BACTERIA EXAMINATION OF WATER
The bacteriological
examination of water is performed routinely by water utilities and many
governmental agencies to ensure a safe supply of water for
drinking,bathing,swimming and other domestic and industrial uses. The
examination is intend to identify water sources, which have been contaminated
with potential disease causing microorganisms. Such contamination generally
occurs directly by human or animal feces or indirectly through improperly
treated either sewage or improperly functioning sewage treatment systems. The
organisms of prime concern are the intestinal pathogens, particularly those
that cause typhoid fever and bacillary dysentery.
Since human fecal
pathogens vary in kind with micro-organism (Virus,bacteria,protozoa etc) and in
number ,it would be impossible to test each water sample for each
pathogen.Instead,it is much easier to test for the presence of non pathogenic
intestinal organisms such as E.coli.
E.coli is a normal
inhabitant of the intestinal tract and is normally not found in fresh water.
Therefore, if it is detected in water, it can be assumed that there has been
fecal contamination of the water.
In order to determine
whether water has been contaminated by fecal material, a series of Test are
used to demonstrate the presence or absence of coliforms.The coliform group is
comprised of Gram-negative,nonspore-forming,aerobic to facultatively anaerobic
rods,which ferment lactose to acid and gas.Two organisms in this group include
E.coli and Enterobacter aerogenes;However,the only true fecal coliform is
E.coli,which is found only in fecal material from warm-blooded animals.The
presence of this organism in a water supply is evidence of recent fecal
contamination and is sufficient to order the water supply closed until tests no
longer detect the presence of E.coli.
TESTING WATER SAMPLES FOR THE PRESENCE OF COLIFORMS
There are (3) Three
principal tests used to detect and identify the presence of Coliforms in water
samples.
- v PRESUMPTIVE
- v COMFIRMED
- v COMPLETED TESTS
PRESUMPTIVE TEST
In the presumptive test, a series of
lactose broth tubes are inoculated with measured amounts of the water sample to
be tested. The series of tubes may consist of three or four groups of three,
five or more tubes. The more tubes utilized, the more sensitive the test. Gas
production in anyone of the tubes is Presumptive evidence of the presence of
coliforms. The number of positive tubes can estimate the most probable number
of coliforms in 100 ml of the water sample.
Material:
- § Nine tubes of double strength lactose broth
- § 10,1.0 and 0.1 ml pipettes
- § Water samples
- § Durham tubes
- Procedures
- Take a water sample (Dilute as instructed in some cases) and inoculate three tubes of lactose broth with 10 ml, three tubes with 1.0 ml and three tubes with 0.1 ml.
- In each tubes add a Durham tube such that it sink at the bottom with no air space or bubbles inside
- Incubate all tubes at 37°C for 24hours
Observation
Observe
the number of tubes of with gas production inside by checking if there is air
or gas in the Durham tubes.(Remember they were sink such that no air bubble was
seen inside)
This presumptive
test presumes the presence of lactose fermenting bacteria releasing gas and acid found
in the Durham tubes.Note that fermenting lactose releasing gas and acid change the PH of the medium reason why there is also a color change.
COMFIRMED TEST
1.
Inoculate an EMB plate with material or sample
from a tube from the presumptive test containing gas
2.
Invert the plate and incubate the plate at 37°C
for 24 hours
OBSERVATION
Observe EMB agar plates,
A positive confirmed test is indicated by small colonies with dark centers and
a green metallic sheen (E.coli).
COMPLETED
TEST
- Inoculate a lactose broth tube and a nutrient agar slant with organisms from the EMB plate.
- Inoculate the broth tube and agar slant at 37°C for 24 hours
OBSERVATION
Check for gas production
in the lactose broth tube
Make a gram stain from
the organisms on the nutrient agar slant.
Conclusion
Since E.coli is used as
an indicator of Fecal contamination of water, detection of these microorganism
serves as a complete conclusion that the water is not fit for drinking. The
presence of E.aerogenes does not indicate fecal water contamination since these
bacteria is almost found everywhere but not in the G.I.T of man or animal.
To totally conclude that
you have isolated E.coli,Biochemical test such as Indole,catalase can also be
done to confirm but A green metallic sheen color on EMB is a typical
identification of E.coli.
Also note E.coli is Escherischia
coli not Entamoeba coli
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