Classification of Bacteria Based on Temperature Requirement


 The temperature at which the growth of organisms is maximum and most rapid is called the optimum growth temperature. The range of temperature between which the organisms can grow is called the temperature range for growth. Shift in temperature on either side of optimum value retards growth. Usually the maximum temperature up to which the organisms can grow, is close to the optimum growth temperature. Whereas the minimum temperature required for growth can be much lower than the optimum value. Minimum, optimum and maximum growth temperatures are called cardinal temperatures.

   Cardinal temperatures vary greatly between microorganisms. Optima normally range from 0°C to 75°C; whereas growth can take place between -20°C to 100°C. Organisms having a narrow range of growth temperature are called stenothermal while the organisms having wide range of growth temperature are called eurythermal.

Based on the temperature requirements for growth, bacteria can be divided into three groups:

  1. Psychrophiles
  2. Mesophiles
  3. Thermophiles


The organisms able to grow at low temperature i.e. below 10°C, are called psychrophiles. They can grow even at 0°C or even lower, if the medium is not frozen due to high salt concentration. The optimum temperature for growth is 15°C or lower. The maximum temperature of growth is 20°C. There are two types of psychrophiles.

  1. Obligate or true psychrophiles :  These are the organisms which grow at 0°C or lower with optimum growth temperature 15°C or below. e.g. Vibrio marinus, Vibrio psychroerythreous.
  2. Facultative psychrophiles or psychrotroph : These are the organisms which can grow at 0°C. But optimum temperature for growth is between 20°C to 30°C. e.g. Pseudoinonas flourescens.

The physiological factors for the psychrophilic nature of organisms are not much clear. But it is observed that their ribosomes and other cellular enzymes are unstable at high temperature. Hence, they cannot grow at higher temperature. Even their membrane permeability is altered at temperature above optimum value, leading to leakage of cell material, impairment of membrane permeability and hence results in loss of viability.


  These are the organisms that can grow at moderate temperatures of incubation, i.e. within range of 25°C to 40°C. Their optimum growth temperature falls within this range. All pathogenic bacteria for humans and warm blooded animals grow best at body temperature, i.e. at 37°C.


The organisms capable of growing best at temperature above 45°C are called thermophiles. They can be grouped into two categories.

1) Facultative thermophiles

These organisms can grow even in the mesophilic range of temperature.

2) Stereothermophiles or hyperthermophiles

  These bacteria cannot grow in the mesophilic range of temperature. They grow well between temperatures 45°C to 70°C. There are some organisms which can grow even at 110°C. These bacteria are also referred to as strict or obligate thermophiles. These bacteria are normally found from hot water springs, salt lakes etc.
e.g. Bacillus coaggulans, B. stereothermophilus. Thermus aquaticus can grow even at 100°C.

The basis of thermal resistance of these bacteria is the thermal stability of most of their cellular proteins.