Identification of sites of injury in Lactobacillus bulgaricus during heat stress

Heat resistance of Lactobacillus bulgaricus in skimmed milk at 62#&176;, 64#&176;, 65#&176; and 66#&176;C was studied. The response to increasing temperatures in this range was not linear, with temperatures at 65#&176;C and above giving a lower survival rate than would be predicted from experiments at lower temperatures. To identify sites of injury at these temperatures, chemical markers were used. Heating at 64#&176;C and below resulted in damage to the cytoplasmic membrane. At temperatures of 65#&176;C and above chemical markers also indicated damage in the cell wall and proteins. Using differential scanning calorimetry analysis of whole cells of Lact. bulgaricus seven main peaks were observed (l-51, m 1 -61, m 2 -73, n-80, p-89, q-100,r-112#&176;C). Three of these peaks (l r , m r and p r ) were the result of reversible reactions. Analysis of cell fractions identified the cell structure involved in giving rise to each of the three reversible peaks; l r , cell membrane lipids, m r , ribosomes, and p r , DNA. The evidence presented in this paper shows that irreversible reactions in the cell ribosomes are a critical site of damage in Lact. bulgaricus during heat stress in liquid media at 65#&176;C and above.