Hardy Survivors in the Microbial Kingdom

Heating effectively kills virtually all kinds of microbes, but on occasion it is found that some can survive this harsh treatment. Except for certain “extremophiles,” the microbes that survive exposure to highly elevated temperature are usually species of bacteria that are able to produce specialized structures called “endospores.” Some species of disease-producing bacteria form endospores, and their long-term persistence in soils or other natural reservoirs can pose public health problems (for example, anthrax is caused by the endospore-forming bacterium Bacillus anthracis). Endospores—referred to hereafter simply as spores—are formed mainly by three genera, commonly found in soil: Bacillus: rod-shaped aerobes, Clostridium: rod-shaped anaerobes, and Thermoactinomyces: aerobic bacteria that grow best at slightly elevated temperatures (50№C). Although clostridial spores are very resistant to heat, they can be killed by appropriate heating procedures—for example, by superheated steam under pressure. Cryptobiosis, or “latent life,” is defined as the state of an organism when it shows no visible signs of life and when its metabolic activity becomes hardly measurable or comes (reversibly) to a standstill. A number of studies have shown that spores of Bacillus anthracis and Clostridium tetani can remain viable for at least 50 to 70 years. Bacteria of the genus Thermoactinomyces produce heat-resistant spores and grow optimally at a temperature of about 50°C (122°F). They are present in most soil samples and sporulate profusely in habitats such as compost, haystacks, and stored cereals.