█ BRIAN HOYLE
A spore is a hard casing that contains the genetic material of those bacteria and other microorganisms that are able to form the structure. This physically and chemically resilient package protects the genetic material during periods
when the environmental conditions are so harsh that the growing form of the microbe would be killed.
The effect of temperature on bacterial and spore survival provides a good example of the resilience of bacterial spores. Temperatures of 80 to 90° Celsius (176–194°F) typically kill bacteria that are growing and dividing within minutes. These high temperatures cause structural components of the bacteria to dissolve, and strands of genetic material to separate from one another. A group of bacteria known as thermophilic bacteria can survive these temperatures, but temperatures of 120°C (248°F) kill even thermophiles. In contrast, spores can survive exposure to 120°C for several hours.
Spores of bacteria that subsequently could be revived into the growing form have been recovered from materials that are over a century old. Thus, spores offer an extraordinary form of protection to bacteria. Anthrax spores that could germinate into living bacteria were recovered on Gruinard Island, an island off the coast of Scotland that was used for biological weapons testing by the British government during World War II.
Spores are noteworthy in terms of security because of the threat they pose in the hands of terrorists. Bacillus anthracis , the bacterium that causes anthrax, is a spore former. The spores are very light and tiny. As a result, they can be readily dispersed through the air and can be easily inhaled into the lungs. The resulting lung infection, which is called inhalation anthrax, is almost always fatal without prompt medical treatment. Anthrax spores were used as a mechanism of bioterrorism to target United States citizens by deliberate dispersal in the mail system in late 2001.
Another prominent example of a bacterial spore former of concern is Clostridium botulinum . The bacterium and the spore are widespread in nature; for example, they are a common inhabitant of the soil. This bacterium can also survive in canned foods for extended time periods, even when the food has been heated or is acidic. When the food is eaten, the dormant bacteria begin to grow again and produce a variety of potent toxins that disrupt the nervous system, causing serious illness.
The contamination of foods by terrorists is a significant security concern, especially in the United States. Because the spores are hardy and can be transported virtually undetected, they could be taken to food plants or supermarkets, where the food could be contaminated. The spores would survive to cause illness.
Other microorganisms of human concern that form spores include protozoa (e.g., Microsporidia ) and fungi (e.g., Actinomycetes).
Formation of bacterial spores. The multistep process of forming a spore is known as sporulation. The process begins when a bacterium senses that the environmental conditions are becoming life threatening. Bacteria are equipped with a whole battery of sensing proteins and other compounds that monitor environmental conditions of temperature, pH of the surrounding fluid, water content, and availability of food, as some examples. After monitoring the environment for a period of time, the deteriorating conditions trigger the microbe to begin the change from a growing and dividing cell to a dormant spore.
The genetic material of the bacterium is duplicated. Then, the membrane coat that surrounds the inside of the bacterium pinches inward until the ends of the inward growing membrane meet. This isolates one of the copies of the genetic material from the remainder of the bacterium. This smaller cell is called a daughter cell. The remainder of the bacterium is called the mother cell.
In the next stage of spore formation, the membrane that surrounds the mother cell surrounds the daughter cell. This creates a daughter cell that is surrounded by two layers of membrane. Between these two membranes a think layer of a rigid material forms. This layer is called peptidoglycan. Peptidoglycan is normally present in the bacterial cell wall, but not in nearly the same amount. The thick peptidoglycan makes the double membrane layer very tough and hard to break apart. Finally, this tough membrane is coated on the outer surface by proteins. The proteins are also resistant to breakage.
The remnants of the mother cell dissolve away leaving the spore. The spore is essentially in hibernation. There is very little chemical activity. Nevertheless, the spore is able to monitor the external environment and, when conditions are sensed as being more favorable, the conversion from the spore form to the growing organism begins.
The threat from spores. The threat from spores, particularly anthrax spores, lies in their small size and powdery texture once they have been dried. As shown in the anthrax attacks in the United States in 2001, anthrax spores can be delivered to someone in a letter. The spores escape detection using methods like an x-ray. When the letter is opened, the spores can be dispersed in the air and breathed in.
Studies in animal models have shown that even the inhalation of a few spores is enough to cause an infection. The lung is an ideal environment for the anthrax bacterium. Food is available and the atmosphere is warm and moist. When the spores germinate into growing bacteria, the resulting infection can feel similar to the flu at first. Thus, a victim may not seek treatment, believing that the illness will pass in a few days. By the time the true nature of the infection is discovered, the infection can be so advanced as to be fatal.
Anthrax spores could also potentially be dispersed from an airplane or a balloon. Indeed, the terrorists responsible for the September 11, 2001 attacks on the World Trade Center and the Pentagon had explored the use of crop dusting aircraft. Models developed by the U.S. government have predicted that a few hundred pounds of anthrax spores released upwind of Washington, D.C. could cause at least several hundred thousand deaths within a few days.
The growing of the amounts of bacteria necessary to prepare large amounts of powdered spores and the preparation of the spores is not an easy task. Nonetheless, many microbiologists are capable of the task, and the construction of a facility that is large enough to house the needed equipment is not overly difficult. In the past century, nations including the U.S. and Russia had active anthrax weaponization programs. Prior to Operation Iraqi Freedom, Iraq was suspected of having an anthrax weapons development program.
Protection from spores. The threat posed by the use of spores in the mail is difficult to counter. Researchers are working to develop sensors that detect the spores, based on the reaction of antibodies with target proteins on the surface of the spores. However, such detection requires physical contact with the spores. Methods that do not require the opening of letters, such as irradiation, are being tested and refined in the field and in the laboratory.
Another tact is the use of compounds that can destroy the spore. For example, in 2002, researchers discovered that an enzyme called PlyG lysin will chemically crack apart the spore coat. The spore contents are released and disintegrate. Until such sophisticated detection and protection methods are perfected, the treatment of a site contaminated with spores will continue to include the use of bleach.
█ FURTHER READING:
Fischetti, Vincent, Richard P. Novick, Joseph J. Ferretti, and Danile A. Portnoy. Gram-Positive Pathogens. Washington: American Society for Microbiology Press, 2000.
Storz, Gisela, and Regine Hengge-Aronis. Bacterial Stress Responses. Washington: American Society for Microbiology Press, 2000.
Caipo, M.L., S. Duffy, L. Zhao, et al." Bacillus megaterium Spore Germination is Influenced by Inoculum Size." Journal of Applied Microbiology. no. 92 (2002): 879–84.
American Society for Microbiology. "Microbial Spore Formation." Microbe.org. 1999. < http://www.microbe.org/microbes/spores.asp >(10 January 2003).