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Reasons Why Endospores Are Resistant to Heat

Microorganisms, such as bacteria, survive by detecting and adapting to changes in their environment. Endospore formation is a survival strategy used by several types of bacteria. Endospores are dormant, highly resistant structures that preserve the bacterium's genetic material during times of extreme stress. They can survive environmental conditions that would normally kill the organism, including extreme heat. Because they are not easily destroyed, endospores play a significant role in disease transmission.

  1. Structure

    • The endospore unique cellular structure contributes to its resilience. It is a sphere composed of a central core surrounded by multiple layers of resistant coats. The core houses the cell's genetic information, or DNA, ribosomes and other substances needed for cell function once it returns to its non-dormant, vegetative state. It also contains large amounts of dipicolinic acid. An inner membrane and germ cell wall separate the core from the cortex. The cortex is composed of large, structural peptidoglycan molecules. Outside the cortex is a spore coat composed of more than 22 different types of proteins.

    Heat Resistance

    • Bacterial endospores are resistant to antibiotics, most disinfectants and physical agents, including heat, radiation and drying. Core dehydration is essential for heat resistance. Water is removed from the core by the cortex. The dipicolinic acid within the core plays a role in wet heat resistance. Small acid-soluble proteins, or SASPs, tightly bind and condense endospore DNA to protect it from dry heat. Finally, if DNA does suffer heat damage, the endospore contains special enzymes that make necessary repairs when the cell germinates.

    Destruction

    • Endospores are heat-resistant, not heat-proof. Water boils at 100 degrees Celsius. Although this temperature is sufficient for killing vegetative bacteria in 10 minutes or less, up to 20 hours of boiling may be required to kill endospores. Consequently, boiling is not a dependable means of sterilization. Endospores are killed more efficiently using an autoclave. The device is an enclosed chamber that uses steam under pressure to kill endospores in 15 to 20 minutes. The most common autoclave setting is 15 pounds of pressure and 121 degrees Celsius. Baking, flaming and incineration use dry heat to kill endospores. Though effective, dry heat sterilization methods require higher temperatures and longer exposures than moist heat methods.

    Disease Transmission

    • Bacterial endospores are harmless until they germinate. They are problematic because they allow disease-causing organisms to survive in the environment until suitable growing conditions, such as those offered by a human host, trigger germination. Bacteria commonly associated with endospore formation include the genera Bacillus and Clostridium. Inhalation, ingestion or direct contact with B. anthracis spores causes anthrax. These endospores received considerable media attention during the United States' anthrax scare of 2001. Ingested C. botulinim spores germinate in the intestine. The vegetative bacteria produce a toxin that causes botulism. C.perfrigens and C. tetani cause gas gangrene and tetanus, respectively. Spores enter the body through traumatic or postoperative wounds.

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