Microbiology

Question 1

Differentiate between Gram Positive and Gram Negative Bacteria?

Answer 1

Gram positive:

• Stains purple
• Double membrane

Gram negative:

• Stains pink
• Single membrane

Question 2

Explain the process of Gram Staining?

Answer 2

• Apply a primary stain such as crystal violet (purple) to heat fixed bacteria
• Add iodine which binds to crystal violet and helps fix it to the cell wall
• Decolourise with ethanol or acetone
• Counterstain with safranin (pink)

Question 3

What does the Ziehl-Neelsen stain differentiate?

Answer 3

• TB are acid-fast bacilli bacilli that DO NOT take up
  the gram stain
• TB/ Mycobacteria is acid fast bacilli which will stain RED with Ziehl-Neelsen

Question 4

What colour will non-acid fast bacilli stain with Ziehl-Neelsen?

Answer 4

Blue - e.g. E.coli

Question 5

What is the Catalase test?

Answer 5

- Essentially add H2O2 to bacteria and see for
BUBBLING REACTION (positive reaction)

• Used to differentiate between staphylococci
  and streptococci

Question 6

What is the result of Staphylococci

and Streptococci with the Catalase test?

Answer 6

• Staphylococci are catalase POSITIVE
• Streptococci are catalase NEGATIVE

• Many GRAM NEGATIVE bacteria e.g. E.coli and fungi (aspergillus
spp.) are catalase POSITIVE

Question 7

What test differentiates staphylococcus. aureus from other staphylococci?

Answer 7

COAGULASE TEST

• Coagulase is an enzyme produced by Staphylococcus. aureus that
  converts (soluble) fibrinogen in plasma to (insoluble) fibrin
• Other staphylococci DO NOT PRODUCE
  COAGULASE e.g. staphylococcus. epidermis
• Thus this test distinguishes staphylococcus.
  aureus (coagulase positive) from other
  staphylococci (coagulase negative)