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Discussion of S. Epidermidis and E. Aerogenes | Micro Unknown Lab Report

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The Unknown #101 was a mixture of two different bacterium.  The unknown bacterium that was identified in #101 was classified as a gram negative red rod.  After observing the bacteria and performing many biochemical tests the Unknown A bacterium was identified as Enterobacter aerogenes.  After a gram stain was performed on Unknown A the process of running biochemical tests began.  The first test that was performed was a Simmons Citrate Test, which came back positive.  This bacterium uses citrate as its sole carbon source.  This allowed the elimination of Proteus vulgaris and Escherichia coli.  Next, the Mannitol Test was performed.  This test came back positive for producing an acid, which allows the elimination of Pseudomonas aeruginosa.  This leaves only two gram-negative bacteria, Klebsiella pneumoniae and Enterobacter aerogenes.  To differentiate between the two, a urea test was done.  This test result came back negative for urease production.  By this result, it eliminated Klebsiella pneumonia, leaving Enterobacter aerogenes.  To make sure the test results led to the right bacterium a Methyl Red Test was performed.  The results of all the other biochemical tests matched the Methyl Red Test.  The unknown A #101 was indeed Enterobacter aerogenes.

Unknown B only took two tests to identify.  Since the gram stain revealed purple coccus bacteria Bacillus Cereus and Bacillus subtilis could be eliminated immediately due to them having the shape of rods.  By eliminating these two bacteria that leaves Staphylococcus eidermidis, Staphylococcus auerus, and Entercoccus faecalis.  After eliminating these bacteria biochemical tests were performed.  The first test that was performed was a Manntiol Test.  This tested to see if the unknown B bacterium can ferment mannitol.  This test result came back negative for mannitol fermentation.  Because this test was negative, it allowed the bacteria Staphylococcus aureus and Enterococcus faecalis to be eliminated.  By eliminating Enterococcus faecalis and Staphylococcus aureus it leaves Staphlyococcus epidermidis.  To prove that unknown B bacterium was Staphlyococcus epidermidis a Urea Test was performed.  This test came back with a positive result for the production of the enzyme urease which breaks down urea.  It was confirmation that unknown B of #101 was Staphylococcus epidermidis.  After consulting with Professor Snaric, it was confirmed that Unknown A gram negative bacterium was Enterobacter aerogenes and Unknown B gram positive bacterium was Staphylococcus epidermidis.

Staphlycoccus epidermidis was discovered in 1884 by Friedrich Julius Rosenbach by noticing a difference between Staphylococcus aureus and Staphylococcus epidermidis. (2) This bacteria is part of the human’s normal skin flora and is naturally found on the body. The appearance of Staphylococcus aureus is round grape-like structures. This bacteria can grow by aerobic respiration or by fermentation because it is a facultative aerobe. The most virulence of this bacteria is seen in the fact that it has the ability to form biofilms among plastic surfaces which most often causes problems in catheters or other plastic surfaces (2). The reason this causes such high virulence is because typically antibiotics are not as effective in treating bacteria with biofilms. Staphylococcus epidermidis is most often treated with Vancomycin. However, this bacteria is typically non-pathogenic (3). It is harmless to healthy people but it is an opportunistic bacteria and will cause problems in individuals with compromised immune systems. Some of these people may be newborns, the elderly, intravenous drug users, and catheter users (3). This disease may cause other diseases to occur such as endocarditis. The types of symptoms that may occur when an infection with this bacteria is present are fever, fatigue, pain or tenderness at the site of an implant. Rapid breathing, rapid heartbeat, and sweating are usually signs of any typical infection (3). The faster this bacterial infection is detected, the better, because it cuts down the time of the bacteria’s ability to form biofilms and be successfully treated.



  1. “Health Care Associated Infections.” Centers for Disease Control and Prevention. Centers for Disease Control and Prevention, 02 Apr. 2013. Web. 22 Apr. 2014.
  2. “Staphylococcus Epidermidis.” Microbiology Glossary. WikiSpaces, 27 Feb. 2011. Web. 24 Apr. 2014.


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