Temperature

Microorganisms as a group are able to grow over a wide range of temperatures, from around freezing to above boiling point. For any organism, the minimum and maximum growth temperatures define the range over which growth is possible; this is typically about 25-30 °C. Growth is slower at low temperatures because enzymes work less efficiently and also because lipids tend to harden and there is a loss of membrane fluidity. Growth rates increase with temperature until the optimum temperature is reached, then the rate falls again (Figure 5.4). The optimum and limiting temperatures for an organism are a reflection of the temperature range of its enzyme systems, which in turn are determined by their three-dimensional protein structures (see Chapter 6). The optimum temperature is generally closer to the maximum growth temperature than the minimum. Once the optimum value is passed, the loss of activity caused by denatu-ration of enzymes causes the rate of growth to fall away sharply (see also Figures 6.3 and 6.4).

Temperature For Microbial Growth Images

enzyme activity and membrane fundity)

Figure 5.4 Effect of temperature on microbial growth rate. The factors governing the minimum, optimum and maximum temperatures for a particular organism are indicated. The curve is asymmetrical, with the optimum temperature being closer to the maximum than the minimum enzyme activity and membrane fundity)

Figure 5.4 Effect of temperature on microbial growth rate. The factors governing the minimum, optimum and maximum temperatures for a particular organism are indicated. The curve is asymmetrical, with the optimum temperature being closer to the maximum than the minimum

Psychrophile

Thermophile

Mesophile

Extreme thermophile

Mesophile

Extreme thermophile

Extreme Thermophile

Temperature

Figure 5.5 Microorganisms can be categorised according to the temperature range at which they grow

Temperature

Figure 5.5 Microorganisms can be categorised according to the temperature range at which they grow

The majority of microorganisms achieve optimal growth at 'middling' temperatures of around 20-45 °C; these are called mesophiles (Figure 5.5). Contrast these with ther-mophiles, which have become adapted to not only surviving, but thriving at much higher temperatures. Typically, these would be capable of growth within a range of about 40-80 °C, with an optimum around 50-65 °C. Extreme thermophiles have optimum values in excess of this, and can tolerate temperatures in excess of 100 °C. In 2003, a member of the primitive bacterial group called the Archaea (see Chapter 7) was reported as growing at a temperature of 121 °C, a new world record! Psychrophiles occupy the other extreme of the temperature range; they can grow at 0°C, with optimal growth occurring at 15 °C or below. Such organisms are not able to grow at temperatures above 25 °C or so. Psychrotrophs, on the other hand, although they can also grow at 0 °C, have much higher temperature optima (20-30 °C). Members of this group are often economically significant due to their ability to grow on refrigerated foodstuffs.

In the laboratory, appropriate temperatures for growth are provided by culturing in an appropriate incubator. These come in a variety of shapes and sizes, but all are thermostatically controlled and generally hold the temperature within a degree or two of the desired value.

Microorganisms are strongly influenced by the prevailing pH of their surroundings. As with temperature, we can define minimum, optimum and maximum values for growth of a particular type (Figure 5.6). The pH range (between minimum and maximum values) is greater in fungi than it is in bacteria. Most microorganisms grow best around neutrality (pH 7). Many bacteria prefer slightly alkaline conditions but relatively few pH

Microbe Growth Temperature Graph

Figure 5.6 Effect of pH on microbial growth rate. Individual species of microorganism occupy a relatively narrow range of pH. Although for most species this is around neutrality, both acidophilic and alkalophilic forms exist. The shape of the curve reflects the properties of a particular organism's enzymes and other proteins

Figure 5.6 Effect of pH on microbial growth rate. Individual species of microorganism occupy a relatively narrow range of pH. Although for most species this is around neutrality, both acidophilic and alkalophilic forms exist. The shape of the curve reflects the properties of a particular organism's enzymes and other proteins are tolerant of acid conditions, and fewer still are acidophilic. Fungi, on the other hand, generally prefer slightly acid conditions and therefore tend to dominate bacteria when these prevail. The reason for the growth rate falling away either side of the optimum value is again due to alterations in three-dimensional protein structure.

The pH value of growth media is adjusted to the desired value by the addition of acid or alkali during its preparation. The metabolic activities of microorganisms often means that they change the pH of their environment as growth proceeds, so it is important in a laboratory growth medium that a desirable pH is not only set but maintained. This is achieved by the use of an appropriate buffer system. Phosphate buffers are widely used in the microbiology laboratory; they enable media to minimise changes in their pH when acid or alkali is produced (see Box 5.3).

Acidophilic = 'acid-loving'; a term applied to organisms that show optimal growth in acid conditions (pH < 5.5).

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Responses

  • gregorio
    Why is the optimum temperature closer to the maximum than the minimum?
    7 years ago
  • MEDHANIE
    Why is the growth optimal closer to the maximum in microorganisms?
    7 years ago
  • olga
    Why is optimal cardinal temperature closer to the maximum?
    7 years ago
  • esa
    Why is optimal temperature closer to maximum than minimum?
    7 years ago
  • conan
    Why is optimal cardinal temperature closer to maximum than minimum?
    7 years ago
  • Elisa
    Why the optimum temperature of an organism is closer to its maximum than minimum?
    7 years ago
  • kris
    Why is the curve relating enzyme function to temp asymmetrical?
    7 years ago
  • Sampsa
    How does maximum temperature affect microbial growth?
    7 months ago
  • edvige
    What is the highest range of cardinal temperature of microorganisms?
    5 months ago
  • stuart curran
    Why is the optimum temperature for growth is usually near the max temperature?
    1 month ago

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