Bacterial growth defined
• Since individual cells double in size, then divide into
two, the meaningful increase is in the population size.
• Binary fission: cell divides into two cells. No nucleus,
so no mitosis.
• Cells do not always fully detach; produce pairs,
clusters, chains, tetrads, sarcina, etc.
• “GROWTH” = increase in number of bacteria (over
time)
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Mathematics of bacterial growth
• Because bacteria double in
number at regular intervals,
they grow exponentially:
• N = N0 x 2n where N is the
number of cells after n
number of doublings and N0
is the starting number of
cells.
• Thus, a graph of the Log of
the number of bacteria vs.
time is a straight line.
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The Bacterial Growth Curve
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log (# of bacteria)
• Bacteria provided with an abundant supply of
nutrients will increase in number exponentially, but
eventually run out of nutrients or poison themselves
with waste products.
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tim e
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1. Lag phase
2. Exponential or
Log phase
3. Stationary
phase
4. Decline or
Death
phase.
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• Lag phase: growth lags; cells
are acclimating to the medium,
creating ribosomes prior to
rapid growth.
• Log phase: cells doubling at
regular intervals; linear graph
when x-axis is logarithmic.
log (# of bacteria)
Growth curve (continued)
time
•Stationary phase: no net increase in cell numbers,
some divide, some die. Cells preparing for survival.
•Decline phase: highly variable, depends on type of
bacteria and conditions. Death may be slow and
exponential.
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More about Growth
• The Growth curve is true under ideal conditions; in
reality, bacteria are subject to starvation, competition,
and rapidly changing conditions.
• Generation time: the length of time it takes for the
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population to double.
•Growth of bacteria is nonsynchronous,
not every bacterium is dividing at the
same time.
•Instead of stepwise curve, smooth
curve
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Measurement of growth
• Direct methods: cells
actually counted.
– Petroff-Hausser
counting chamber
(right), 3D grid. Count
the cells, multiply by a
conversion factor.
– Dry a drop of cells of
known volume, stain,
then count.
• Coulter-counter: single-file
cells detected by change
in electric current.
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Coulter Counter
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Coulter-counter: singlefile cells detected by
change in electric
current.
Measurement of growth -2
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• Viable plate count
– Relies on bacteria being alive,
multiplying and forming
colonies.
– Spread plate: sample is spread
on surface of agar.
– Pour plate: sample is mixed
with melted agar; colonies form
on surface and within agar.
• “Alive” means able to multiply.
biology.clc.uc.edu/.../Meat_Milk/ Pour_Plate.htm
Filtration:
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•Membrane filters are very thin
with a defined pore size, e.g.
0.45 µm.
•Bacteria from a dilute sample are
collected on a filter; filter placed
on agar plate, colonies counted.
http://dl.clackamas.cc.or.us/wqt111/coliform-8.jpg
http://www.who.int/docstore/water_sanitation_health/labman
ual/p25bs.jpg
Spectrophotometry
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• Bacteria scatter light, making a turbid (cloudy)
suspension.
• Turbidity is usually read on the Absorbance scale
– Not really absorbance, but Optical Density (OD)
• More bacteria, greater the turbidity (measured as OD)
Based on www.umr.edu/~gbert/ color/spec/Aspec.html
More about Spectrophotometry
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– Does NOT provide an actual number unless a
calibration curve (# of bacteria vs. O.D.) is created.
• Indirect counting method
– Quick and convenient, shows relative change in the
number of bacteria, useful for determining growth
(increase in numbers).
– Does NOT distinguish between live and dead cells.
To create a calibration curve, best to plot OD vs.
number of cells determined with microscope (not
plate count).
Exponential growth
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• “Balanced growth”
– Numbers of bacteria are doubling at regular
intervals.
– All components of bacteria are increasing in amount
at the same rate
• 2x as many bacteria = 2x as much protein, 2x as
much peptidgolycan, 2x as much LPS, etc.
– During exponential growth, bacteria are not limited
for any nutrients, i.e. they are not short of anything.
Biomass:
• Measure the total mass of cells or amount of any
component such as protein, PS, DNA, KDO.
• Especially when cells are doubling,
the amounts of all the components
of a cell are increasing at the same
rate, so any could be measured.
– Not so in stationary phase.
In this example, total biomass
increases exponentially over time.
http://www.pubmedcentral.nih.gov/pagerende
r.fcgi?artid=242188&pageindex=10#page
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