The dynamic spore

Abstract

Most bacteria can, at least to some degree, hunker down during periods of stress and wait for good times to return. No cells, however, do this as effectively as those Bacilli and Clostridia that form spores. Dormant cells produced by these bacteria can survive most environmental challenges found on earth and even a few in outer space and can remain dormant in excess of millions of years (1, 2). Nonetheless, when suitable conditions are present once again, spores rapidly germinate and resume vegetative growth. The spore is composed of a set of protective structures arranged in a series of concentric shells (Fig. 1); each component contributes in some essential way to spore durability. Key functions of these structures include locking the DNA into a stable, relatively desiccated crystalline state and excluding toxic molecules via an armored external shell (3). The conception of the spore as a dormant, dry, and hardened vehicle designed to preserve DNA has strongly suggested the notion of a cell that is largely static. In this view, the spore has no moving parts; dormancy depends on being inert. What a surprise, then, to learn from Westphal et al. (4) that Bacillus thuringiensis spore dimensions can change significantly depending on relative humidity, a parameter that is likely to vary in natural environments where spores are found, such as the soil. The authors used a novel automated technique, formerly applied to nonbiological problems such as relativistic heavy ion physics, to nondestructively measure spore dimensions. Their technique gives extremely accurate relative size information and, importantly, allows them to collect multiple measurements on the same spore as environmental conditions are varied. They show that swelling occurs in two phases, the first taking place in less than 1 min, and the second requiring about 8 min. These two phases may reflect …