Microbiology

Question 1

Gram stain procedure

Answer 1

Stain with crystal violet for 1 minute
Rinse and stain with iodine for 1 minute
Rinse and decolourise with alcohol for 2-10 seconds
Rinse and stain with safranin or carbol fuschin for 1 minute

Question 2

Gram stain morphology

Answer 2

Pink - gram negative
Purple - gram positive
Circles - coccus
Sausages - bacillus
Spirals - spirochetes

Question 3

Catalase reaction

Answer 3

Tests for catalase enzyme which converts hydrogen peroxide to water and gaseous oxygen.
The enzyme is present in most cytochrome containing aerobic and facultatively anaerobic organisms.

Question 4

Catalase procedure

Answer 4

Add hydrogen peroxide to a sample of bacteria on a glass slide.
Positive - bubbles produced
Negative - no bubbles produced

Question 5

Indole reaction

Answer 5

Tests for bacteria that are able to produce indole from amino acid tryptophan.

Question 6

Indole procedure

Answer 6

Place indole reagent on filter paper and transfer a colony to it.
Positive - brown/red colour
Negative - no colour change

Question 7

Oxidase reaction

Answer 7

Tests for bacteria that have the cytochrome oxidase enzyme.

Question 8

Oxidase procedure

Answer 8

Touch colony to be tested with oxidase detection strip and wait up to 10 seconds.
Positive - blue/purple colour
Negative - no colour change

Question 9

Coagulase (staphylase) reaction

Answer 9

Tests for the presence of free or bound coagulase or clumping factor.

Question 10

Coagulase (staphylase) procedure

Answer 10

Add bacterial sample to reaction card, vigorously mix reagents and add control reagent and test reagent to separate samples.
Positive - clumping mixture
Negative - homogenous mixture

Question 11

Dichotomous key

Answer 11

A guide to identifying bacterial samples based on gram staining and biochemical tests