Microbiology Flashcards
On gram stain, what colour will appear if gram positive
Purple
Why do gram positive bacteria stain purple
Contains peptidoglycan that is stained by crystal violet
On gram stain, what colour will appear if gram negative
Pink
Why do gram negative bacteria stain pink
Has a lot less peptidoglycan that is not stained, by instead stained by safranin
Appearance of Staphylococcus
Gram-positive cocci in clusters
Appearance of Streptococcus
Gram-positive cocci in chains
What is another way of distinguishing between staph and strep (not clusters or chains)
Catalase test
Describe catalase test
Hydrogen peroxide + Staphylococci -> Gas bubbles
+ve for staph
-ve for not staph (or strep)
You know that bacteria is Staphylococcus as positive catalase test. How would you test for Staph.aureus
Coagulase test
+ve = staph aureus as it produces coagulase that converts fibrinogen to fibrin
-ve = coagulase -ve staph
Example of a coagulase negative staphylococcus
Staph epidermis
You know a bacteria is an Alpha Haemolytic Streptococcus. How would you further differentiate between species
Optochin test:
Sensitive means Strep. pneumoniae
Resistant means Viridans strep
How can you tell if a Strep is beta, alpha or gamma haemolytic
Blood agar
Beta haemolysis = complete haemolysis
Alpha = partial
Gamme - none
Appearance of Streptococcus pneumoniae on microscopy
Gram Positive Diplococci
How would you further differentiate between beta haemolytic streptococci
Lancefield test - groups bacteria based on carbohydrate composition of bacterial antigen on cell wall, normally for beta-haemolytic strep
Examples of beta haemolytic streptococci and their group
A: Streptococcus pyogenes
B: Streptococcus agalactiae
D: Enterococcus, Strep bovis (group D is not beta-haemolytic)
Examples of gram positive bacilli
Listeria monocytogenes
Propionibacterium acne – acne
Clostridium difficile – diarrhoea from antibiotic overuse (those starting with C)
What does MacConkey agar contain
Bile salts, lactose and neutral red (pH indicator)
What is the purpose of MacConkey agar
Identifies bacteria that can ferment lactose (normally gram -ve bacilli or enteric bugs)
How does MacConkey agar work
Ferment lactose -> produce acid -> pH drop <6.8= pink, or if pH>6.8, remains normal colour
Therefore:
Lactose fermenting gram -ve bacilli = pink
Non-lactose fermenting gram -ve bacilli = yellow
Examples of bacteria (bacilli) that appear pink on MacConkey agar
(Enterobacteriaceae (coliforms)):
Escherichia coli
Klebisella pneumoniae
Examples of gram -ve non-lactose fermenting bacilli and what colour they appear on MacConkey agar
Appear YELLOW
salmonella, shigella, pseudomonas aeruoginosa, proteus mirabilis
Further differentiation between Gram negative non-lactose fermenting bacteria
Oxidase test:
Black/purple = +ve = pseudomonas (additional anti-pseudomonal snesitivity tests required to identify this) or Neisseria(note it is cocci not bacilli)
Colourless = -ve = Shigella, salmonella, proteus
How could you specifically identify Proteus mirabilis (Gram-ve bacilli that is oxidase negative and non-fermenting)
Urease test
+ve
Examples of gram -ve cocci
Neisseria meningitidis, gonorrhoea
Moraxella catarrhalis
