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Physical Requirements continued
©2000 Kenneth Todar, University of Wisconins-MadisonThe Effect of Temperature on Growth
Microorganisms have been found growing in virtually all environments where there is liquid water, regardless of its temperature. In 1966, Professor Thomas D. Brock at Indiana University, made the amazing discovery in boiling hot springs of Yellowstone National Park that bacteria were not just surviving there, they were growing and flourishing. Boiling temperature could not inactivate any essential enzyme. Subsequently, procaryotes have been detected growing around black smokers and hydrothermal vents in the deep sea at temperatures at least as high as 115°C. Microorganisms have been found growing at very low temperatures as well. In supercooled solutions of H2O as low as -20°C, certain organisms can extract water for growth, and many forms of life flourish in the icy waters of the Antarctic, as well as household refrigerators, near 0°C.
For more information on Professor Brock's amazing discoveries at Yellowstone, check out Life at High Temperatures
A particular microorganism will exhibit a range of temperature over which it can grow, defined by three cardinal points in the same manner as pH (Figure 3, cf. Figure 2). Considering the total span of temperature where liquid water exists, the procaryotes may be subdivided into several subclasses on the basis of one or another of their cardinal points for growth. For example, organisms with an optimum temperature (T) near 37°C (the body temperature of warm-blooded animals) are called mesophiles. Organisms with an optimum T between about 45°C and 70°C are thermophiles. Some Archaea with an optimum T of 80°C or higher and a maximum T as high as 115°C, are now referred to as extreme thermophiles or hyperthermophiles. The cold-loving organisms are psychrophiles defined by their ability to grow at 0°C. A variant of a psychrophile (which usually has an optimum T of 10-15°C) is a psychrotroph, which grows at 0°C but displays an optimum T in the mesophile range, nearer room temperature. Psychrotrophs are the scourge of food storage in refrigerators since they are invariably brought in from their mesophilic habitats and continue to grow in the refrigerated environment where they spoil the food. Of course, they grow slower at 2°C than at 25°C. Think how fast milk spoils on the counter top versus in the refrigerator.
Psychrophilic bacteria are adapted to their cool environment by having largely unsaturated fatty acids in their plasma membranes. Some psychrophiles, particularly those from the Antarctic have been found to contain polyunsaturated fatty acids, which generally do not occur in procaryotes. The degree of unsaturation of a fatty acid correlates with its solidification T or thermal transition stage (i.e., the temperature at which the lipid melts or solidifies); unsaturated fatty acids remain liquid at low T but are also denatured at moderate T; saturated fatty acids, as in the membranes of thermophilic bacteria, are stable at high temperatures, but they also solidify at relatively high T. Thus, saturated fatty acids (like butter) are solid at room temperature while unsaturated fatty acids (like canola oil) remain liquid in the refrigerator. Whether fatty acids in a membrane are in a liquid or a solid phase affects the fluidity of the membrane, which directly affects its ability to function. Psychrophiles also have enzymes that continue to function, albeit at a reduced rate, at temperatures at or near 0°C. Usually, psychrophile proteins and/or membranes, which adapt them to low temperatures, do not function at the body temperatures of warm-blooded animals (37°C) so that they are unable to grow at even moderate temperatures.
Thermophiles are adapted to temperatures above 60°C in a variety of ways. Often thermophiles have a high G + C content in their DNA such that the melting point of the DNA (the temperature at which the strands of the double helix separate) is at least as high as the organism's maximum T for growth. But this is not always the case, and the correlation is far from perfect, so thermophile DNA must be stabilized in these cells by other means. The membrane fatty acids of thermophilic bacteria are highly saturated allowing their membranes to remain stable and functional at high temperatures. The membranes of hyperthermophiles, virtually all of which are Archaea, are not composed of fatty acids but of repeating subunits of the C5 compound, phytane, a branched, saturated, "isoprenoid" substance, which contributes heavily to the ability of these bacteria to live in superheated environments. The structural proteins (e.g. ribosomal proteins, transport proteins (permeases) and enzymes of thermophiles and hyperthermophiles are very heat stable compared with their mesophilic counterparts. The proteins are modified in a number of ways including dehydration and through slight changes in their primary structure, which accounts for their thermal stability.
Figure 3. Growth rate vs temperature for four environmental classes of bacteria. Most bacteria will grow over a temperature range of about 30°C. The curves exhibit three cardinal points: minimum, optimum and maximum temperatures for growth. There is a steady increase in growth rate between the minimum and optimum temperatures, but slightly past the optimum a critical thermolabile cellular event occurs, and the growth rates plunge rapidly as the maximum T is approached.
Table 9. Terms used to describe microorganisms in relation to temperature requirements for growth.
Temperature for growth (°C)
Table 10a. Minimum, maximum and optimum temperature for growth of bacteria.Temperature for growth (°C)
Table 10b. Optimum growth temperature of some procaryotes.
Table 10c. Hyperthermophilic Archaea.
Temperature for growth(°C)
Water is the solvent in which the molecules of life are dissolved, and the availability of water is therefore a critical factor that affects the growth of all cells. The availability of water for a cell depends upon its presence in the atmosphere (relative humidity) or its presence in solution or a substance (water activity). The water activity (Aw) of pure H2O is 1.0 (100% water). Water activity is affected by the presence of solutes such as salts or sugars, that are dissolved in the water. The higher the solute concentration of a substance, the lower is the water activity and vice-versa. Microorganisms live over a range of Aw from 1.0 to 0.7. The Aw of human blood is 0.99; seawater = 0.98; maple syrup = 0.90; Great Salt Lake = 0.75. Water activities in agricultural soils range between 0.9 and 1.0.
The only common solute in nature that occurs over a wide concentration range is salt [NaCl], and microorganisms are named based on their growth response to salt. Microorganisms that require some NaCl for growth are halophiles. Mild halophiles require 1-6% salt, moderate halophiles require 6-15% salt; extreme halophiles require 15-30% NaCl for growth. Bacteria that are able to grow at moderate salt concentrations, even though they grow best in the absence of NaCl, are called halotolerant. Although halophiles are "osmophiles" (and halotolerant organisms are "osmotolerant") the term osmophiles is usually reserved for organisms that are able to live in environments high in sugar. Organisms which live in dry environments (made dry by lack of water) are called xerophiles.
The concept of lowering water activity in order to prevent bacterial growth is the basis for preservation of foods by drying (in sunlight or by evaporation) or by addition of high concentrations of salt or sugar.
Figure 4. Growth rate vs osmolarity for different classes of procaryotes.
Osmolarity is determined by solute concentration in the environment. Osmolarity is inversely related to water activity (Aw), which is more like a measure of the concentration of water (H2O) in a solution. Increased solute concentration means increased osmolarity and decreased Aw. a is the growth rate of a normal (nonhalophile) such as E. coli or Pseudomonas. b is the growth rate of a halotolerant bacterium such as Staphylococcus aureus. c is the growth rate of an extreme halophile such as Halobacterium, the predominant organism in the Great Salt Lake.
Table 11. Limiting water activities (Aw) for growth of certain procaryotes.
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