subtilis endospores and also contain large amounts of dipicolinic acid. These spores possess all of the characteristic protective layers seen in B. Some Epulopiscium-like surgeonfish symbionts form mature endospores at night. The endospore will remain dormant until it senses the return of more favorable conditions. Finally, the mother cell is destroyed in a programmed cell death, and the endospore is released into the environment. This is followed by the final dehydration and maturation of the endospore (Stages VI+VII). The activities of the mother cell and forespore lead to the synthesis of the endospore-specific compounds, formation of the cortex and deposition of the coat (Stages IV+V). Next (Stage III), the peptidoglycan in the septum is degraded and the forespore is engulfed by the mother cell, forming a cell within a cell. Intercellular communication systems coordinate cell-specific gene expression through the sequential activation of specialized sigma factors in each of the cells. These two cells have different developmental fates. This results in the creation of two compartments, the larger mother cell and the smaller forespore. As a cell begins the process of forming an endospore, it divides asymmetrically (Stage II). Key morphological changes in the process have been used as markers to define stages of development. Endospore development requires several hours to complete. The model organism used to study endospore formation is Bacillus subtilis. The process of forming an endospore is complex.

Other species-specific structures and chemicals associated with endospores include stalks, toxin crystals, or an additional outer glycoprotein layer called the exosporium. These proteins tightly bind and condense the DNA, and are in part responsible for resistance to UV light and DNA-damaging chemicals. Small acid-soluble proteins (SASPs) are also only found in endospores. This endospore-specific chemical can comprise up to 10% of the spore's dry weight and appears to play a role in maintaining spore dormancy. The center of the endospore, the core, exists in a very dehydrated state and houses the cell's DNA, ribosomes and large amounts of dipicolinic acid. The inner membrane, under the germ cell wall, is a major permeability barrier against several potentially damaging chemicals.

This layer of peptidoglycan will become the cell wall of the bacterium after the endospore germinates. A germ cell wall resides under the cortex. Proper cortex formation is needed for dehydration of the spore core, which aids in resistance to high temperature. Beneath the coat resides a very thick layer of specialized peptidoglycan called the cortex. The outer proteinaceous coat surrounding the spore provides much of the chemical and enzymatic resistance. The resilience of an endospore can be explained in part by its unique cellular structure. A variety of different microorganisms form 'spores' or 'cysts', but the endospores of low G+C Gram-positive bacteria are by far the most resistant to harsh conditions.

The extraordinary resistance properties of endospores make them of particular importance because they are not readily killed by many antimicrobial treatments. These stresses include high temperature, high UV irradiation, desiccation, chemical damage and enzymatic destruction. It allows the bacterium to produce a dormant and highly resistant cell to preserve the cell's genetic material in times of extreme stress.Įndospores can survive environmental assaults that would normally kill the bacterium. This complex developmental process is often initiated in response to nutrient deprivation. One example of an extreme survival strategy employed by certain low G+C Gram-positive bacteria is the formation of endospores. When favored nutrients are exhausted, some bacteria may become motile to seek out nutrients, or they may produce enzymes to exploit alternative resources. Microorganisms sense and adapt to changes in their environment.

B subtilis endospore stain