Identifying Gram-Positive Organisms

Question 1

If a gram-positive organism is catalase positive, what tests are conducted next?

Answer 1

Growth Characteristics and Novobiocin Sensitivity (both on an MH plate)

DNAse Agar

MSA

PYR

Question 1

How does a catalase test work?

Answer 1

A drop of hydrogen peroxide is added to the organism on a slide. If the organism produces the enzyme Catalase, white bubbles will appear as it converts the hydrogen peroxide into water and O2

Question 2

If a gram-positive organism is catalase negative, what tests are conducted next?

Answer 2

Growth Characteristics, SXT sensitivity, and hemolytic pattern (all on a blood plate)

BEA slant

PYR

Question 3

DNAse Agar

Answer 3

Differentiates organisms based on their ability to secrete the exoenzyme DNAse, which hydrolyzes DNA into nucleotides

Question 4

What does a positive DNAse agar look like?

Answer 4

Clear zone in agar around bacterial growth

Question 5

What does a positive DNAse agar with methyl green dye look like?

Answer 5

the green/blue coloring of the agar will have a clear zone around bacterial growth

Question 6

MSA Agar

Answer 6

Mannitol Salt Agar

7.5% NaCl: selects for halotolerants

Mannitol

pH indicator: Phenol red

Question 7

What color will an MSA Agar become if the organism can ferment mannitol?

Answer 7

Yellow: acidic products

Question 8

PYR test

Answer 8

Tests if an organism can produce the enzyme PYRW

Question 9

What does a positive PYR test look like?

Answer 9

Dark red

Question 10

BEA Slant Test

Answer 10

Tests if an organism can hydrolyze esculin (carb)

Question 11

What does a positive BEA slant look like?

Answer 11

More than 50% of the agar is black/grey