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Kinetics
(rates) of fungal growth
R =
D
in some measure of growth
D t
Focus on 5* of 6 situations
that concern practical measurements of
fungal biomass
1.* Growth of populations of
unicells
(reproductive growth)
2.* Hyphal growth by apical
extension & branching (nonreproductive
growth)
3.* Unrestricted growth
4.
Restricted growth (some factor
limiting)
5.* Surface growth
6.* Submerged growth
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Types
of growth as related to rates (kinetics)
1.
Unicellular growth - usually involves
doubling rates (exponential rates)
2.
Hyphal growth - may or may not
involve doubling rates
3.
Unrestricted growth - maximum rates
&/or no limiting factors
4.
Restricted growth - less than maximum
rates
5.
Surface growth - air-substrate
interface growth (somewhat restricted
because substrate not equally
available to all parts of fungus
6.
Submerged growth - may or may not be
at max rates
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Question?
What to measure for index of fungal
growth?
Key - parameter measured must
be known to be proportional to criterion of
growth being considered
Measured parameter must
directly correlate with increase in biomass
(protoplasm)
Could measure* -
1.
D in linear dimensions (colonies, hyphae)
2.
D in mass or weight
3.
D in cell #
4.
D in volume
5.
D in metabolic activity
6.
D in quantity of cell constituent
7.
D in absorbance
*
all have been used for fungi
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Practical
considerations of measuring fungal growth
(which is best?)
1.
Linear measurements - e.g.
a. Increase in colony
diameter*
B.
Increase in colony margin*
in
"race tube"
c. Increase in hyphal
length using microscopic techniques **
Advantages:
1.
Simplicity
2.
Nondistructive
Disadvantages:
1. No necessary
correlation between increase in colony
diameter and increase in total biomass
* Use - good for evaluating growth
rates of strains of same species
**
Excellent, but difficult
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2. Dry weights - most
widely used parti-cularly for molds - or as
correlates* of other measures for yeasts
(e.g. absorbance
x = y mg dry wt)
*
Should correlate with balanced growth.
Advantages:
a.
Probably one of best measures for
mold growth except when linear
measurements appropriate
b.
Excellent for yeasts, but time
consuming
Disadvantages:
a.
Destructive
b.
Requires culture sampling which must
be precise
c.
Near impossible to uniformly sample
mold culture*
* Why
- Hard to pipette mold mycelium
- Mycelium not always uniformly
distributed in media
- Culture homogeneity difficult to
insure
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3.
Estimates of cell #s
a.
Viable counts (plate counts)
b.
Particle counts (haemocytometer,
coulter counters, etc.)
Good for unicells - yeasts
& spore counts
Poor for molds - because:
1)
Hypha of any length would yield only
one (1) viable or
Particle count
2)
Difficult to sample
3)
Correlations inaccurate
Most often used for yeast
measures, and estimates of spore, germling
or blended hyphal inoculum levels.
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4.
Cell volume - (packed cells)
o.k. for unicells & spores,
sometimes germlings
poor in general for molds because
packing is dependent on:
a.
Llength of hyphae
b.
Degree of branching
c.
Endogenous substrate reserves
5.
Turbidity or absorbance
a.
Extremely useful for spores, unicells,
germlings or germination activity
b.
Poor for molds
1)
Hyphae do not always form
homogeneously
2)
Filaments do not uniformly scatter
light
Can get good data with yeasts.
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6.
Metabolic activity
- Good in well-defined situations
e.g., o2
| co2
| during
"balanced growth"
- Problems:
data may be misleading -e.g., because
of generally high levels of endogenous
reserves, fungi often show high rates of o2
uptake and co2
evolution in the absence of growth
- Same for product formation
- Substrate utilization
- Enzymatic activity
7.
Compositional changes same
- Good for certain balanced growth
situations, e.g.>dna
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Review
Kinetics
(rates) of fungal vegetative growth
Rate = D in some measure of growth
D t
Comparisons of reproductive
growth (yeast growth) and nonreproductive
growth (mold growth) involve different
considerations
Practical considerations:
What to measure?
parameter must directly correlate
with increase in biomass.
Measures:
1.
Linear dimensions
2.
Mass or weight
3.
Cell # or viable #
4.
Volume
5.
Metabolic activity
6.
Cell constituent (e.g. protein)
7.
Absorbance or transmittance
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Questions
to ask related to the kinetics of fungal
growth
1.
What conclusions with fungi,
particularly molds, have resulted from the
collection of data by techniques reviewed?
2.
How fast do fungi grow?
a.
Do fungi grow as fast as bacteria?
b.
Do molds grow exponentially?*
*
Can/do they ever double all their
cell constituents proportionally with time?
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How
fast do molds grow?
What controls rates of mold growth?
Conclusions* of Henderson Smith ~ 1920:
1.
Rate of growth of a mold during any
one period of time is not a function
necessarily of total fungal
biomass -although this may contribute
2.
Rate controlled by # of hyphal tips
3.
Rates at which tips are supplied
nutrients either by absorption or
translocation
4.
Rates of branching
* Not readily apparent because mold growth most
easily demonstrated to show constant growth
rates (arithmetic increases)
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Conclusions
1.
Both yeasts and molds can be
demonstrated to grow logarithmically
2.
Usually mold exponential growth
occurs for only relatively short period
while colony establishes itself into circle
or sphere shape*, **
3.
Mold exponential growth is correlated
with rapid branching to -->
2 or 3D colonies
4.
Rapid growth rates must involve
protoplasm contributions and syntheses from
areas not actually
involved in apical extension (tips)
* then by definition is
restricted
** use of strains that
fragment at high rates extends periods of
demonstrable exponential growth (strain
selection in industry)
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Zolakar's calculations (1950) w/ N.
crassa
1.
N. crassa extends
its tip at maximum rates of 100 µm/min (6
mm/hr).
-
he did this by measuring microscopically
& in race tubes.
2. Hyphae needed 2 hours
to double protein content (..) took 2 hrs to
double protein, but only an hr to
double length
3. Means protein
synthesized behind tip contributes to
elongation rate (translocation)
4. Need a minimum of 12
mm of hyphal length to sustain an elongation
rate of 100 µm/min
100
µm
= 12 mm length
100
µm (per min extension rate) x 120 min (time
to double protein) = 1200 µm = 12 mm
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