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Aseptic Technique


An answer should be submitted for each of the following short answer questions. These questions are based on the essential components of the three practical classes and signify aspects that should have been included in the original practical portfolio submission.

Aseptic technique (5% of grade)

  1. With reference to laboratory science, define what is meant by the term “aseptic technique”.
  2. Describe what precautions should be adopted to ensure aseptic technique when manipulating microorganisms in a laboratory. Streak plate method (5% of grade)
  3. What is the purpose of streaking bacteria on growth media?
  4. Describe how the streak plate method is carried out, indicating how & when the loop is sterilized or a fresh sterile loop used. You may include an original diagram (prepared by you) to support your text.

Escherichia coli (10% of grade)

The species E. coli contains diverse strains with different properties.

  1. What is the natural habitat where E. coli is typically found?
  2. What range of infections can be caused by strains of E. coli?
  3. Are all strains of E. coli equally able to cause disease? Explain the reason for this.
  4. With your answer to question 3 (above) in mind, what properties of the E. coli strain used in the first year practical make it suitable for untrained microbiologists to study?
  5. Why were plates inoculated with presumed E. coli incubated at 37°C? Selective and differential growth media (5% of grade) MacConkey agar is an example of both a selective and differential growth media.
  6. What is the definition of a selective growth media and a differential growth media?
  7. What is the expected appearance of Escherichia coli growing on MacConkey agar?
  8. What property does the colour of E. coli growing on MacConkey agar indicate?

Contamination (5% of grade)

During the practical a number of students reported colonies of different size and colour on the nutrient agar plates, indicating possible contamination.

  1. Why may you expect more contaminants to grow on nutrient agar plates than on MacConkey agar?
  2. What are the possible sources of contamination and how could this be prevented in future experiments? Note: there is certainly more than one possible source.

Gram stain (5% of grade)

The Gram stain is a differential stain used in conjunction with a light microscope to determine properties of bacteria.

  1. On the basis of the Gram stain, bacteria can be classified as gram positive or gram negative. What is the difference between these two groups of bacteria?
  2. E. coli is a gram negative organism. What is the expected appearance of this species under a light microscope following the Gram stain? Include both colour and cell shape in your answer.
  3. What is the expected appearance under a light microscope of Bacillus subtilis following Gram stain? Serial dilution & spread plate (5% of grade)
  4. What are the essential differences between the spread plate method and the streak plate method described above? Include in your answer a comparison of the aims and the specific steps in each procedure.
  5. We performed a procedure called a viable plate count. In this context what is the meaning of “viable”?
  6. Would an estimate of cell numbers established by the viable plate count give similar results to an estimate based on total cell counts using a microscope?

Explain the reason for your answer.

  1. Explain the meaning of “probiotic” and how this relates to Yakult.
  2. What species of microorganism is found in Yakult and what quantity is found in each 65 ml bottle?

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