LAB 10 – GRAM POSITIVE BACILLI AND CANDIDA

Question 1

What would be of concern for us for Bacillus sp. on a plate? I.e.. What would indicate B.
anthracis?

Answer 1

• Non-hemolytic “medusa head” colonies indicates Bacillus anthracis
• Anthrax is associated with necrotic lesions (black eschars)

Question 2

Name some key characteristics of Candida

Answer 2

• endogenous to digestive tract + mucocutaneous areas of body (opportunistic)
• most common cause of serious fungal disease (candidiasis)
• can cause vaginitis

Question 3

Since Listeria monocytogenes colonies look similar to strep, what quick tests could you use to
differentiate them? What are the expected results?

Answer 3

Catalase = (+)

Question 4

What are the presumptive ID tests and expected results for Listeria? What are the
confirmatory tests for ID of listeria?

Answer 4

1. Colony morph = sm β hem
2. Gram = g+b, pleo
3. Catalase = (+)
4. Bile esculin = (+)
5. CAMP = (+) is confirmatory
6. Motility = tumbling (+) at RT, (-) at 37°C

Question 5

What can happen if the Germ tube test is incubated for longer than 2-2.5 hours?

Answer 5

Longer incubation can promote pseudohyphae production, which can be confused for germ tubes (no constriction at point of attachment)

Question 6

Pathogenic Corynebacterium sp. are isolated from the throat. They will grow on BAP and
CAP but there are 2 types of media that they like best. What are they? What are the
components?

Answer 6

1. Tellurite media (ie. Tinsdale’s agar):
   - potassium tellurite inhibits normal flora from respiratory tract
   - potassium tellurite is reduced by C. diphtheriae = metallic tellurium stains colonies brown-black
   - production of H2S = presumptive C. diphtheriae withbrown-black “halos”
2. Loeffler’s Blood Serum (LBS):
   - enriched with serum and egg to enhance growth and formation of metachromatic granules

Question 7

Explain the principle of the Elek test

Answer 7

• differentiates between atoxigenic and toxigenic C. diphtheriae
• filter paper is infused with an antitoxin and placed across streaks of bacteria
• precipitation lines form between diphtheria toxin and anti-toxin at optimum concentrations (forms an X)

Question 8

What type of agar is required to show hemolysis for Gardnerella vaginalis?

Answer 8

human BAP

Question 9

What is in Tinsdale media? What is the function of those components?

Answer 9

• contains potassium tellurite
  -inhibits normal flora from the respiratory tract
• is reduced by C. diphtheriae = brown-black “halo” colonies due to H2S production

Question 10

. You see clue cells in a vaginal smear. What organisms do you expect to see and what
organisms do you expect will be absent? What are their expected grams?

Answer 10

• Gardnerella vaginalis is expected (gram variable to negative coccobacilli)
• absence of Lactobacilli (parallel g+b)

Question 11

Why is Gardnerella vaginalis gram variable?

Answer 11

• has a cell wall with gram positive characteristics
• but gram variable due to pleomorphic nature of coccobacilli