Media/Agar Exam 1

Question 1

Describe the elements of the Gram Stain

Answer 1

Crystal Violet- Initial Stain-stains all organisms present.
Gram’s Iodine-Mordant- fixes initial stain in all gram positive organisms.
Decolorizer- ETOH, acetone mixer- washes away CV
Safranin-secondary (counter) stain-takes up by Gram neg organisms

Question 2

When do you use Acid Fast Stain?

Answer 2

For bacteria that don’t stain well with the gram stain. High amount of teichoic acid

Question 3

Why are they called acid fast bacteria?

Answer 3

Fast refers to the quality or state of being fixed and/or the resistance to the action of a specific stain or de-staining agent.

Question 4

What is a pure culture?

Answer 4

Only one bacterial sp. is represented on a culture plate

Question 5

What are the 4 media classifications?

Answer 5

Supportive, Enrichment, Selective, and Differential

Question 6

Describe enrichment media

Answer 6

Contains specific nutrients to promote growth of a particular type of organism.

Question 7

What are some additives that can be added to selective media?

Answer 7

Dyes, bile salts, alcohols, acids, and antibiotics.

Question 8

How does differential media work?

Answer 8

Contains additives allowing organisms to be differentiated by metabolic characteristics as evidenced by distinct colonial appearances on the media. Usually contains an indicator and a fermentable carbohydrate.

Question 9

What is Sheep Blood Agar?

Answer 9

Enriched and Differential

Question 10

What is Chocolate Agar?

Answer 10

Enriched; has X and V factors (supports the growth of Neisseria gonorrhoeae, Haemophilus spp. Neither grows well on SBAP).

Question 11

What is Phenylethyl Alcohol Agar?

Answer 11

Selective for gram + cocci
PEA inhibits growth of g- bacilli
Inhibits swarming of Proteus
Good for anaerobic work

Question 12

What is MacConkey Agar?

Answer 12

Selective and Differential for Gram - bacilli, both lactose fermenting (pink) and non-lactose fermenting (clear).

Question 13

What is Modified Thayer-Martin Media?

Answer 13

Highly enriched (chocolate agar + hemoglobin) and selective (antibiotics) for Neisseria gonorrhoeae and Neisseria meningitidis.

Question 14

What are the antibiotics added to Modified Thayer-Martin Media and what do they inhibit?

Answer 14

Colistin- inhibits gram -
Vancomycin- inhibits gram +
Nystatin- Inhibits yeast
Trimethoprim- Inhibits spreading of Proteus

Question 15

What is New York City Agar?

Answer 15

Selective media for Neisseria gonorrhaeae, Ureaplasma urealyticum, and Mycoplasma spp.

Question 16

What is Xylose-Lysine Desoxychoolate (XLD) Media?

Answer 16

Selective and differential for Salmonella spp. and Shigella spp. (both are gnr and enteric pathogens). They stay red on the media. Salmonella produces hydrogen sulfide and has black centers.

Question 17

What does XLD media contain?

Answer 17

Contains sodium desoxycholate which inhibits growth gram - bacilli that aren’t enteric pathogens and also inhibits gram + cocci. Contains xylose, lactose, and sucrose sugars.

Question 18

What is Thioglycollate broth enriched with?

Answer 18

Casein, yeast and beef extracts, and vitamins.

Question 19

What is LIM broth selective for?

Answer 19

Promotes growth of Group B strep especially from specimens from the female genital tract.

Question 20

What 3 tests should you do for Gram + organisms?

Answer 20

Catalase, Coagulase, and PYR

Question 21

What 3 tests should you do for Gram - organisms?

Answer 21

Indole, oxidase, and urease

Question 22

What is spot indole test presumptive ID for?

Answer 22

E. coli

Question 23

What is urease test presumptive ID for?

Answer 23

Proteus spp.

Question 24

Sensitivity means?

Answer 24

The % of people in whom the organism is present actually test positive.

Question 25

Specificity means?

Answer 25

The % of people without disease that test negative for the organism.