Microbiology Practical

Question 1

E.coli?

Answer 1

Gram-negative (pink) rod-shaped bacilli.
Common cause of urinary tract infections (UTIs).
Oxidase negative.

Question 2

S.aureus?

Answer 2

Gram-positive (purple) grape-like cocci clusters.
Common cause of skin and soft tissue infections.
Catalase positive.
Coagulase positive.

Question 3

S.epidermis?

Answer 3

Gram-positive (purple) cocci.
Part of normal skin flora but can stick to plastic/metal devices so can cause infection in surgical patients.
Catalase positive.
Coagulase negative.

Question 4

Strept. pyogenes?

Answer 4

Gram-positive (purple) cocci that form chains.
Large zone of beta (complete) haemolysis.
Cause of pharyngitis, cellulitis and necrotising fasciitis.
Catalase negative.

Question 5

Pseudomonas aeruginosa?

Answer 5

Gram-negative (pink) rod-shaped bacilli.
Not part of the normal flora but can colonise patients who have antibiotic therapy or are immunocompromised.
Oxidase positive.

Question 6

Test used to distinguish between Staph. and Strept. gram-positive bacteria?

Answer 6

Catalase Test
Positive Result = Staph. species
Negative Result = Strept. species

Question 7

Test used to distinguish between Staph. aureus and other forms of Staph. (eg. S. epidermis)?

Answer 7

Coagulase Test
Positive Result = S. aureus
Negative Result = other Staph. eg. S. epidermis

Question 8

Test used to distinguish between gram-negative bacteria?

Answer 8

Oxidase Test
Positive Result = Pseudomonas eg. P. aeruginosa
Negative Result = Escherichia eg. E. coli