IDENTIFICATION OF SITES OF INJURY IN LACTOBACILLUS-BULGARICUS DURING HEAT-STRESS

Heat resistance of Lactobacillus bulgaricus in skimmed milk at 62 degr ees, 64 degrees, 65 degrees and 66 degrees C was studied. The response to increasing temperatures in this range was not linear, with tempera tures at 65 degrees C and above giving a lower survival rate than woul d be predicted from experiments at lower temperatures. To identify sit es of injury at these temperatures, chemical markers were used. Heatin g at 64 degrees C and below resulted in damage to the cytoplasmic memb rane. Ar temperatures of 65 degrees C and above chemical markers also indicated damage in the cell wall and proteins. Using differential sca nning calorimetry analysis of whole cells of Lact. bulgaricus seven ma in peaks were observed (l-51, m(1)-61, m(2)-73, n-80, p-89, q-100, r-1 12 degrees). Three of these peaks (l(r), m(r) and p(r)) were the resul t of reversible reactions. Analysis of cell fractions identified the c ell 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 degrees C and above.