Question

1. Sulfide, Indole, Motility (SIM): Bacteria that have the enzyme tryptophanase, can convert the amino acid tryptophan to indole. Indole reacts with Kovac’s reagent to form a Black Color. T or F

2. GROWTH ON EMB & MAC: Which Observations are INCORRECT?

1. both EMB and MAC contain dyes that inhibits gram-positive bacteria.
2. both EMB and MAC differentiate gram negative organisms based on their ability to ferment lactose
3. both EMB and MAC indicate that the pH becomes basic (alkaline) because of fermentation
4. all the above are incorrect
5. none of the above

3. GROWTH On MSA (7.5% NaCl). Which Observations are CORRECT?
   1.    MSA is selective for Staphylococci
   2. Staphylococci species are Gram +
   3.     MSA is differential based on acid production
   4. Acid production on MSA changes the phenol red (acid indictor) to yellow
   5.    All the above

4. Blood Agar is differential with respect to Hemolysis. Which of the following is CORRECT?
   1. hemolytic enzymes are produced in produced hemolysis
   2. Beta (ß) hemolysis and results in the complete breakdown of the RBCs
   3. Alpha (a) hemolysis Partial destruction of the RBCs leads to a greenish brown color
   4. Gamma(γ)hemolysis causes no damage to the RBCs
   5. all the above are correct

5. Blood Agar Plates have the enzyme catalase. We use blood agar to determine whether an unknown bacterium is catalase positive. T or F

6. Differential and/or Selective Media include EMB, MSA, MAC, Blood Agar Plates, Simmons Citrate & SIM Tubes. Regarding what was observed on these media, which is CORRECT?
   1.     a change in color on any of those media means we are looking at the result of enzyme activity
   2.   acid production, or no -acid production indicate the presence or absence of specific enzymes
   3.     all the above are correct
   4.   none of the above are correct

7. A pH sensitive dye (in media) responds to acid production with a change in color, and
   1. MacConkey (MAC) media turns reddish
   2. Eosin Methylene Blue (EMB) agar media inoculated with Escherichia coli produces a green metallic sheen
   3. Mannitol-salt agar (MSA) turns yellow
   4. all the above
   5. none of the above

8. Page 284 of Benson’s summaries some “characteristics of common Gram-positive and Gram-negative bacterial Species.”. The Table identifies a list of Gram (-) bacteria that are all catalase +. Which of the following makes sense?
   1.    the catalase test can be used to categorize bacteria., i.e. they can be categorized as catalase (+ or -)
   2. the catalase test alone could be used to identify a specific Gram (-) bacteria on that list
   3.   all the above makes sense
   4. none of the above makes sense

1. If we inoculated MSA (which has 7.5% NaCl) with Gram negative bacteria, what would be the result? We would find that the growth of MOST Gram-negative bacteria is inhibited by the 7.5% NaCl. T or F

2. If we inoculated MAC with Staphylococcus aureus, what would be the result?

1. we would observe growth and acid production (fermentation)
2. we would observe growth, but no acid production (fermentation)
3. we would observe no growth
4. none of the above

3. If we inoculated EMB with Staphylococcus aureus, what would be the result?

1. we would observe growth and acid production (fermentation)
2. we would observe growth, but no acid production (fermentation)
3. we would observe no growth
4. none of the above

4. S. aureus is catalase positive, which means 2H₂O+ catalase -> O₂ (gas) + 2H₂     T or F

5. Selective & Differential media have specific carbohydrate substrates. Which is CORRECT?
   1. the carbohydrate substrate in MAC is mannitol
   2. the carbohydrate substrate in EMB is eosin
   3. the carbohydrate substrate in MSA is mannitol
   4.   all the above
   5. None of the above

Answer 1

Ans 1) False.

Sulfide, Indole, Motility (SIM): Bacteria that have the enzyme tryptophanase, can convert the amino acid tryptophan to indole. Indole reacts with Kovac’s reagent to form a red colour due to rosindole dye.

This media is Differential for the members of Enterobacteriaceae test reduces to H​​​​​​2​​​​​S which provides black colour to medium and it also gives indole.

Ans 2) Option (c) is correct.

​​​​​​Eosin Methylene blue and MacConkey Agar, are differential media as they will distinguish between the gram negative bacteria based on their ability to ferment lactose. Also, the gram positive bacteria cannot grow in EMB and MAC, as it contain dyes that are toxic to the species of gram positive bacteria.

If the lactose fermentation occurs, strong acid is produced and as acid acts on dye the pH is lowered as a result of which the colonies turn dark purple.

Ans 3) Option (e) is correct.

Mannitol Salt Agar is used to select the gram positive bacteria specifically the Staphylococcus species. After the fermentation of mannitol an acid is produced in the medium. Phenol red, a pH indicator is added in the medium and as a result of which yellow colonies are being produced.

Ans 4) Option (e) is correct.

Blood Agar is a medium which will differentiate between the bacterial species which is based on the hemolytic properties. Blood supplies nutrition for the growth of bacteria generally streptococci.
Products of hemolysis are-
Alpha hemolysis- In this, Red Blood Cells are partially lysed and thus greenish brown colour is produced around the colony.
Beta hemolysis- In this, Red Blood Cells are completely lysed
Gamma Hemolysis- In this, no breakdown of red blood cells or we can say no hemolytic reactions occur in gamma hemolysis. Thus no colour change is observed.

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