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Microbiology | Staphyloccocus epidermidis

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Unknown Lab Report
Report by Jessica Unruh

The study of living organisms through a microscope is
known as microbiology. Knowing what microorganisms are
and how they survive and reproduce is beneficial in
knowing how to control them. In the healthcare system,
knowing the identity of a microorganism can help in
determining which antibiotic needs to be prescribed.

Materials and Methods
An unknown test tube labeled as number 106 was
given out by the instructor. There were two unknown
bacteria in the test tube. To determine which two
bacteria were in this tube, procedures, and tests were
done. These procedures and tests were done as indicated
in the laboratory course manual. The first procedure done
was using the T streak method on a Nutrient Agar (NA)
plate from the unknown test tube labeled number 106.

This was done to isolate two colonies. After incubation two
different colonies needed to be grown on the NA plate to isolate the bacteria on two separate NA plates.
Proper incubation was done again and there were two
separate plates labeled unknown A and unknown B. After
observation and recording, a Gram stain was done to
determine a Gram-negative or Gram-positive bacteria.
Once the Gram stain was performed, biochemical tests were
done to further determine which bacteria were in the test
tube labeled unknown number 106. Biochemical tests
were done referencing the lab manual for microbiology.

Unknown A was determined to be a Gram-negative rod. A
Simmons citrate, mannitol, and urea tests were performed.
Unknown B was determined to be a Gram-positive cocci.
A nitrate, mannitol, and urea test were performed as well.
All of the following tests were performed on unknown
number 106:
1. Mannitol Salt Agar
2. Simmons Citrate
3. Urea
4. Nitrate

Once the Gram stain was performed and a Gram-negative rod was determined, a series of tests were done
to find out the unknown bacteria A. A mannitol test was
done to determine if the unknown bacteria fermented
mannitol. This was done by streaking a Mannitol Salt Agar
(MSA) plate with the isolated unknown A. Once incubated,
if the medium changed from a red color in the agar to
yellow, the bacteria fermented mannitol and produced the
acid, making it a positive reaction.

The next test that was
performed was a Simmons citrate test. The unknown was
streaked on a Simmons citrate slant test tube to determine
if the unknown bacteria use citrate as its main carbon
source. If carbon is used the slant turns from a green to a
blue color. Unknown bacteria A changed colors, resulting

in Unknown B – Staphylococcus epidermidis
in a positive reaction. The last test that was performed was
a urea test. This test was done to determine if the
unknown bacteria produce the enzyme urease to ferment
urea, changing the urea test tube to a bright pink color.

The first test done was determined to be a false negative,
as per the instructor. A second urea test was performed
and a true positive resulted. After these three tests were
done, it was determined that the unknown A bacteria was a
Gram-negative rod, Klebsillea pneumoniae.

The unknown bacteria B was given by the instructor
as complete isolation. This was done because of
inconclusive tests done from the thought to be isolated
agar plate for Gram-positive. Once the unknown bacteria
B was determined to be a Gram-positive cocci there were
three biochemical tests performed using the microbiology
lab manual as a reference. The first test done was a nitrate
test. This was done by inoculating a nitrate broth with the
unknown bacteria B. After incubation, the test came back
as a positive result. Unknown bacteria B reduced nitrate to
nitrite, changing the broth color from yellow to red

The second test performed was a mannitol test. This
was done to determine if the unknown bacteria B
fermented mannitol. Mannitol fermenters change the
medium of the agar plate to a yellow color. Non-fermenters
do not change the color, leaving the medium red in color.
After these tests, it was determined that unknown bacteria
B was Staphylococcus epidermidis.

A last test was
performed to verify that this was correct. A urea test was
done to determine if the unknown bacteria produced
urease, an enzyme that ferments urea and produces acids
that change the pH in the urea test tube, resulting in a pink
broth color. Staphylococcus epidermidis does produce
urease and caused the broth to turn pink in color, which
concluded that S. epidermidis was the unknown bacteria
B. This was verified by the instructor.

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Staphylococcus epidermidis is from the genus
Staphylococcus and the family Staphylococcaceae. It is
found in the human flora and mucous membranes, as well
as in animals. S. epidermidis is a Gram-positive bacteria. It
is not usually pathogenic but can cause problems in
individuals with weakened immune systems. S.
epidermidis is known to produce biofilms. These biofilms
are what help S. Epidermidis to spread easily, especially
through catheters and medical equipment, such as plastic
devices implanted in the human body.

Antibiotics that treat
S. epidermidis are very hard to find due to the production
of biofilms, which, in turn, lead to resistant strains. These
resistant strains are normally found in the intestines but
can live directly on the skin as well. These types of strains
can be treated with the drug Vancomycin.

2. McDonald, Virginia, Mary Thoele, Bill Salsgiver, and Susie Gero. Lab
Manual for General Microbiology, St. Louis Community College, STLCC (2011).



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