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Classifying Microorganisms by Experiment

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http://www.diffen.com/css/img/magnify-clip.png At the end of the Gram Staining process, gram positive and gram negative bacteria vary from each other according to the colour of their stain. Both slides, one containing gram positive bacteria and the other containing gram negative bacteria, are washed with ethanol after being washed with the crystal violet dye. It is observed that gram negative bacteria slides are stained either red or pink, whereas the gram positive bacteria slides protect the purple/blue stain which they were exposed to when washed with crystal violet. The reason behind the variation in stain colour is due to the difference in the gram positive and gram negative cell wall structure. Gram negative bacteria possess an outer membrane unlike the gram positive bacteria which lack this layer in their cell wall. Instead, gram positive bacteria?s cell wall contains a much thicker peptidoglycan layer just after the exterior of the cell, as seen in the drawings of a gram negative and gram positive cell wall structure; the peptidoglycan layer retains the crystal violet dye, allowing the gram positive bacteria to appear purple/blue underneath the microscope. ...read more.


The reason behind this is because when the slides with bacteria samples were placed under the microscope, they were easily identified. The results for sample A was that the shape of the bacteria were long rod and the colour was purple (we know that the colour purple is stained on gram positive bacteria); there was only one specie that matched our observations, which was the Bacillus subtilis. Similarly, sample B was observed as purple and cocci shaped; this was easily identified because again there was only one expected result that matched the observations and that was Staphylococcus albus. The MacConkey plate test was the second confident test I carried out. In the results for sample A, I observed no growth, however there are two expected samples that match this, they are: Bacillus subtilis and Lactobacillus bulgaricus. From these results further identification was necessary for sample A, but considering the gram staining test, Bacillus subtilis was the specie of bacteria I elected from the two expected results. As for sample B, the only result was poor growth in the MacConkey plates, therefore the inference made straight away was Staphylococcus albus. ...read more.


There were contradictions we came across, such as the antibiotic susceptibility test as mentioned above. The conclusion that I can make is, I would recommend the antibiotics Chloramphenicol, Erythromycin, Oxacillin, Novobiocin, Streptomycin and Tetracycline to the patients of sample A. Furthermore, to confirm the identification of Bacillus subtilis, a ?starch hydrolysis? test could be performed. Bacillus subtilis will have a zone of clearing in the plate where starch is spread out; the zone will indicate that the starch is broken into maltose and glucose. As for Staphylococcus albus, there is a confirmation test that could be performed. However, I have used its substitute ?Staphylococcus aureus? to determine the results of the further tests. The coagulase test will determine Staphylococcus aureus from Staphylococcus epidermidis. When clotting is observed at the bottom of the broth in one of the test tubes- one test tube holds Staphylococcus aureus and the other holds Staphylococcus epidermidis- that test tube is identified as holding Staphylococcus aureus. The reason behind this is the Coagulase enzyme clots the plasma used as the reagent in order to prevent an attack by the defences of the host. ...read more.

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