Unknown # __5__
Observation Result Explanation of Observation/Result Organism or Organisms Indicated
Gram reaction &
Shape Purple, spheres in clumps and clusters
Glucose Durham Tube Fermentation Broth is Yellow and there is no air bubble in the Durham tube
Lactose Durham Tube Fermentation Broth is Yellow and there is no air bubble in the Durham tube
Mannitol Durham Tube Fermentation Broth is Yellow and there is no air bubble in the Durham tube
After addition of Kovac’s reagent a no red ring is visible
Methyl Red After addition of Methyl red, broth is red
Voges -Proskauer After addition of Barritt’s reagent, broth is yellow
Agar is green
No black precipitate in agar
Agar is has a sharp edge around the stab
Catalase No bubbles seen after addition of hydrogen peroxide
Lab Report for the Identification of Bacteria Associated with the Gastrointestinal Tract
Directions: Your gastrointestinal tract unknown is one of the following bacteria: Escherichia coli, Enterobacter aerogenes, Enterococcus faecalis, Salmonella typhimurium, or Staphylococcus aureus. Your lab report will consist of the following:
1- A Table (30 points)
a) Record your observations in the table for each test performed to identify your unknown. In the column labeled “Observations” write what you saw (for example, a color change, the presence or absence of growth, a particular growth pattern).
b) Record your results in the table for each test performed to identify your unknown. In the column labeled “Result” write whether your unknown is either positive or negative for a particular enzyme or metabolic activity.
c) In the column labeled “Explanation of Observation/Result”, write a brief explanation of the reaction observed. (meaning what reaction took place resulting in an observable change)
d) In the column labeled “Organism(s) Indicated”, write the name(s) of all the organism(s) indicated by the result you wrote on your table. You need to include all of the organisms that give the result you listed in (b).
e) For your report, you will submit a typed version of your table.
2- A Written Report
You will attach a discussion to your table. Your report is the most important part of the unknown identification process. In your discussion, focus on the results you obtained and how you identified your unknown based on a process of elimination. The discussion portion of your report is to be typed, double spaced. You will staple the table to the written report (table first followed by the report). Please do not include information about the procedures you conducted. The steps for writing your discussion are:
a) List all possible GI bacteria used in this laboratory exercise. (5 points)
b) Discuss your Gram staining results; describe if your Gram staining results allowed you to eliminate any of the possible bacteria as your unknown. (10 points)
Note: once you have eliminated bacteria based on Gram staining, you do not need to continue to eliminate them.
c) Discuss the results for your Durham tube fermentation tests. Describe how you used the three fermentation tubes to rule out certain bacteria. You need to describe phenol red glucose, lactose and mannitol results as one series of tests; do not describe each sugar fermentation test separately. Be sure to clearly list which bacteria are included and/or excluded based on the series of Durham tube fermentation results and give clear reasons why. (20 points)
Note: once you have eliminated bacteria based on Durham tube fermentation, you do not need to continue to eliminate them.
d) Discuss several tests that clearly allowed you to identify your unknown (these are known as definitive tests). For this part of your report, you cannot include Gram staining and phenol red glucose, lactose and mannitol since you have already discussed them in parts b and c. Give reasons why certain organisms are excluded, and how you reached your specific decision on the identity of your unknown. You should describe how you excluded bacteria that remained as your possible unknown after Gram staining and Durham tube fermentation. Do not include a description of all the tests you performed. Include only the most relevant tests and explain how they allowed you to eliminate the remaining bacteria that were not previously excluded and determine the identity your unknown. (20 points)
e) State your unknown number and the identity of your unknown. (10 points)
f) Conclude your report with a summary statement of how you were able to identify your unknown. (5 points)