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Endospores are refractile - light cannot penetrate them - so that they are very easy to see in the phase microscope and this makes it easy to detect them. Most endospores have no measurable metabolism and are really a form of suspended animation. Endospores can survive for a very long time, and then return to a growing state, a process termed germination. Endospores that were dormant for thousands of years in the great tombs of the Egyptian Pharaohs were able to germinate and grow when placed in appropriate medium. Several scientists have been able to recover viable endospores from bees trapped in amber that is 25-40 million years old. The microbe isolated was found to be most closely related to Bacillus sphaericus. There are even claims of endospores that are over 250 million years old being able to germinate when placed in appropriate medium, but these claims still need to be verified. Endospores are found everywhere, are easily dispersed throughout the environment and can be difficult to remove. The anthrax scare of 2001 in the United States is ample evidence of the insidiousness of endospores and their impressive resistance.
Endospores are resistant to heat (>100 °C), radiation, many chemicals (i.e. acids, bases, alcohol, chloroform), and desiccation. The mechanisms that account for this resistance include the impermeability of the endospore coat, the dehydration of the cytoplasm and the production of special proteins that protect the spore's DNA. Figure 2-51 shows the major structures of an endospore.
Figure 2.51 An endospore. An electron micrograph of an endospore of Bacillus subtilis showing the core, cortex and coat. (Source: M. Serrano, et al. 1999. J. Bacteriol. 181:3632-3643)
The formation of an endospore is clearly a great advantage for these bacteria and enables them to endure extreme stress. At a later time, even much later, when conditions are favorable, they can reemerge and flourish. Endospores enable a species to spread easily from one suitable environment to another an many endospore-forming bacteria are ubiquitous in the environment. Endospores are a particular problem in the food industry where great care must be taken to insure either the destruction of endospores or suitable preservation methods so that endospore-forming bacteria (and other microbes) cannot grow.
Endospores can be divided into several important parts (Fig 2-51). The center of the endospores contains the core and it consists of the cytoplasm, DNA, ribosomes, enzymes and everything that is needed to function once returned to the vegetative state. The core is dehydrated, which is essential for heat resistance, long-term dormancy and full chemical resistance. Calcium dipicolinate is a major component of the core and has been shown to play a role in resistance to wet heat and UV light. The cortex surrounds the core and is composed of two layers, a thin more densely staining layer that is similar in structure to the vegetative cell wall and a thicker less dense layer containing modified peptidoglycan. Two major modifications are present. First, there is less cross-linking with only 3% of the muramic acid present in the peptidoglycan of the cortex participating, in comparison to 40% of muramic acid in the vegetative cell wall. Second, much of the muramic acid is modified to a muramic-β-lactam structure. Both of these modifications of the cortex appear to be important in germination. Muramic-β-lactam serves as a specific target for lytic enzymes that are activated during germination and the lower cross-linking enables easier outgrowth. Outside of the cortex is the spore coat containing several protein layers that are impermeable to most chemicals. The coat is composed of more than two dozen different types of proteins and there is some evidence that these proteins are connected by cross-links. This covalent connection between coat proteins probably contributes to the spores' resistance.[Prev] | [Next]