Bacterial Endospore: Structure and Development

A number of bacteria, mainly the Bacillus and Clostridium genera produce a special reproductive structure called the endospore. Endospore formation is a complex developmental process that occurs when microbes sense and adapt to changes in the environment. In response to nutrient deprivation, the bacterium produces a dormant and highly resistant cell to preserve the cell’s genetic material in times of extreme stress.

Endospores are so named because they are formed intracellularly, although they are eventually released from this mother cell or sporangium as free spores. Endospores have proven to be the most durable type of cell found in nature and they can remain viable for extremely long periods of time. Mature spores have no detectable metabolism, a state that is described as cryptobiotic. They can survive extreme environmental stresses like high temperature, high UV irradiation, desiccation, chemical damage and enzymatic destruction. The extraordinary resistance properties of endospores make them of particular importance because they are not readily killed by many antimicrobial treatments. Although cryptobiotic, they retain viability indefinitely such that under appropriate environmental conditions, they germinate into vegetative cells. Endospores are formed by vegetative cells in response to environmental signals that indicate a limiting factor for vegetative growth, such as exhaustion of an essential nutrient. They germinate and become vegetative cells when the environmental stress is relieved. Hence, endospore-formation is a mechanism of survival rather than a mechanism of reproduction.

The endospore appears as a highly refractive body inside of a mother cell usually called sporangium. Endospores strongly resist application of simple stains or dyes and hence appear as nonstaining entities in Gram-stain preparations. However, once stained, endospores are quite resistant to decolorization. This is the basis of several spore stains such as the Schaeffer-Fulton staining method which also differentiates the spores from sporangia and vegetative cells. Spores formed in the vegetative cell may be central, terminal or subterminal in position.

Structure of Endospore

The endospore has a complex ultra structure as compared to vegetative cells. The center of the endospores contains the core and it consists of the cytoplasm, DNA, ribosomes, enzymes and everything that is needed to function once returned to the vegetative state. The core is dehydrated, which is essential for heat resistance, long-term dormancy and full chemical resistance. Calcium dipicolinate which forms 10-15% of the dry weight of the endospore is a major component of the core and has been shown to play a role in resistance to wet heat and UV light. The core also contains small acid soluble proteins (SASPs) that bind to the DNA and protect it. The core is surrounded by the cortex, which consists of two layers, a thin dense layer that is similar in structure to the vegetative cell wall and a thicker less dense layer containing modified peptidoglycan. Two major modifications are present. First, there is less cross-linking with only 3% of the muramic acid present in the peptidoglycan of the cortex participating, in comparison to 40% of muramic acid in the vegetative cell wall. Second, much of the muramic acid is modified to a muramic-β-lactam structure. Both of these modifications of the cortex appear to be important in germination. Muramic-β-lactam serves as a specific target for lytic enzymes that are activated during germination and the lower cross-linking enables easier outgrowth. Outside of the cortex is the spore coat containing several protein layers that are impermeable to most chemicals. The coat is composed of more than two dozen different types of proteins are connected by cross-links. This covalent connection between coat proteins probably contributes to the spores’ resistance. Depending on the species, an exosporium may be present.

Endospore Formation

The formation of endospores is a complex and highly-regulated form of development in a relatively simple (procaryotic) cell. The model organism used to study endospore formation is Bacillus subtilis. Endospore development requires about 8 hours. The endospore formation process can be divided into several stages.

The vegetative cell (stage 0) begins spore development when the DNA coils along the central axis of the cell as an “axial filament” (stage I). The DNA then separates and the cell begins to divide asymmetrically (stage II). In this stage one of the chromosomes becomes enclosed in plasma membrane to form a protoplast. This results in the creation of two compartments, the larger mother cell and the smaller forespore. These two cells have different developmental fates. Intercellular communication systems coordinate cell-specific gene expression through the sequential activation of specialized sigma factors in each of the cells. Next (Stage III), the peptidoglycan in the septum is degraded and the forespore is engulfed by the mother cell membrane, forming a cell within a cell. In this stage the synthesis of cortex region is initiated. Exosporium is also synthesized. Dehydration of the spore also begins. Stage IV involves the synthesis of the spore coat layer that surrounds the endospore. The synthesis of the cortex continues. There is accumulation of dipicoloninc acid (DPA), incorporation of calcium and production of SASPs in the core. Stage V involves maturation of the spore, final dehydration and development of resistance to heat and chemicals. Finally, the mother cell is destroyed in a programmed cell death, and the endospore is released into the environment (stage VII).