Besides biofilm formation, microorganisms such as Bacillus sp., Actinomyces, and Myxobacteria form very stable dormant cells, the so-called spores, if nutrient availability becomes limited and cells are crowding.
Spores are stable against desiccation, chemicals, and, to some extent, heat and ultraviolet (UV) radiation. As spores, some microorganisms are able to survive for many years because they are in either a metabolically limited or a dormant state with no detectable metabolism. Protection against threats from the environment is provided by several adaptations.
A spore is engulfed with proteinous layers that form the so-called spore coat. The coat proteins are highly ordered and provide good protection, for example, against lysozyme (a cell-wall-degrading enzyme), H2O2, and UV light. The peptidoglycan membrane surrounding the core spore is almost impermeable for most molecules, preventing them from gaining access to the inside. In addition, the DNA of spores is specially protected by DNA-binding proteins that surround it and there are specialized DNA-repair systems that become active during the germination of the spore. A key role in protection of the spore is also its high degree of dehydration that prevents or slows down any chemical reactions.
Spores are formed by different types of cellular programs. For example, Bacillus subtilis forms endospores. This means a spore cell is generated inside a so-called mother cell. Streptomyces instead form spores by differentiation of their aerial mycelium.
Generally, during sporulation, the major amount of cells is lost by cell lysis, probably in order to provide the necessary extra energy for the spore formation process and to give chemical protection by release ofantibiotics in the surrounding. Once the sporulation program is started, there is no way for the organism to stop it even if the environmental conditions change to favor normal growth. This might be a drawback of sporulation, because it would provide other microorganisms in the neighborhood a time advantage to outcompete spore-forming microorganisms.
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