Ultrastructural alterations induced by moist heat in Bacillus cereus

The ultrastructural alterations induced in vegetative, exponentially growing Bacillus cereus cells suspended in broth, by heating at 65 C for 2 and 15 min and 100 C for 5 min, were studied by electron microscopy of thin sections. The following alterations were observed: (i) change in the triple-layered profile of the membranes from the normal asymmetric to a symmetric geometry, appearance of fractures in the membranes, occurrence of prominent myelin-like systems of concentric membranes; (ii) disappearance of the ribosomes in most cells heated at 65 C and in all cells heated at 100 C; (iii) cytoplasmic precipitation resulting in the appearance of dense blocks of Pronase-sensitive material. The cell wall appears unaffected in most preparations. No signs of deoxyribonucleic acid damage are observed. These ultrastructural data are discussed in relation to the alterations in the chemistry and physiology of heated bacteria described in the literature.