Question

1. a positive citrate test is shown by:

a. blue color on the slant
b. green color remaining
c. black precipitate formation
d. gas bubbles in the butt

1. the urease test uses phenol red to detect:

a. ammonia production from urea hydrolysis
b. acid production from glucose fermentation
c. indole production
d. sulfur reduction

1. in triple sugar iron (tsi) agar, black precipitate indicates:

a. glucose fermentation only
b. hydrogen sulfide (h₂s) production
c. gas formation
d. lactose fermentation

1. the indicator used in phenol red carbohydrate broths is:

a. neutral red
b. phenol red
c. bromcresol purple
d. methyl orange

Explanation:

• Question 6: In the citrate test, bacteria that can utilize citrate as the sole carbon source will grow. The medium contains bromthymol blue as an indicator. When citrate is metabolized, alkaline by - products are produced, which turn the medium blue.
• Question 7: The urease test is based on the hydrolysis of urea by the enzyme urease. The reaction \( \text{CO(NH}_2\text{)}_2+\text{H}_2\text{O}\xrightarrow{\text{Urease}}\text{CO}_2 + 2\text{NH}_3\) produces ammonia (\(\text{NH}_3\)). Phenol red is a pH indicator. As ammonia is produced (increasing the pH), the phenol - red turns from yellow (at acidic pH) to pink (at basic pH).
• Question 8: In TSI agar, black precipitate is formed due to the reaction of hydrogen sulfide (\(\text{H}_2\text{S}\)) with ferrous ions (\(\text{Fe}^{2 +}\)). The reaction is \(\text{Fe}^{2+}+\text{H}_2\text{S}\to\text{FeS}\downarrow+\text{2H}^+\), where \(\text{FeS}\) is the black precipitate.
• Question 9: The name "Phenol Red Carbohydrate Broths" implies that the indicator used is phenol red. Phenol red changes color in response to pH changes that occur during carbohydrate fermentation (acid production) or other metabolic processes in the broth.

Answer:

1. A. Blue color on the slant
2. A. Ammonia production from urea hydrolysis
3. B. Hydrogen sulfide (\(\text{H}_2\text{S}\)) production
4. B. Phenol red