Microbial cells cannot control their temperature and therefore assume the ambient temperature of their natural habitats. So, to survive, they must adapt to whatever temperature variations are encountered in their habitat. The range of temperatures for microbial growth can be expressed as three cardinal temperatures. The minimum temperature is the lowest temperature that permits a microbe’s continued growth and metabolism; below this temperature, its activities are inhibited. The maximum temperature is the highest temperature at which growth and metabolism can proceed. If the temperature rises slightly above maximum, growth will stop. If it continues to rise beyond that point, the enzymes and nucleic acids will eventually become permanently inactivated—a condition known as denaturation—and the cell will die. This is why heat works so well as an agent in microbial control. The optimum temperature covers a small range, intermediate between the minimum and maximum, which promotes the fastest rate of growth and metabolism. Only rarely is the optimum a single point.
Depending on their natural habitats, some microbes have a narrow cardinal range, others a broad one. Some strict parasites will not grow if the temperature varies more than a few degrees below or above the host’s body temperature. For instance, rhinoviruses (one cause of the common cold) multiply successfully only in tissues that are slightly below normal body temperature (33°C to 35°C or 91°F to 95°F). Other microbes are not so limited. Strains of Staphylococcus aureus grow within the range of 6°C to 46°C (43°F to 114°F), and the intestinal bacterium Enterococcus faecalis grows within the range of 0°C to 44°C (32°F to 112°F).
Another way to express temperature adaptation is to describe whether an organism grows optimally in a cold, moderate, or hot temperature range. The terms used for these ecological groups are psychrophile, mesophile, and thermophile (figure 1).
Fig1. Ecological groups by temperature of adaptation. Psychrophiles can grow at or near 0°C and have an optimum below 15°C. As a group, mesophiles can grow between 10°C and 50°C, but their optima usually fall between 20°C and 40°C. One type of mesophile, termed a psychrotroph, is capable of growth at temperatures below 20°C. Generally speaking, thermophiles require temperatures above 45°C and grow optimally between this temperature and 80°C. Extreme thermophiles (hyperthermophiles) are bacteria and archaea with optima above 80°C. Note that the extremes of the ranges can overlap to some extent.
A psychrophile is a microorganism (bacterium, archaea, fungus, or alga) with an optimum temperature below 15°C (59°F) but can generally grow at 0°C (32°F). It is obligate with respect to cold and generally cannot grow above 20°C (68°F). Laboratory work with true psychrophiles can be a real challenge. Inoculations have to be done in a cold room because ordinary room temperature can be lethal to these organisms. Unlike most laboratory cultures, storage in the refrigerator incubates rather than inhibits them. As one might predict, true psychrophiles are not capable of surviving in the human body and do not cause infections. Instead, they populate some of the coldest places on earth, including snowfields (figure 2), polar ice, permafrost, and the deep ocean. Recently, bacteria were isolated from an undersea lake in Antarctica that were living at −15°C (5°F). True psychrophiles must be distinguished from psychrotrophs or facultative psychrophiles that grow slowly in cold but have an optimum temperature above 20°C (68°F). Psychrotrophs such as Staphylococcus aureus and Listeria monocytogenes are a concern because they can grow in refrigerated food and cause food-borne illness.
Fig2. Red snow. (a) The surface of an Alaskan glacier provides a perfect habitat for psychrophilic photosynthetic organisms such as Chlamydomonas nivalis. (b) Microscopic views of this snow alga, which is classified as a “green” alga although a red pigment dominates at this stage of its life cycle (600×). (a and b): Nozomu Takeuchi
The majority of medically significant microorganisms are mesophiles, organisms that grow at intermediate temperatures. Although an individual species can grow between the extremes of 10°C and 50°C (50°F and 122°F), the optimum growth temperatures (optima) of most mesophiles fall into the range of 20°C to 40°C (68°F to 104°F). Organisms in this group inhabit animals and plants, as well as soil and water in temperate, subtropical, and tropical regions. Most hu man pathogens have optima somewhere between 30°C and 40°C (human body temperature is 37°C or 98°F). Thermoduric microbes, which can survive short exposure to high temperatures but are normally mesophiles, are common contaminants of heated or pasteurized foods. Examples include heat-resistant cysts such as Giardia or endospore-formers such as Bacillus and Clostridium.
A thermophile* is a microbe that grows optimally at temperatures greater than 45°C (113°F). Such heat-loving microbes live in soil and water associated with volcanic activity, in compost piles, and in habitats directly exposed to the sun. Ther mophiles generally range in growth temperatures from 45°C to 80°C (113°F to 176°F). Most eukaryotic forms cannot survive above 60°C (140°F), but a few bacteria and archaea, called hyperthermophiles, are adapted to life in extreme temperatures. They can grow at between 80°C and 121°C (250°F), currently thought to be the highest temperature limit endured by enzymes and cell structures. Strict thermophiles are so heat tolerant that researchers can use a heat-sterilizing device to isolate them from non-thermophiles in culture.
Currently, biotechnology companies are intensely interested in thermal microorganisms. One of the most profitable discoveries so far was a strict thermophile Thermus aquaticus, which produces an enzyme that can make copies of DNA even at high temperatures. This enzyme—Taq polymerase—is now an essential component of the polymerase chain reaction or PCR, a process used in many areas of medicine, forensics, and biotechnology.
