Classifying Microorganisms by Experiment

Authors Avatar by zeynep_merve10hotmailcouk (student)

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. On the other hand, due to the fact that gram negative bacteria have a strong outer membrane the crystal violet is not absorbed. Usually a counter stain - generally Safranin- is added to the slides after the crystal violet, all gram negative bacteria have a red or pink colour due to the Safranin.

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 Also, according to the gram staining results along with the shape combination, the bacteria species could be identified. In this case, the observation we made was purple and long rod in appearance. Purple staining indicates gram positive from what we have learnt and only one species of bacteria matched the combination of long rod and gram positive, therefore the inference we made was “Bacillus Subtilis”. As for the second sample, we observed purple and cocci bacteria in appearance. The inference we made for this was the “Staphylococcus Albus” because there was only one combination with gram positive and cocci shaped ...

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