Download Microbial growth (part 3) Measurement of Microbial Growth: 1. A

Microbial growth (part 3)
Measurement of Microbial Growth:
1. A standard plate count reflects the number of viable microbes and
assumes that each bacterium grows into a single colony; plate
counts are reported as number of colony-forming units (CFU)
(Whether a single cell or a clump of cells or whatever, what grows
into an isolated colony is termed a colony-forming unit) (Figure 1).
2. A plate count may be done by either the pour plate method or the
spread plate method (Figure 2).
a- In the pour plate method of addition of cells to solid medium
contained within a Petri dish, cells are added to melted (but
not too hot) solid medium. The melted solid medium is then
poured into Petri dish and allowed to harden. Colonies appear
within, beneath or on top of the agar.
b- In the spread-plate method of addition of cells to solid
medium, a small volume of culture is dropped onto the surface
of agar that has already hardened in a Petri dish. The volume is
then spread around the agar surface. Colonies will grow solely
on the surface of the agar. This technique is advantageous
particularly when cells are sensitive to exposure to relatively
high temperatures plus the method does not require a prior
melting of the solid medium.
3. In filtration, bacteria are retained on the surface of a membrane
filter and then transferred to a culture medium to grow and
subsequently be counted.
4. The degree of turbidity (cloudiness) exhibited by a broth culture
gives an indication of the number of organisms present. Degree of
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turbidity varies with organisms, conditions, and phase of growth
so use of turbidity as a form of enumeration requires previous
standardization.
5. In a direct microscopic count, the microbes in a measured volume
of a bacterial suspension are counted with the use of a specially
designed slide. This method's limitations are that only relatively
high concentrations of bacteria may be enumerated and the
method cannot distinguish living from dead bacteria (Figure 3)
6. An indirect way of estimating bacterial numbers is measuring the
metabolic activity of the population (for example, acid production
or oxygen consumption).
7. For filamentous organisms such as fungi, measuring dry weight is
a convenient method of growth measurement.
Figure (1): A standard plate count method.
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Figure (2): the pour plate and spread plate methods.
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Figure (3): A direct microscope count.
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