Bacterial sporulation

BACTERIAL SPORULATION

In a few bacterial genera, notably Bacillus and Clostridium, a unique process takes place in which the vegetative cell undergoes a profound biochemical change to give rise to a specialized structure called an endospore or spore (Figure 3.6). This process of sporulation is not part of a reproductive cycle, but the spore is a highly resistant cell that enables the producing organism to survive in adverse environmental conditions such as lack of moisture or essential nutrients, or exposure to toxic chemicals, radiation or high temperatures. Because of their extreme resistance to radiation, ethylene oxide and heat, all sterilization processes for pharmaceutical products have been designed to destroy the bacterial spore . Removal of the environmental stress may lead to germination of the spore back to the vegetative cell form.

Endospore structure

Endospores are differentiated cells that possess a grossly different structure to that of the parent vegetative cell in which they are formed. The structure of the spore is much more complex than that of the vegetative cell in that it has many layers surrounding a central core (Figure 3.7). The outermost layer is the exo-sporium composed of protein; within this are the spore coats, which are also proteinaceous but with a high cysteine content, the cortex that consists of loosely crosslinked peptidoglycan and the central core that contains the genome. Characteristic of the spore is the presence of dipicolinic acid and high levels of calcium ions which complex together. The core is also partially dehydrated, containing only 10–30% of the water content of the vegetative cells. Dehydration has been shown to increase resistance to both heat and chemicals. In addition, the pH of the core is about 1 unit lower than the cytoplasm of the vegetative cell and contains high levels of core-specific proteins that bind tightly to the DNA and protect it from potential damage. These core-specific proteins also function as an energy source for the outgrowth or germination of a new vegetative cell from the endospore.

Endospore formation

During endospore formation the vegetative cell undergoes a complex series of biochemical events in cellular differentiation, and many genetically directed changes in the cell that underpin the conversion occur in a series of distinct stages. Sporulation requires that the synthesis of some proteins involved in vegetative cell function cease and that specific spore proteins are made. This is accomplished by activation of a variety of spore-specific genes such as spo and ssp. The proteins coded by these genes catalyse a series of events leading ultimately to the production of a dry, metabolically inert but extremely resistant endospore. The whole process can take only a matter of hours to complete under optimal conditions.

Endospore germination

Although endospores can lie dormant for decades, they can revert back to a vegetative cell very rapidly. Activation of the process may occur through removal of the stress inducer that initiated sporulation. During germination loss of resistance properties occurs along with a loss of calcium dipicolinate and cortex components, and degradation of the core-specific proteins. Outgrowth occurs, involving water uptake and synthesis of new RNA, proteins and DNA until eventually, after a matter of minutes, the vegetative cell emerges from the fractured spore coat and begins to divide again.