Factors effecting microbial growth include:
At lower or greater temperatures or concentrations the organism will grow slower and when too low or high growth will not occur at all. These are the Minimum and Maximum conditions for growth. Between these extremes there is a Range of tolerance.
There is usually some kind of microbe that can grow at any temperature even below zero and above 100 C. Each species of bacteria will have a particular minimum, maximum and optimum. For convenience they are classified into three groups in regard to their requirement for optimum growth:
Mesophiles have an optimum around 37 C
Psychrophiles have an optimum less than 25 C
These groups may have a wide range of overlap where all three can grow even if they are not at their optimum conditions. Optimum conditions for any of these could not be used to isolate any one group from the others due to this over lap. To isolate one group it is necessary to select a temperature when only one grows. ie. Near the limits of tolerance for the desired isolate.
The amount of oxygen is an important condition for determining which group of bacteria grow in food this is measured as the oxidation-reduction potential or Eh.
Aerobic bacteria are those that need a high Eh or plenty of oxygen
Anaerobic bacteria grow without oxygen
Facultative bacteria can grow if oxygen is there or not.