- Chemical and physical state of the environment greatly affects the activities of the microorganisms.
- Many environmental factors can be considered among which four key factors control the growth of all microorganisms: temperature, pH, water availability and oxygen.
- Some other factors can also potentially affect the growth of microorganisms like pressure and radiation.
- It is probably one of the most important environmental factors that affects the growth and survival of microorganisms.
- The microorganisms will not be able to grow or even die if the temperature is too cold or too hot.
- The minimum and maximum temperatures for growth vary among the different microorganisms and usually it reflects the temperature range and average temperature of their habitats.
- It affects the microorganisms in two opposing ways.
- The chemical and enzymatic reaction in the cell proceeds at more rapid rates with the rise in temperature that also makes growth faster.
- But, there is a certain temperature above which the cell components may be irreversibly damaged.
- As the temperature keeps on increasing within a given range, growth and metabolic functions increase up-to a point where denaturation reaction set in.
- Above that range the cell functions fall to zero.
- There is thus a minimum temperature for all microorganisms below which there is no growth, an optimum temperature where growth is rapid and maximum temperature above which again the growth ceases.
- The optimum temperature is always nearer the maximum than the minimum.
- So, these three temperatures are called the cardinal temperatures and is characteristic for any given microorganisms.
- The cardinal temperatures of different microorganisms vary widely.
- Some of them have temperature optima as low as 40C and some has higher than 1000C.
- The temperature range throughout which microorganisms grow is even wider than this, from below freezing to well above boiling point of water.
- However, there is no microorganism that can grow over this whole temperature range. As the range is between 25-40 degrees.
- The maximum growth temperature of an organism indicates the temperature above which there is denaturation of one of the important cell components such as a key enzyme.
- The factors controlling an organism’s minimum growth temperature are not so clear.
- However, the cytoplasmic membrane exists in fluid state for transport and for other important function to take place.
- An organism’s minimum temperature may be governed by membrane functioning i.e. the organism cannot grow if the cytoplasmic membrane stiffens to the point that it no longer functions properly in nutrient transport.
- Development of proton motive force cannot take place as well.
- The growth temperature optimum reflects a state in which all or most of the cellular components function to the maximum rate.
Temperature classes of organisms
- On the basis of very low temperature optima and high temperature optima of the given microorganisms, the microorganisms can be distinguished at least in four groups in relation to their growth temperature optima.
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- Psychrophiles are those microorganisms that have low temperature optima.
- Mesophiles have mid-range temperature optima, thermophiles have high temperature optima and hyperthermophiles have very high temperature optima.
- Among them mesophiles are wide spread in nature as they are found in warm-blooded animals and in terrestrial and aquatic environments in temperate and tropical latitudes.
- Psychrophiles and thermophiles are common in unusually cold and unusually hot environments respectively.
- Hyperthermophiles are found in extremely hot habitats such as hot springs, geysers, and deep sea hydrothermal vents.
Effect of temperature on microbial growth