Bacterial growth defined

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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.
• 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

• 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.

• Lag phase
• Exponential or
• Log phase
• Stationary phase
• Decline or Death
• phase.

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Growth curve (continued)

• 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.

• 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.

• 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
Coulter-counter single-file 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.
• Alive means able to multiply.

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
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th/labmanual/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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Exponential growth

• 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.

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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.

In this example, total biomass increases
exponentially over time.
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