Morphology of extremely heat-resistant spores from Bacillus sp. ATCC 27380 by scanning and transmission electron microscopy.

Bacillus sp. ATCC 27380 is a recently discovered aerobic mesophile, isolated from surface soil, that produces spores with extreme resistance to dry heat: the length of time to 90% kill is 139 hr at 125 degrees C and 13-17 hr at 138 degrees C. Values for spores from other known species range from 5 to 100 min. The molecular basis for this extreme heat resistance is unknown. We report a structural analysis of the internal and external mature spore morphology obtained by both scanning and transmission electron microscopy. Both modes of microscopy delineate a morel-like structure characterized by irregular, but distinct, polygonal ridges suggestive of extreme dehydration. Some spores also possess an appendage resembling the bun of a brioche. This bun-like body is possibly unique to this species. In cross section the spore exhibits a many-layered structure, each layer with a characteristic fine structure. These morphological characters do not suffice to explain the observed resistance to dry heat at extreme temperatures. They do form a basis for the chemical characterizations which will be necessary to understand this heat resistance at the molecular level. The concept of "solid state spore" is put forward as a generalization that may be useful towards understanding this resistance.