Lab Day 7
May 22
Part 1 of today's lab: checking our
experimental results from yesterday
1: Indole (Tryptophan Degradation) Test
Purpose: to determine the ability of
some bacteria to split the amino acid trypotphan into indole and
pyruvic acid.
Result: positive
We added 15 drops of Kovac's reagent. A
red layer formed on the top of our sample and this indicates a
positive test for the presence of indole. This means that Trypotphan
was hydrolyzed.
2: Urea Hydrolysis Test
Purpose: to determine the ability of a
bacterium to hydrolyze a urea.
Result: Negative
A few bacteria use the enzyme urease to
rapidly degrade urea into CO2 and Ammonia. The color didn't change
from from the original color and this signified a negative test. If
ammonia was produced, the color would've been pink in alkaline ph due
to the breaking down of urea with the urease enzyme.
3: Citrate Utilization Test
Purpose: to determine if bacterium can
utilize citrate as its sole source of carbon and energy.
Result: negative
The ph indicator bromothymol blue turns
blue in alkaline ph. A change in color of the medium from green to
blue denotes a positive test. Our color did not change and therefore,
our bacteria didn't have the enzyme to break down citric acid.
4: Nitrate Reduction Test
Purpose: to determine if a bacterium
is able to reduce nitrate ions to either nitrite ions or to nitrogen
gas.
Result: positive
Nitrate accepts electrons instead of
oxygen. We were looking for a nitrate reduction enzyme.
This test can be two part:
Nitrate ----> nitrate
Nitrite ---> ammonia
However, because our test was positive
for nitrites upon adding Sulfanilic Acid and
Dimethyl-alpha-naphthylamine, we did not have to test further in case
our bacterial nitrate broth had undergone both reactions and turned
into ammonia. It turned a cherry red which signifies that it produced
nitrites and is therefore positive.
5: MR-VP
Purpose: to determine the ability of
some bacteria to ferment glucose via mixed-acid fermentation.
MR result: positive – VP result:
negative
If our MR-VP broth used glucose and
fermented it for acid, upon adding methyl red (a ph indicator), you
should see a color change to red. Ours reacted this way when it was
added. VP test tests butanediol for non-acidic end product but if MR
test is positive, VP is negative.
6: Litmus Milk Reactions
Purpose: to differentiate among
bacteria as to their ability to utilize lactose, protein, and litmus
in litmus milk.
Results: The curds that were present
yesterday, were even more of a pink color today. The pink color
signifies that our bacteria can ferment lactose. The cracks in the
curds signify the presence of gas.
When testing for acidic and non-acidic
curds, we turned the tube upside down. If they were to come down, it
would be non-acid curd but being that our curds remained immotile
when turned, we can tell that our bacteria form an acid curd in
litmus milk.
Part 2 of todays lab: Selective and
Differential Plates
The procedures for each of these tests
were the same. Practicing aseptic technique throughout, we inoculated
each of the following plates.
1: MacConkey Agar
Purpose: to detect and differentiate
among gram-negative enteric bacilli, based on their ability to grow
on the medium and to ferment lactose.
2: Eosin Methylene Blue Agar
Purpose: To isolate and differentiate
gram-negative enteric bacilli.
3: Phenylethyl Alcohol Agar
Purpose: To isolate gram-positive
bacteria from a specimen containing a mixture of gram-positive and
-negative bacteria
4: Blood Agar Plate
Purpose: 1. To isolate and support the
growth of fastidious bacteria. 2. To differentiate among bacteria
based on their ability to lyse red blood cells.
5: Mannitol Salt Agar
Purpose: to isolate bacteria based on
their salt tolerance and differentiate among these isolates for
mannitol fermentation.
After inoculating each of these plates,
we placed them in an incubator set at 37 degrees Celsius.
Joanna Dawyot, Cassie Livingston, Mary Rose Capara
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