Effects of heat shock treatment on the survival rate of Lactobacillus acidophilus after freeze-drying

Herein we investigated the effects of heat shock treatment on the resistance of Lactobacillus acidophilus ATCC4356 to freeze-drying and the underlying mechanisms. We assessed the survival rate, cell morphology, enzyme activities, and metabolites in glycometabolism and energy metabolism. Heat shock treated at 45 degrees C for 30 min has increased the survival rate from 39.1% to 56.3% and had a certain protective effect on the integrity of the cell wall and membrane after freeze-drying. Activities of key enzymes, namely glucose-6-phosphate isomerase and lactate dehydrogenase in the glycolytic pathway; phosphoglucomutase, UDP-glucose pyrophosphorylase, and glycosyltransferases in the glycogen biosynthetic pathway; and Na+ -K+ -ATPase in energy metabolism were significantly altered. Further, the utilization rate of extracellular glucose in the broth decreased 7.59% but the conversion rate of intracellular glucose increased 24.04%, which led to the production of lactic acid and energy. Meanwhile, the production of polysaccharides with potential protectant function was increased by 47.6% and the proportion of glucose in the monosaccharide fraction decreased from 21% to 17%. However, the production of galactose increased from 17% to 26%, consequently enhancing the activities and survival rate of bacterial cells in a freeze-drying environment. This is the first study to determine the potential mechanisms and metabolic changes induced by heat shock treatment that make LAB tolerant to freeze-drying, and providing a new insight on the anti-adversity for LAB during the process.