Microorganisms sense and adapt to changes in their environment. When favourite nutrients are depleted, some bacteria may become mobile to seek out nutrients or may produce enzymes to exploit alternative resources. An example of an extreme survival strategy employed by certain low G+C Gram-positive bacteria is endospore formation. This complex developmental process often begins in response to nutrient deprivation. It allows the bacterium to produce a highly resistant, dormant cell to preserve the cell’s genetic material in times of extreme stress.
The endospores can survive environmental attacks that would normally kill the bacteria. These stresses include high temperature, high UV radiation, desiccation, chemical damage, and enzymatic destruction. The extraordinary resistant properties of endospores make them of particular importance because many antimicrobial treatments do not kill them easily. A variety of different microorganisms form “spores” or “cysts”, but the endospores of Gram-positive bacteria with low G+C are by far the most resistant to adverse conditions.
The resilience of an endospore can be explained in part by its unique cellular structure. The proteinaceous outer coat surrounding the spore provides much of the chemical and enzymatic resistance. Beneath the cape resides a very thick layer of specialized peptidoglycan called the cortex. Proper shell formation is necessary for dehydration of the spore nucleus, which aids resistance to high temperatures. A wall of germ cells resides below the cortex. This peptidoglycan layer will become the cell wall of the bacteria after the endospore germinates. The inner membrane, below the germ cell wall, is an important permeability barrier against a number of potentially harmful chemicals.
The centre of the endospore, the nucleus, exists in a highly dehydrated state and houses the cell’s DNA, ribosomes, and large amounts of dipicolinic acid. This endospore-specific chemical may constitute up to 10% of the spore’s dry weight and appears to play a role in maintaining spore dormancy. Small acid-soluble proteins (SSPs) are also found only in endospores. These proteins bind tightly to and condense DNA, and are partly responsible for resistance to ultraviolet light and DNA-damaging chemicals. Other species-specific structures and chemicals associated with endospores include stalks, toxin crystals, or an additional outer glycoprotein layer called an exospore.
The process of bacterial spore formation is complex. The model organism used to study endospore formation in Bacillus subtilis. Endospore development requires several hours to complete. Key morphological changes in the process have been used as markers to define stages of development. When a cell begins the process of forming an endospore, it divides asymmetrically (Stage II). This results in the creation of two compartments, the larger mother cell and the smaller spore. These two cells have different developmental fates. Intercellular communication systems coordinate the expression of cell-specific genes through the sequential activation of specialized sigma factors in each cell.
Then (Stage III), the peptidoglycan in the septum degrades and the spore is taken up by the parent cell, forming a cell within a cell. The activities of the mother cell and the spore lead to the synthesis of the specific compounds of the endospore, the formation of the cortex and the deposition of the coat (Stages IV+V). This is followed by the final dehydration and maturation of the endospore (Stages VI+VII). Finally, the stem cell is destroyed in programmed cell death and the endospore is released into the environment. The endospore will remain dormant until it feels the return of more favourable conditions. [A sigma factor is a small protein that directs RNA polymerase to specific points on DNA to initiate gene expression.]
Endospores and Epulopiscium
Some Epulopiscium-like surgeonfish symbionts form mature endospores at night. These spores possess all of the characteristic protective coatings seen on B. subtilis endospores and also contain large amounts of dipicolinic acid. These are the largest endospores described so far, with the largest being more than 4000 times larger than a Bacillus subtilis endospore.
Endospore formation may help maintain the symbiotic association between these Epulopiscium-like symbionts and their surgeonfish hosts. Since endospore formation coincides with periods when the host surgeonfish are not actively feeding, the cells do not need to compete for the limited nutrients present in the gut at night. The protective properties of endospores also allow them to survive passage to new surgeonfish hosts. Fish may also benefit from this relationship because they can maintain stable microbial populations that aid in digestion and can receive a nutritional gain from microbial products released during stem cell death and spore germination.
Endospore formation in some Epulopiscium-like symbionts follows a daily cycle:
- Polar septa form at the poles of the cell.
- The forespores are engulfed.
- The forespores gradually increase in size within the mother cell throughout the day.
- In the late afternoon, final preparations for endospore dormancy.
- Endospores mature and remain dormant for most of the night.
- Just before dawn, the endospores germinate and are released from the mother cell to repeat the cycle.