Role of food lipids in cold adaptation of Bacillus cereus in absence of oxygen
Résumé
Production of heat resistant spores combined with the ability of some strains to grow at temperatures of refrigeration, makes B. cereus a hazard in cooked chilled foods. Cooked chilled foods are mostly packaged anaerobically to reduce oxidative spoilage. In air, B. cereus adapts its membrane to cold through fatty acids desaturation, which is not possible in absence of oxygen.
The impact of absence of oxygen on growth of B. cereus at low temperature, and the potential role of food lipids was studied.
Bacillus cereus was grown in cooked spinach and in synthetic media with spinach extracts. B. cereus lipids were extracted and characterized.
Anaerobiosis inhibited growth of Bacillus cereus at low temperatures in synthetic medium but not in cooked spinach. Adding a lipid extract of cooked spinach to the synthetic medium restored growth of B. cereus, whereas an aqueous extract had no such effect. The lipid extract of spinach contained a high proportion of unsaturated, low melting point fatty acids, which were presumably integrated in B. cereus membrane, increased its fluidity, and permitted B. cereus growth at low temperature. When added to the synthetic medium, triglycerides and various phospholipids rich in unsaturated fatty acids restored growth of B. cereus at cold and anaerobiosis, but not free unsaturated fatty acids and phospholipids containing only saturated fatty acids. In the case of phospholipids rich in unsaturated fatty acids such as soy lecithin, 25 µg ml-1 in the growth medium were sufficient to observe an effect, and 125 µg ml-1 were sufficient for the maximal effect. Lipids from B. cereus membranes grown at low temperature anaerobically in presence of soy lecithin, were at least as fluid as those from membranes of B. cereus grown aerobically. Triglycerides were integrated unmodified in B. cereus membranes, whereas phospholipids were integrated in diverse ways, mostly as diacylglycerol. Presence of these very unusual lipids in its membrane did not prevent multiplication of B. cereus, but may explain the distorted morphology of the cells observed under electronic transmission microscopy.
Ability of B. cereus to grow at cold temperatures in absence of oxygen depends on the presence of lipids rich in unsaturated fatty acids. Lipids are widespread in foods and should not be overlooked when predicting the response of pathogenic bacteria to stress conditions.