Throat Culture Unknown
For this unknown identification, I was directed to select a culture of a microbe at random. Then,
inoculate or swab samples of the culture onto Tryptic Soy agar (TSA), Blood Agar, Mannitol Salt Agar
(MSA), DNase Agar, Natural Agar and into Rabbit Plasma. These specimen were then placed in the
incubator. The results for these tests would prove the bacteria to be one of either Staphylococcus
Aureus or Staphylococcus Epidermidis.
The first test I used was the gram reaction. I used an inoculating loop to obtain a sample of the
microbe from its growth on the TSA. Next, I prepared a gram stain and viewed it under the oil immersion
lens of the microscope. Both Staphylococcus aureus and Staphylococcus epidermidis are gram positive,
coccus cells, arranged mostly in staphylococci and diplococci. The only difference in observation
between the two is that the cells of Staphylococcus Epidermidis are slightly larger. Under the slide I
created, I observed dark purple, circular cells mainly arranged in grape like clusters and some in pairs.
Due to the color, shape and especially size of the cells, I think I may have Staphyloccous epidermidis.
However, since the two are both gram positive staphylococci, this test is not reliable for distinction.
Secondly, I observed the Blood Agar, which determines hemolysis. In the case of Staphylococcus
aureus which undergoes beta hemolysis, the blood agar should demonstrate a zone of inhibition or
clearing around the growth of the bacteria. This is created by the breakdown of RBC’s, leakage of
hemoglobin and clearing in agar, due to the cessation of the red pigment. On the other hand,
Staphylococcus epidermidis test negative for hemolysis. Since there will be no hemolytic activity, the
RBC’s should not be effected and there should be growth on the agar. On my Blood Agar plate, I observe
cloudy, white colonies growing on the agar, where it was swabbed. This indicates a negative result for
hemolysis indicative of Staphylococcus epidermidis.
Next is the Mannitol Salt Agar, which determines fermentation. The Mannitol Salt Agar before
the inoculation was a clear, pale pink color. Staphylococcus aureus ferments mannitol into acids, which
decreases pH and turns the phenol red indicator in the agar yellow. Staphylococcus epidermidis does
not ferment, pH increases and the phenol red indicator will brighten in color. The observation of my
MSA plate is a bright pink in color with off white colonies growing on the agar. This tests negative for
fermentation, which is indicates Staphylococcus epidermidis. The S. epidermidis uses protein respiration
to break peptide bonds, remove amino groups and produce ammonia and keto acid.
The DNase agar determines the presence of DNase activity and requires the addition of HCl in
order to obtain a reaction and determine results. In a culture of Staphylococcus aureus, the agar should
remain clear, testing positive for DNase activity. A culture of Staphylococcus epidermidis should test
negative and the area surrounding the bacterial growth should be cloudy. My DNase plate shows
cloudiness around the growth of the bacteria. This indicates a negative test for DNase activity.
Staphylococcus epidermidis lacks the enzyme to break down DNA. Therefore, the DNA clumps up and
coagulates around the growth. I suspect that my unknown is Staphylococcus epidermidis.
Lastly, the coagulase test is the definitive for determining the unknown. The test tube of Rabbit
Plasma is a clear, colorless liquid. A positive coagulase test would be indicative of Staphylococcus
aureus. This would demonstrate a solid gel like form of the plasma, make by the transformation of
fibrinogen into fibrin. A negative test would be indicative of Staphylococcus epidermidis and the Rabbit
Plasma would remain liquid form. The Rabbit Plasma for...
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