Microbiology

Question 1

Describe the results of the gram stain and what they mean

Answer 1

Gram stain:

• Purple = gram positive
• Pink = gram negative

Question 2

You are looking down a microscope after performing a gram stain… which type of microorganism could be responsible for the following?

a) Gram positive cocci in clusters
b) Gram positive cocci in chains

Answer 2

a) Staph

b) Strep

Question 3

Which test is used to determine whether a bacteria is staph or non-staph? Explain the results

Answer 3

Catalase test (hydrogen peroxide + bacteria):

• +ve result (i.e. forms gas bubbles) = staph
• -ve result (i.e. no gas bubbles) = non-staph

Question 4

Which test is used to determine whether bacteria is staph AUREUS? Explain the results

Answer 4

Coagulase test:

• +ve result (forms clumps as it converts fibrinogen to fibrin) = staph aureus
• -ve result = coagulase negative staph, e.g. staph epidermis

Question 5

You have just performed a gram satin and identified gram positive cocci in chains (strep)… Which test is used to determine what kind of strep this is?

Answer 5

Haemolysis on blood agar:

• Alpha (partial) haemolysis
• Beta (full) haemolysis

Question 6

What test is performed in alpha haemolytic strep?

Answer 6

Optochin test:

• +ve result (i.e. clear demarcated ring around optochin disc) = strep pneumoniae
• -ve result = strep viridans

Question 7

How does strep pneumoniae appear on microscopy?

Answer 7

Gram positive diplococci

Question 8

What test is performed on beta haemolytic strep?

Answer 8

Lancefield grouping
A = strep pyogenes
B = strep agalactiae

Question 9

What test is performed on gram negative bacilli?

Answer 9

MacConkey agar (fermentation of lactose)

• +ve result (ferments lactose) turns PINK, e.g. E. coli, Klebsiella
• -ve result (does not ferment lactose) stays YELLOW, e.g. salmonella, shigella

Question 10

a) What is the causative microorganism for TB?
b) What kind of culture medium is used to identify causative microorganism for TB?
c) What kind of stain is used to identify the causative microogranism for TB?

Answer 10

a) Mycobacterium tuberculosis
b) Lowenstein-Jensen medium
c) Ziehl-Neelsen stain