Transcript Bacterial Growth and Nutrition

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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.
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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.
The Bacterial Growth Curve
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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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)
tim e
•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
population to double.
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•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: singlefile cells detected by
change in electric current.
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Measurement of growth -2
• 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.
biology.clc.uc.edu/.../Meat_Milk/ Pour_Plate.htm
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Filtration:
•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
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Spectrophotometry
• 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
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More about Spectrophotometry
– 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).
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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.
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Note different ratios
of carbohydrate
(slime) to protein in
these cells.
C:P Ratio
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Bacterial Isolates