Mic 428 Lecture

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Mic 428 Lecture 3 Microbial Growth II
Please add section 6.6 to your syllabus.
Outline
Changes that occur during a batch culture brief
review from last class. Comparison of batch
culture and continuous culture. Characteristics
of continuous growth. Turbidostats and
chemostats. Chemostats and nutrient limitation.
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Outline (continuation)
Steady-state growth. Growth rate and growth yield
control. Growth at low and high dilution
factors. Basic equations. Dilution rate and
growth rate effects. Growth yields and
maintenance energy. Synchronous culture.
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Changes that happen during microbial growth in
batch culture.
In batch culture, growth is UNBALANCED.
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If we want a constant environment
CONTINUOUS CULTURE

• A constant microbial population is present at all
  times
• The environment is constant.
• The rate of growth is both constant and
  controlled.

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In continuous culture, regulation of cell
population, waste product removal and nutrient
addition can be controlled in two ways

• Turbidostatic control (internally controlled).
• Chemostatic control (externally controlled).

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Chemostats
The population is controlled through nutrient
limitation
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It controls growth rate and population density
independently.
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Continuous culture the chemostat. Experimental
uses and advantages.
A continuous culture is a flow system of constant
volume to which fresh medium is added
continuously and spent culture medium removed
continuously at a constant rate.
Once such a system is in equilibrium, cell number
and nutrient status remain constant, and the
system is said to be in steady state.
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The population is controlled through nutrient
limitation since the other nutrients are supplied
in excess.
External control
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In batch culture, nutrient concentration can
affect both growth rate and growth yield.
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by adjusting the dilution rate
In a chemostat, both growth rate and growth yield
are controlled independently.
by varying the concentration of limiting nutrient
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What happens
at very low D?
A large fraction of cells may die from starvation
at high D?
The culture washes out
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d X µ X - D X d t
The change in cell population or mass as a
function of time is equal to the difference
between the product of the growth rate (µ) and
the product of the dilution rate and the existing
population.
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d X X (µ - D) d t
The critical matter in regard to population
change is not the absolute value of the growth
rate or the dilution rate but whether or not they
are equal.
And when µ equals D?
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Dilution rate and growth rate effects
When the dilution rate exceeds the organisms
maximum growth rate
The ability of an organism to maintain a constant
population is lost.
Complete nutrient utilization is no longer
possible because of the decreasing population.
The concentration of nutrient in the chemostat
increases until it is the same of that of the
nutrient reservoir.
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Growth yields and maintenance energy
The yield of cells obtained during a microbes
growth reflects both the amount of available
nutrient and the proportion of that nutrient used
for growth since not all the nutrient is used for
this purpose.
Maintenance
At a very small substrate concentration that fail
to allow growth, all of the available substrate
is simply used for maintenance.
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Mathematical relationship between total change in
substrate concentration and the proportion used
for growth and maintenance
d C d C d C d t d tg d tm
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The yield of microbes obtainable from a given
amount of substrate is influenced by the
proportion of substrate used for maintenance.
d X Y d C - a X d t dtg
Y yield A maintenance rate constant