A2C1 - Chapter 4 - Microbiology Flashcards
(22 cards)
What are bacterial cells made up of?
3D Mesh of peptidoglycan (murein), a polymer of amino acids and sugars.
What is gram staining?
A technique used to differentiate between gram negative and gram positive bacteria.
Outline the process of gram staining.
- Stain with crystal violet. Remove and rinse with water.
- Add iodine solution. Rinse after 1 min.
- Alternate washes of alcohol and water every 30s.
- Counterstain with red safranin for 1 min.
- Dry and examine under microscope.
Define gram positive bacteria.
Bacteria with thick peptidoglycan wall and purple appearance following gram staining.
Why do gram positive bacteria appear purple after gram staining?
The thick peptidoglycan layer retains the crystal violet when rinsed with alcohol.
Define gram negative bacteria.
Bacteria with thin peptidoglycan wall and an outer lipopolysaccharride membrane and a red appearance following gram staining.
Why do gram negative bacteria appear red after gram staining?
On treatment with alcohol, the lipopolysaccharide layer is lost and the crystal violet washes away. The counterstain safranin stains the thin peptidoglycan layer red.
What is an obligate anaerobe?
An organism requiring oxygen for metabolism.
What is an obligate anaerobe?
An organism only able to survive in environments lacking oxygen.
Define facultative anaerobe.
An organism that normally respires aerobically.
Capable of switching to anaerobically in the absense of oxygen.
What are aseptic techniques?
A range of techniques used to culture microorganisms under sterile conditions in order to minimise contamination.
List the basic aseptic techniques.
- Wipe surfaces with antibacterial cleanser.
- Setup bunsen burner nearby, the convection current stops airborne bacteria from entering the culture.
- Flame inoculating loop.
- Minimise time the vessels containing bacteria are open.
- Sterilise equipment using autoclave.
- Wear protective clothing
- When inoculating petri dish, ensure lid is held at angle to prevent microbes in air contaminating the agar.
Outline how to culture microorganisms.
- Sterilise inoculating loop in bunsen flame
- Lift dish lid and keep at angle.
- Transfer bacteria to agar plate using sterile inoculating loop or pipette.
- Tape on lid on two ends, invert dish and incubate.
- In school lab, ensure dish is not airtight and do not incubate above 25C to avoid growth of pathogens.
Explain the difference between a spread plate and a streak plate.
Spread plate = Microorganisms distributed evenly with a sterile spreader.
Streak plate = aims to obtain single colonies by rotating the plate to build layers of the culture on at least 3 different streaks.
What is nutrient media?
A solid or liquid-rich medium used in the cultivation of microorganisms.
Describe the composition of nutrient media.
Contains carbon source, nitrogen source, water and growth factors (eg: salts and vitamins)
Describe the conditions used when culturing microorganisms.
- Optimum temperature
- Constant pH
- Nutrient supply
- Aerobic conditions
What is the difference between total cell count and viable cell counts?
In a given area or volume, total cell count is the total number of cells (both living and dead) whereas viable cell count is the total number of living cells.
Describe how a viable count is conducted.
Add known volume of organisms to agar plate.
Incubate plate
Count number of colonies.
What is assumed when conducting viable cell count.
It is assumed one cell gives rise to a single colony.
What is the problem with the ‘one cell one colony’ assumption?
It does not count for the clumping of cells in the original inoculum. This may result in a lower estimate of the number of cells.
What is a serial dilution?
A sequence of dilutions, in which the dilution factor is constant, used to dilute a stock solution.
