Microbiology- Identifying Unknowns: Micro. Lueteus

1178 Words5 Pages
Will Numair
Bacterial Unknown Project:
Unknown #18
The purpose of this lab was to identify the species of an organism from a given TSB broth culture. Each student was given an unknown sample, and each sample had an assigned ‘sample number’ for post-lab verification by the professor. My sample number was #18. A list of possible organisms was given before any tests were run and a chart with the accepted test results for each organism was given as well. To correctly identify the organism, multiple biochemical tests were performed until enough test results were collected to single out the correct organism from the given list. Also given at the beginning of the lab was each samples results for the VP test (negative for
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Because of this, 2 more tests were run to confirm that the organism was gram positive. 4) A MSA plate test was run on a sample of the organism and the results were consistent with the given results for M. Luteus. The results showed a fair amount of growth on the plate, and the color of the agar around the growth remained red. The MSA test is selective in that the salt will inhibit most gram negative organisms and select for gram positives. If there is growth and the color of the agar turns yellow around the growth, this would mean that mannitol was fermented by the organism and the acid waste released by the bacterium lowered the pH around the growth. Since there was growth and no color change, the sample is said to be gram positive and unable to ferment mannitol (negative for differential). This result was also consistent with the given test results for M. Luteus. 5) One final test was run on the sample and that was the EMB test. Results of this test showed little to no growth on the EMB plate while the color of the colony (although barely visible) remained the same color as the agar. This test is selective for gram negative bacteria but inhibits the growth of gram positives. It also is differential in that it can test whether a bacteria can ferment lactose. Due to the fact that there was virtually no growth (a very faint haze) and no color change of the agar, the sample can be said to be gram

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