Lectrue 9 (Linda Stewart) - Techniques In Microbiology Flashcards
Give an example of a microorganism in each of the different bio safety levels
Level 1 - Bacillus subtilis
Level 2 - Steptococcus pyogenes, Escherichia cold
Level 3 - Myobacterium tuberculosis, HIV
Level 4 - Ebola virus, drug-resistant Myobacterium tuberculosis
Describe the process of aseptic technique
- Sterilise the inoculating loop by flaming it.
- Remove the tube cap.
- Flame the tube neck to sterilise the surface.
- Only the sterilised portion of the inoculating loop enters the tube.
- Reflame the tube neck.
- Recap the tube and resterilise the loop.
How is a bacterial smear prepared
- A thin film of bacteria is spread on a microscope slide.
- The film of bacteria is air dried.
- The air dried film is fixed to prevent cells washing off during staining and to denature bacterial enzymes that could digest the cell wall.
- Stains can then be applied.
What are the limitations of microscopic counts of bacteria
- Cannot distinguish between living and dead cells.
- Small cells can be overlooked.
- Cell suspensions of low density are hard to count.
- Motile cells need to be immobilised.
- Debris in sample can be mistaken for cells.
What are the two methods for plate counts
- Spread-plate method
- Pour-plate method
Describe the spread-plate method
- The sample is pipettes onto the surface of an agar plate.
- The sample is spread evenly over the surface of the agar using a sterile glass spreader.
What is the great plate anomaly
Direct microscopic counts of natural samples reveal far more organisms than those recoverable on plates.
Why does the great plate anomaly exist
- Microscopic methods count dead cells whereas viable methods do not.
- Different organisms may have vastly different requirements for growth.
What are the pros and cons of turbidity measurements of microbial growth
- Quick and easy to perform.
- Typically do not require destruction or significant disturbance of the sample.
- Sometimes problematic (clumps or biofilms in liquid medium).
