General bacteriology

Question 1

Gram staining steps

Answer 1

1. Crystal Violet
2. Lugol solution
3. 96% Ethanol
4. Water
5. Fuchsin or Safranin
6. Water

Question 2

What does gram POS bacteria look like and why

Answer 2

Stained strongly purple by crystal violet that is trapped as a complex by iodide inside the thick peptidoglycan cell wall

Question 3

What does gram NEG bacteria look like and why

Answer 3

Not stained by any particular colour because the alcohol takes away the lipopolysaccharide membrane of the gram negative bacteria and this also takes away the crystal violet-iodide complexes inside. The counterstain (fuchsin or safranin) leaves the bacteria a pink colour.

Question 4

Alpha, beta and gamma haemolysis

Answer 4

Alpha: incomplete/partial
Beta: complete
Gamma: none

Question 5

What is the difference between nutrient agar and nutrient broth cultures and what categories do they both fall into?

Answer 5

Nutrient agar: solid gel-like agar
Nutrient broth: liquid nutrient culture
Both: simple culture

Question 6

Mannitol-salt agar

Answer 6

Selective medium
High salt level (10%) and mannitol fermentation
Phenol red to detect pH change (S.aureus)
Detects coagulase positive Staphylococci

Question 7

Baird-Parker agar

Answer 7

Selective medium
Looks yellow with black colonies
Na-tellurite, glycine, pyruvic acid, egg
Detects coagulase positive Staphylococci1

Question 8

CAMP test

Answer 8

Test to detect streptococci agalactiae (in presence of staphylococci aureus)
S. agalactiae will produce some hemolysis but in the presence of beta-hemolysin (prod by S. aur), there will be a large amount of hemolysis!

Beta haemolysis! Also used for R.equi and L. monocytogenes

Question 9

Soil bacteria are generally not …?

Answer 9

Fastidious!!!