These highly resistant structures are formed in response to adverse conditions by two genera of medically important gram-positive rods: the genus Bacillus, which includes the agent of anthrax, and the genus Clostridium, which includes the agents of tetanus and botulism. Spore formation (sporulation) occurs when nutrients, such as sources of carbon and nitrogen, are depleted (Figure 1). The spore forms inside the cell and contains bacterial DNA, a small amount of cytoplasm, cell membrane, peptidoglycan, very little water, and most importantly, a thick, keratin-like coat that is responsible for the remarkable resistance of the spore to heat, dehydration, radiation, and chemicals.
Fig1. Bacterial spores. The spore contains the entire DNA genome of the bacterium surrounded by a thick, resistant coat.
Once formed, the spore is metabolically inactive and can remain dormant for many years. Upon exposure to water and the appropriate nutrients, specific enzymes degrade the coat, water and nutrients enter, and germination into a potentially pathogenic bacterial cell occurs.
The medical importance of spores lies in their extraordinary resistance to heat and chemicals. As a result of their resistance to heat, sterilization cannot be achieved by boiling. Steam heating under pressure (autoclaving) at 121°C, for at least 15 minutes, is required to ensure the sterility of products for medical use. Spores are often not seen in clinical specimens recovered from patients infected by spore-forming organisms because the supply of nutrients is adequate.
Table 1 describes the medically important features of bacterial spores.
Table1. Important Features of Spores and Their Medical Implications
