Spores

Illness and death can occur from pathogenic (disease-causing) microbial infections. Thus knowledge of the ways infections spread and the myriad of symptoms that can develop are a vital part of forensic science. This is especially important when the infection is a serious threat to health and is easily spread from person to person. One important contributor to the spread of infection by certain bacteria (including the infamous cause of anthrax) is the spore.

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 176–199°F (80 –90°C) 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 (248°F) 120°C kill even thermophiles. In contrast, spores can survive exposure to 248°F 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.

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.

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.

Other microorganisms of human concern that form spores include protozoa (e.g., Microsporidia) and fungi (e.g., Actinomycetes).

The multi-step 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, such as temperature, pH of the surrounding fluid, water content, and availability of food. 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 thick 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 as is present in a spore. 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.

SEE ALSO Anthrax; Bacterial biology; Pathogens.