
Bacillus cereus sensu lato (B. cereus sl), a major cause of foodborne outbreaks in Europe (EFSA-BIOHAZ, 2016), is composed of closely related species (B. cereus sensu stricto, Bacillus thuringiensis, Bacillus cytotoxicus, Bacillus weihenstephanensis, Bacillus wiedmanii, Bacillus toyonensis, Bacillus mycoides, and Bacillus pseudomycoides) able to grow over a wide range of temperatures and in various environments (Guinebretiere et al, 2008)
The melting point of membrane fatty acid (FA) at cold temperatures is lowered by the synthesis of short-chain FAs (SCFAs) (Kaneda, 1972), increase in unsaturated FAs (UFAs) relative abundance, and increase in anteiso/iso branchedchain FAs (BCFAs) ratio (Kaneda, 1977; Freese et al, 2008)
The first three sampling times for FA analysis at 0.5, 1, and 2 h corresponded to a phase of rapid growth, the fourth sampling time at 4 h corresponded to the end of rapid growth, the fifth to seventh sampling points (6, 7, and 24 h) to stationary phase
Summary
Bacillus cereus sensu lato (B. cereus sl), a major cause of foodborne outbreaks in Europe (EFSA-BIOHAZ, 2016), is composed of closely related species (B. cereus sensu stricto, Bacillus thuringiensis, Bacillus cytotoxicus, Bacillus weihenstephanensis, Bacillus wiedmanii, Bacillus toyonensis, Bacillus mycoides, and Bacillus pseudomycoides) able to grow over a wide range of temperatures and in various environments (Guinebretiere et al, 2008). A crucial mechanism for bacterial growth at cold temperatures is change in fatty acid (FA) composition of the membrane to reduce its melting point, maintain fluidity, and exchanges with the extracellular environment (Haque and Russell, 2004; Russel, 2008; de Sarrau et al, 2012). Mesophilic B. cereus growth at relatively cold temperatures (12 or 15◦C) was characterized by an increase in UFA and SCFA relative abundance (Haque and Russell, 2004; Brillard et al, 2010; de Sarrau et al, 2012; Chazarreta Cifre et al, 2013; Diomande et al, 2015a). A few studies investigated FA changes during the stationary phase (Annous et al, 1999) and during the whole growth at optimal and high temperatures (Lopes et al, 2019a,b), but none considered the whole cold adaptation and cold growth process, from lag to late stationary
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