Make your own free website on
Lecture Seven

Physical Factors effecting Microbial Growth.

Factors effecting microbial growth include:
Physical Factors such as Temperature, pH, water activity, light and atmosphere
Chemical Factors, such as nutrient supply
Biological Factor, such as there relationship with other organisms in there environment
Measures of Suitability
Rate of growth:
The usual measure of suitability of any condition for microbial growth is the rate of growth produced under those conditions. The faster a microbe grows the better the condition for that microbe. Such measurements are usually based on a measurement of the number of bacteria in a medium after periods of time measured by conducting viable counts or turbidity of the medium or direct microscopic count (total counts).
Total Growth:
The rate of growth can be a misleading measure because when bacteria grow rapidly they may inhibit them selves due to build up of toxic waste near the cells or slow diffusion of material to the cells. This occurs with rapid fermentation of wine when extra vitamins are added.
The maximum number of microbial cells may be a better measure but it is a slower and more cumbersome method.

Normal Growth response

Bacteria usual exhibit maximum growth rate for the best set of conditions these are called the Optimum Conditions.

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:
Thermophiles have an optimum greater than 40 C

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.

Bacteria exhibit a similar rage of growth responses in regard to pH as they do for temperature with most bacteria preferring neutral conditions (pH = 7) but some doing well in acid conditions and others in alkaline conditions. Some (particularly mold) have a wide range of tolerance.
Water Activity All bacteria need water to grow so there must be at least a minimum for even the one loving the driest conditions. However no bacteria can live in absolutely pour water because they all need some dissolve nutrient. Even the most autotrophic bacteria need minerals.
The amount of water is food is not a suitable measure of suitability for microbial growth because it is possible to have a lot of water in a medium and not have it available for the bacteria to use. This is because dissolved substances bind the water so that it is not free for the bacteria to absorb.
The best measure of water availability is the "water activity" ( aW )
The water availability in a salt solution is much less than in a sugar solution because salt is one of the best water binding agents. 10% salt is as good as 90% sugar
Binding agents bind more water is they are small and highly charged
Water binding ability: Salt >>>>>>>> glycerine >>> sugar >>> starch or protein
The water activity can be measured by the humidity of the atmosphere that builds up around a nutrient in a closed cautioner. The aW is RH/100
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.
Facultative anaerobes do not use oxygen even when it is available These grow better with out oxygen than with it.
Facultative aerobes will use oxygen if it is available
These grow better with oxygen than without it.
Obligate anaerobes can not survive in the presence of oxygen
Obligate aerobes can not grow with out the presence of oxygen

The End

Prepared by Barry Brazier