Sample interview questions: Share your knowledge of different microbial identification techniques, such as biochemical tests or DNA-based methods.
Sample answer:
Biochemical Tests:
- Catalase Test:
- Principle: Determines the ability of a microorganism to produce the enzyme catalase, which breaks down hydrogen peroxide into water and oxygen.
- Procedure: A drop of hydrogen peroxide is added to a bacterial culture. If catalase is present, bubbles of oxygen are produced, indicating a positive result.
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Significance: Useful for distinguishing catalase-positive bacteria (e.g., Staphylococcus aureus) from catalase-negative bacteria (e.g., Streptococcus pneumoniae).
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Oxidase Test:
- Principle: Detects the presence of cytochrome oxidase, an enzyme involved in the electron transport chain.
- Procedure: A reagent containing a redox dye (e.g., tetramethyl-p-phenylenediamine) is applied to a bacterial culture. If cytochrome oxidase is present, the dye changes color, indicating a positive result.
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Significance: Useful for differentiating oxidase-positive bacteria (e.g., Pseudomonas aeruginosa) from oxidase-negative bacteria (e.g., Enterococcus faecalis).
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Coagulase Test:
- Principle: Determines the ability of a microorganism to produce coagulase, an enzyme that clots blood plasma.
- Procedure: A drop of blood plasma is added to a bacterial culture. If coagulase is present, the plasma clots, indicating a positive result.
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Significance: Useful for distinguishing coagulase-positive Staphylococcus aureus from coagulase-negative staphylococci.
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Gram Staining:
- Principle: Differentiates bacteria based on their cell wall structure and staining properties.
- Procedure: A smear of bacteria is stained with crystal violet, iodine, alcohol, and safranin. Gram-positive bacteria retain the crystal violet-iodine complex and appear purple, while Gram-negative bacteria lose the complex and are counterstained pink with safranin.
- Significance: Aids in the identification of a wide range of bacteria based on their Gram reaction.
DNA-Based Methods:
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