Methods of Counting Bacteria

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Methods of Counting Bacteria

• Direct microscopic
• Most probable number
• Standard plate count
• Coulter counter
• Turbidity (optical density) this is an indirect
  method

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Standard Plate Count
The basis of the standard plate count is that
dilutions are made of the starting culture and
each of these dilutions is plated onto solid agar
(pour plate or spread plate). Each colony that
grows up represents a single cell. Therefore the
number of colonies can be used to back calculate
the starting population density in cells/ml.
Sometimes it is called cfu/ml (colony forming
units per ml)
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Standard Plate Count Things you must know
1. How to calculate an individual dilution
factor. It is the volume added divided by the
final total volume.
2. How to move back and for between fractions
and scientific notation
3. How to calculate the total dilution factor.
It is the multiplication of each individual
dilution factor.
4. You must include the plating dilution factor
(if there is any) in your final total dilution
factor.
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Standard Plate Count
Some Trial Dilution Problems
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Optical Density
Optical density (OD) is only an indirect measure
of bacterial numbers. By itself, it can only
tell us if one culture is more dense than
another. It cannot give us absolute numbers. It
is based on the fact that when a culture has a
higher number of cells, it scatters more light.
The problem with OD readings is that at high
denisty the cells scatter almost all of the light
and the cell density is no longer proportional to
the optical density.
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Optical Density
However, if you do a standard plate count and an
optical density reading at the same time, these
data can be used to generate a line that will
predict the absolute numbers for that particular
bacterial strain.
OD
Cfu/ml