![]() Our focus here will be the spore coat, a structure present on the exterior of all spores that is crucial for conferring resistance to environmental stress. A variety of spore-forming species are routinely found in soil samples 10, but they have also been isolated from ecosystems as diverse as hot springs 11, arctic sediments 12 and the mammalian gastrointestinal tract 13. ![]() When the spore is released into the surrounding environment it can persist or germinate to re-enter the vegetative cycle. Over the next 8 to 10 hours, the smaller of the two compartments, the forespore (or prespore), develops into a mature spore capable of protecting the genome. Sporulation is a simple example of differentiation: a cell replicates its DNA, divides asymmetrically and places copies of its genome in both compartments. anthracis and Clostridium difficile, and the model organism Bacillus subtilis. Two taxa within the phylum Firmicutes - the aerobic Bacillaceae and the anaerobic Clostridia - form spores via an evolutionarily conserved mechanism 8, 9 (see below). Nevertheless, viable spores have been isolated from specimens such as dried plant samples dating from 1640 onwards 6 and a flask of Bacillus anthracis spores sealed in Louis Pasteur's laboratory for 60 years 7. Although the upper limit of viability is unknown, some researchers have reported the revival of spores from samples ranging in age from decades to several thousands of years 5, a claim that is difficult to prove unambiguously. The exact mechanism of spore persistence is unknown, but in addition to being resistant to ultraviolet (UV) radiation, chemicals (such as peroxide and hypochlorite), extreme heat and other stresses, spores are metabolically dormant and partially dehydrated, which is likely to allow their survival in nutrient-free and harsh environments. Unrelated mechanisms of bacterial sporulation 1 include the formation of exospores, myxospores and akinetes ( Box 1).Įndospores (hereafter referred to as spores) exhibit extraordinary resistance properties and have served as textbook examples of long-term cell survival 2, 3, 4. Endospores are formed and nurtured completely within a mother cell, which must lyse to release the spore into the environment. A more sophisticated and prolonged example of stress response is spore formation, or sporulation, wherein the bacterial genome is sequestered in a safe place (the spore) until environmental conditions improve, upon which the spore quickly germinates and returns to the vegetative state. These strategies frequently involve rapid changes in gene expression that temporarily alter the phenotype of a cell and allow it to survive. Time course analyses of spore coat assembly have revealed that two main steps can be distinguished in coat morphogenesis: the initial recruitment of proteins to the spore surface as a scaffold cap, followed by spore encasement in a series of successive waves.Ĭoat assembly is regulated at the transcriptional level by the sequential expression of individual coat genes and at the protein level by a small group of coat morphogenetic proteins that coordinate both the recruitment of coat proteins to specific coat layers and spore encasement.īacteria have many strategies for surviving environmental challenges. ![]() subtilis, referred to as the spore crust. These studies led to the discovery of the outermost layer of the coat in B. In addition to its protective role, the spore coat influences the process of spore germination and defines the type of interactions that spores can establish with various surfaces in the environment.įluorescence microscopy in combination with high-resolution image analysis has produced a spatially scaled coat protein interaction network indicating that the coat is organized into four distinct layers. The Bacillus subtilis spore coat is a multilayered protective structure composed of more than 70 different proteins.
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