Sporulation (World of Microbiology and Immunology)
Sporulation is the formation of nearly dormant forms of bacteria. In a limited number of bacteria, spores can preserve the genetic material of the bacteria when conditions are inhospitable and lethal for the normal (vegetative) form of the bacteria. The commitment of a bacterium to the sporulation process sets in motion a series of events that transform the cell.
Sporulation ultimately provides for a multilayered structure can be maintained for a very long time. Relative to the norm life span of the microorganism, spores are designed to protect a bacterium from heat, dryness, and excess radiation for a long time. Endospores of Bacillus subtilis have been recovered from objects that are thousands of years old. Furthermore, these are capable of resuscitation into an actively growing and dividing cell. Spores have been recovered from amber that is more than 250 million years old.
Given that resuscitation is possible, sporulation does not result in a completely inert structure. The interior of a spore contains genetic material, cytoplasm, and the necessary enzymes and other materials to sustain activity. But, this activity occurs at an extremely slow rate; some 10 million times slower than the metabolic rate of a growing bacterium.
The sporulation process has been well studied in Bacillus subtilis. The process is stimulated by starvation. Typically, sporulation is a "last resort," when other options fail (e.g., movement to seek new food, production of enzymes to degrade surrounding material, production of antimicrobial agents to wipe out other microbes competing for the food source, etc.). The genetic grounding for the commitment to form a spore is a protein called SpoA. This protein functions to promote the transcription of genes that are required for the conversion of the actively growing bacterium to a spore. The formation of an active SpoA protein is controlled by a series of reactions that are themselves responsive to the environmental conditions. Thus, the activation of SpoA comes only after a number of checkpoints have been passed. In this way a bacterium has a number of opportunities to opt out of the sporulation process. Once committed to sporulation, the process is irreversible.
A similar series of reactions has been identified as a means of regulating the degree of host damage caused by a Bordetella pertussis, the bacterium that causes pertussis, as well as in the response of the yeast Saccharomyces cervesiae to osmotic pressure.
Sporulation begins with the duplication of the bacterial genome. The second copy and some of the cytoplasm is then enveloped in an in-growth of the membrane that surrounds the bacterium. The result is essentially a little spherical cell inside the larger bacterium. The little cell is referred to as the "daughter cell" and the original bacterium is now called the "mother cell." Another membrane layer is laid down around the daughter cell. Between these two membranes lies a layer of peptidoglycan material, the same rigid material that forms the stress-bearing network in the bacterial cell wall. Finally, a coat of proteins is layered around the outside of the daughter cell. The result is a nearly impregnable sphere.
The above spore is technically termed an endospore, because the formation of the membrane-enclosed daughter cell occurs inside the mother cell. In a so-called exospore, the duplicated DNA migrates next to a region on the inner surface of the cell membrane and then a bud forms. As the bud protrudes further outward, the DNA is drawn inside the bud. Examples of endospore forming bacteria include those in the genera Bacillus and Clostridium. Endospore forming bacteria include Methylosinus, Cyanobacteria, and Microsporidia.
When still in the mother cell, the location of the spore (e.g., in the center, near one end or at one pole) is often a distinctive feature for a particular species of bacteria, and can be used as a feature to identify the bacteria.
As the mother cell dies and degrades, the spore will be freed. When conditions become more hospital, the metabolic machinery within the spore will sense the change and a reverse process will be initiated to transform the spore into a vegetative cell.
The type of sporulation described here is different from the sporulation process that occurs in many kinds of fungi and in the bacteria called Actinomyces. The latter spores are essentially seeds, and are used in the normal reproduction cycle of the microorganisms. Bacterial sporulation is an emergency protective and survival strategy.
See also Asexual generation and reproduction; Bacterial adaptation; Bacterial growth and division; Bacterial kingdoms; Bacterial membranes and cell wall; Cell cycle (prokaryotic), genetic regulation of; Desiccation; Extraterrestrial microbiology; Extremophiles; Fossilization of bacteria; Genetic identification of microorganisms; Genetic regulation of prokaryotic cells; Life, origin of; Radiation resistant bacteria