ICS - Microbiology Flashcards
What does gram staining do?
Differentiate between gram positive and gram negative bacteria
Describe the gram staining process
- Fixation to microscope slide (heat/methanol)
- Primary stain - crystal violet
- Mordant - iodine
- Decolourisation - ethanol or acetone
- Counterstain - safranin
What colours does gram staining turn gram positive and negative bacteria?
Gram positive - purple
Gram negative - pink
What bacteria does not stain with gram staining?
Acid fast bacilli, mainly mycobacteria (e.g. TB)
Why do mycobacteria not stain with gram staining?
Have a waxy, lipid cell wall that is impenetrable to the gram stain (also allows them to withstand phagolysosomal killing)
What stain is used for mycobacteria?
Ziehl-Neelsen stain
Describe the Ziehl-Neelsen stain process
Use heat to drive a dye (carbon fuchsin) inside the cell wall
What colours does Ziehl-Neelsen staining turn bacteria?
Acid fast bacteria (positive mycobacterium) - red
Non-acid fast bacteria - blue
Define pathogen
Organism that causes or is capable of causing disease
Define commensal
Organism which colonises the host but causes no disease in normal circumstances
Define opportunist pathogen
Microbe that only causes disease if host defences are compromised
Define virulence/pathogenicity
How easily a pathogen is spread
E.g. high virulence = easily spread e.g. measles
Define asymptomatic carriage
When a pathogen is carried harmlessly at a tissue site where it causes no disease
Describe cocci
Round-shaped bacteria. Can be individual, as pairs, (diplococci), in chains or clusters
Describe bacilli
Rod-shaped bacteria. Can be individual, chains, vibrio (curved), spirochaete (spiral)
Describe blood agar
- Contains sheep/horse blood
- Provides a good medium for growing many different types of bacteria (non selective)
Describe chocolate agar
- Blood agar heated to 80 degrees celsius for 5 minutes
- Releases nutrients into agar and makes it easier to grow certain organisms (fastidious bacteria)
What are 2 examples of bacterias that grow better on chocolate agar?
- Haemophilus influenza
- Neisseria meningitidis
Define fastidious bacteria
Bacteria that is hard to grow because they have complex/restricted nutritional and/or environmental requirements
Describe MacConkey agar
- Contains bile salts (inhibits gram positive), lactose and pH indicator
- Designed primarily to grow gram negative bacilli
- Enables differentiation of gram negative bacilli into lactose fermenters and non-lactose fermenters
What colour are gram negative bacilli on MacConkey agar?
Lactose fermenting - red
Non-lactose fermenting - white/colourless
Describe CLED agar
- Cysteine lactose electrolyte deficient
- Use to differentiate microorganisms in urine
- Enables differentiation of lactose fermenting and non lactose fermenting gram negative bacilli
What colour are gram negative bacilli on CLED agar?
Lactose fermenting - yellow
Non lactose fermenting - blue
Describe Sabouraud’s agar
- For fungi
- Selective media