Physical Factors effecting Microbial Growth.
Factors effecting microbial growth
Physical Factors such as Temperature, pH, water activity, light and
Factors, such as nutrient supply
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).
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
Thermophiles have an optimum greater
than 40 C
Mesophiles have an optimum around 37 C
Psychrophiles have an optimum less than
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
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.
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
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
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
Anaerobic bacteria grow without oxygen
Facultative anaerobes do not use oxygen even when it is available
These grow better with out oxygen than with it.
Facultative bacteria can grow if oxygen is there or
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