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TitleViable Plate Count
TagsColony Forming Unit Earth & Life Sciences Life Sciences Microbiology
File Size1.2 MB
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original broth culture containing E. coli was calculated to be 1.3x108. Other results are
labelled to be TFTC or Too Few To Count (#colonies < 25), Too Many To Count
(#colonies >250). Some plates were contaminated with fungal growth characterized
by the large to moderate sized filamentous growth present in the plates. Errors may be
due to unsuitable culture conditions which may include inappropriate use of medium,
inadequate environment to facilitate growth (Temperature and other conditions). Error
may also arise from procedural errors such as inaccurate transfer or dilutions and
inadequate mixing of culture before inoculation. Cell clumping may also overestimate
the number of bacteria present.
STUDY QUESTIONS:
1. Why do you think it is important to be able to quantify the number of viable

bacteria in a sample?
Quantification of the number of viable bacteria in a sample is one of the methods

of measuring microbial growth which is essential clinically, particularly in the
potency, mechanism and diagnosis of infectious diseases. Also, the total number
of viable counts are important in dairy microbiology, food microbiology, and
water microbiology.

2. What is a CFU?
Colony forming unit or CFU refers to each colony that can be counted in a plate.

In addition, each viable cell can yield one colony.

3. Why is the viable plate count technique considered to be an indirect measurement
of cell density?
Viable plate count technique is done indirectly, in a series of dilution, that is, the
sample is serially diluted then plated out on an agar surface isolating visible

colonies. This is done because most bacterial populations are very large to be
counted directly. Thus, serial dilution can readily estimate the number of bacteria
in the original sample and reveals only information about live bacteria (Tortora,
2010).

4. Give an example of an industrial setting where quantifying viable bacteria would
be a useful tool.
Heterotrophic plate count (HPC) tests play an important role in water

microbiology. HPC measurements are used: to indicate the effectiveness of a water
treatment process; as a measure of numbers of microorganisms that may have a
sanitary significance; and, as measure of possible interference with coliform
measurements in lactose-based culture methods (Allen et al., 2002).

5. Describe turbidimetric method to quantify the number of bacteria in a given
sample.
In this method, turbidity is estimated as a practical way of monitoring bacterial
growth because as bacteria multiply in liquid, the liquid becomes turbid as cells
multiply in the medium. The turbidity is measured by a spectrophotometer or
colorimeter (Tortora, 2010).

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