Under environmental stress, some species of bacteria produce endospores; endospores are encapsulated packages of non-reproductive genetic material. Endospores allow bacteria to survive conditions that would otherwise destroy the single-celled organisms. The word endospore means "inner seed." True to the name, endospores form within bacteria; they are part of the progenitor organism, though, and not true seeds. Endospore-forming bacteria resist efforts to eradicate them, making pathogenic endospores --- such as those responsible for botulism and anthrax --- especially lethal.
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Bacteria that exhaust their nutrient supply form endospores to weather the famine. When host organisms die, endospores form and encapsulate the essential genetic information within each bacterium, allowing it to become dormant until the next host ingests or inhales it. Other endospore-forming bacteria remain in this dormant state outside of any host. The mail-borne anthrax attack that killed five people and sickened more than a dozen others in 2001 contained Bacillus anthracis endospores in a white powder form. The organisms needed no host in their endospore phase.
Radiation affects the genetic material in bacteria just as it does in larger and more complex organisms. Under the onslaught of radiation, endospore-forming bacteria encapsulate their essential genetic information and weather the radioactive storm. Food irradiation readily kills bacteria that cannot form endospores, but will not penetrate the coating of an endospore to render the cell non-viable. Using irradiation as an antimicrobial measure requires longer and greater exposure to the radioactive source to kill off endospores.
Heat treatment denatures cell proteins; for unprotected bacterial cells, high heat kills. Endospores can even endure immersion in boiling water, making them difficult to eradicate while sterilising medical equipment or laboratory tools. Heating items well beyond the boiling point of water will typically kill endospores, but boiling alone could leave pathogens viable after the sterilisation attempt. Ken Alibek, a purported germ warfare expert, made the claim in 2001 that ironing mail with a household steam iron sterilised any possible anthrax contamination, but as Bacillus anthracis survives temperatures higher than those a home iron can produce, this claim was untrue.
Chlorine bleach, alcohol and a host of other disinfectants suffice to kill the great majority of bacteria on contaminated surfaces, but endospores can resist virtually all household cleaners. Their hardy, but relatively non-reactive, protein coat protects them from the oxidising effects of weak bleach solutions. Disinfectants that do penetrate endospores and destroy the dormant organism they contain are themselves potentially toxic, limiting their utility.
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- Cornell University Microbiology Department: Bacterial Endospores
- Ohio State University; Supplemental Lecture - Endospore; Stephen T. Abedon; Apr. 1998
- The Microbial World: 2-33: Bacteria Can Exist in Different Cell States
- "Time"; How Anthrax Is Weaponized; Alice Park; Nov. 5, 2001
- "Salem Evening News"; Germ Warfare Expert Tells Americans to Iron the Mail; Melissa B. Robinson; Oct. 17, 2001