Agar Plates (Blood, MacConkey, XLD) Flashcards
What type of media is blood agar an example of?
Enriched media (blood is added after the agar is autoclaved to avoid cell lysis).
How does blood agar differentiate bacteria?
Based on whether the bacteria can lyse the blood.
How does chocolate agar differentiate to blood agar?
Chocolate agar also contains blood, but is autoclaved with the blood. This lyses the cells and releases the haemoglobin and other useful nutrients from the cells
What are the three types of haemolysis types with blood agar?
Alpha, beta and gamma
What does alpha haemolysis look like?
Partial destruction of RBCs -> alpha haemolytic bacteria -> alpha haemolysin produced -> GREEN COLOUR
What does beta haemolysis look like?
Complete destruction of RBCs -> beta haemolytic bacteria -> beta haemolysin produced -> CLEAR ZONE AROUND BACTERIAL GROWTH
What does gamma haemolysis look like?
No destruction of RBCs -> gamma haemolytic bacteria -> no haemolysin produced -> NO CLEAR ZONE
What type of media is MacConkey Agar?
Selective (isolate a particular bacteria) and indicator (colonies of certain organisms to take on a distinctive appearance).
How does MacConkey agar differentiate bacteria?
It is designed to selectively isolate Gram-negative and enteric (normally found in the intestinal tract) bacteria and differentiate them based on lactose fermentation.
What does bacteria that can ferment lactose look like on MacConkey agar?
Visual indicator is the pH indicator -> neutral red dye which stains microbes fermenting lactose red/pink
What does bacteria that cannot ferment lactose look like on MacConkey agar?
Non-Lactose fermenting bacteria will use peptone instead forming ammonia, which raises the pH of the agar, and leads to the formation of white/colourless colonies.
What type of medium is XLD agar?
Selective growth medium
What bacteria does XLD isolate?
Salmonella and Shigella
What is the appearance of salmonella on XLD agar?
- Ferment Xylose -> produce acid -> decrease pH -> appears yellow
- Decarboxylate lysine -> increases pH again -> red colonies
- Metabolise thiosulfate -> produce hydrogen sulfide -> of colonies with black centers
Final appearance =red colonies, some with black centers
What is the appearance of Shigella on XLD agar?
Cannot ferment xylose -> red colonies
How do different pHs appear on XLD agar?
Alkaline = pink/red
Neutral = yellow
Acid = yellow
