Microbiologist Teresa Thiel of the University of Missouri, St. Louis, offers the following explanation:
Many microorganisms easily survive the environmental stresses of the microbial world, such as heat, cold and desiccation. Probably the best understood are those organisms that produce specialized cells designed to persist in a dormant state in hostile environments.
Most fungi, for example, produce single-celled spores. (You have probably seen these if you have ever picked a mushroom or puffball that was full of a dust-like substance¿each of those tiny particles is a spore.) Winds carry the spores to new spots where they can survive for long periods of time before germinating and sprouting fungal filaments of their own.
Some bacteria produce a special type of spore called an endospore, which can withstand such extremes as boiling and freezing temperatures, and ultraviolet radiation. These bacterial endospores often endure many years of hardship before they find the growth conditions necessary for germination. Several features in particular help to make endospores resistant to environmental stress. They have a low water content, unusual proteins and a tough spore coat that is not present in the mature bacterial cells. Take the bacterium that causes botulism, whose endospores are present in soil. When garden fruits and vegetables, which may contain a few of these endospores, are preserved by canning at boiling temperatures, these spores survive the heat and germinate in the food. As a result, the bacteria grow in the canned food and produce the botulism toxin that can cause fatal cases of food poisoning. (The very high temperatures used in commercial canning destroy all spores, so the canned foods you buy in the grocery store are safe.)
Other dormant cells, produced by some bacteria and protozoa, include various types of cysts. Bacterial cysts are not so resistant to environmental stresses as bacterial endospores are, but they can face up to adverse circumstances, particularly desiccation. For instance, Giardia, a protozoan that can cause severe intestinal disease, produces infectious cysts that tolerate the chlorine in drinking water and can cause outbreaks of a disease called giardiasis.
Even organisms that do not make special dormant cells can survive harsh conditions. As early as 1702, Dutch microscopist Antony van Leeuwenhoek observed that tiny creatures we now know as rotifers come back to life when water is added to dried samples. For some microorganisms¿notably certain cyanobacteria (blue-green algae)¿novel proteins, UV-absorbing pigments and a protective complex polymer coat help them to survive on rocky surfaces where they are exposed to high levels of sunlight without water. The addition of water quickly resuscitates these cells.
Microorganisms are often preserved commercially and in the laboratory using techniques that involve desiccation. For example, a paste of baker's yeast cells is dried on a fine screen mesh using warm air from beneath to produce the yeast sold in the grocery store for making bread. Another method, freeze-drying, involves drying the microorganisms under a vacuum. This preserves most bacteria for many years.
Researchers do not completely understand the mechanisms that contribute to cell survival under adverse conditions. But maintaining the proteins of the cell in an active form is clearly critical to survival. Keeping a very low level of water inside the cell appears to be likewise essential to long-term survival. (Spores, endospores, cysts and desiccated cells all have low water content.) Freezing itself does not usually harm cellular components. Ice crystals, however, are lethal to living cells. Therefore, removing water¿especially while the cells are cold, as is done in freeze-drying¿will usually keep the proteins active. Many bacteria can also be stored frozen at ultracold (-80 degrees Celsius) temperatures¿the secret is to add a substance like glycerol that prevents the formation of ice crystals.