A water supply may be exposed to contamination by

A
group of total coliform which is bacteria frequently found in the environment
such as in soil and water. Total coliform bacteria are not probable to cause
illness, but their presence show that water supply may be exposed to
contamination by more harmful microorganisms. (Owen et al.,2007).The coliform
group consist of several types of bacteria belonging to the family
Enterobacteriaceae. A large number of different bacteria such as
Escherichia, Enterobacter, Klebsiella, Serratia, Citrobacter and Proteus belong
to the total Coliform group. This coliform group is defined as all aerobic and
facultative anaerobic, Gram-negative, nonspore-forming, rod shaped bacteria
that ferment lactose with gas and acid formation within 48 hours at 35° C.Most
E.coli strainsare harmless, but the presence of E.coli in the water indicates
high probability of disease-causing microorganisms. The Coliform microorganisms
do not change the color or taste of the water. The only way to find out if they
are present in the water is performed via a “colorimetric” test. A colorimetric
test is a test which forms a color by using mac conkey broth. A distinctive
color change indicates the presence of Total Coliform Bacteria which is from
purple to cloudy. (Murray,2007)

 

A standard plate count reflects the number of viable microbes and
assumes that each bacterium grows into a single colony. Because it is
impossible to say that each colony actually arose from an individual cell plate
counts are reported as the number of colony-forming units (CFU) instead of the
number of cells. To
insure a countable plate a series of dilutions should be plated. The technique
of dilution to extinction was used in this experiment. The serial dilutions
should give at least one countable plate in the series .Some
samples may be very dilute to begin with, so a plate prepared with undiluted
inoculum may still have too few colonies to be countable. In this case the
concentration of bacteria can be increased by filtering the sample. 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. The most
probable number (MPN) method can be used for microbes that will grow in a
liquid medium. A dilution series such as in figure 2.10 to no growth is
prepared and the combination of positives is used to look the most probable
number up in a MPN
Table. Used for microbes that won’t grow on solid media or are grown in
differential liquid media for identification purposes. (Murray et al., 2007)

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