Question

Which of the following components of urease media is added for the differentiation of rapid and slow urease producers?

Multiple Choice

• Urease

• Yeast extract

• Phenol red

• Phosphate buffer

Which of the following classes of bacteria are being tested for using the urease test?

Multiple Choice

• Gram-negative enterics

• Gram-positive streptococci

• Gram-positive staphylococci

• Gram-negative non-enterics

Which of the following metabolic waste products would be a result of nitrate reductase?

Multiple Choice

• Nitrite

• Nitrous oxide

• Atmospheric nitrogen gas

• Ammonium

Nitrite reductase is produced by bacteria that carry out denitrification in the nitrogen cycle.

True or False

Bacteria positive for nitrite production will remain colorless after the addition of nitrate reagents A and B and HCl.

True or False

Answer 1

Answer 1) The correct option is third (Phenol Red).

Urease test is commonly used for differentiating enteric bacteria. These bacteria hydrolyses urea and produce CO2 and ammonia. Presence of ammonia shifts the pH of the medium towards alkalinity which is detected by PHENOL RED. Color of phenol red at near neutral pH is orange and alkalinity causes color shifting from orange to Pink and this is used as an indicator for differentiation of rapid and slow urease producers.

Option first is incorrect: urease enzyme catalyzes the hydrolysis of urea however they do not give directly any indicator for differentiation between rapid and slow urease producers. Color change is detected due to phenol red.

Option second is incorrect: yeast extract does not give any reaction to be used as indicator in urease test. Hence, this option is incorrect.

Option fourth is incorrect: Phosphate buffer does not give any differentiation reaction during urease test. Hence, this option is incorrect.

Answer 2) The correct option is first (gram negative enterics)

Enterics are the bacteria present in the intestinal track. Enterics are gram-negative rods and have known to possess anerobic metabolism. Urease test is used for differentiation among different enteric bacteria. Hence, option first is correct.

Option second and third are incorrect: Gram-positive streptococci or staphylococci may produces urease enzyme (for example S. thermophilus), however urease test is not for distinguishing gram-positive bacteria. Hence, these options are incorrect.

Option fourth is incorrect: urease test is not generally used as a differentiation test for non-enteric bacteria. Hence, this option is incorrect.

Answer 3) The correct option is first (nitrite)

nitrate reductase is well known enzyme which is responsible for reduction of nitrate in to nitrite. This conversion (by bacteria) is a very crucial reaction for plants as this provide nitrogen for protein synthesis.

Option second is incorrect: One nitrite has formed, it is again reduced by nitrite reductase in to ammonia and nitric oxide which further converted in to nitrous oxide. Hence, nitrous oxide is not a byproduct of nitrate reductase enzyme.

Option third is incorrect: Nitrous oxide further converted in to nitrogen gas which is released in the environment. Hence, nitrogen gas is also not a byproduct of nitrate reductase enzyme.

Option fourth is incorrect: Ammonium is a byproduct of nitrite reductase and not of nitrate reductase. Hence, this option is incorrect.

Answer 4) The statement is True

Denitrification is a mechanism performed by several bacteria where nitrate is converted to nitrite with the help of nitrate reductase. Further, nitrite reductase form ammonia and through nitrous oxide, nitrogen gas is formed. Hence, the statement that nitrate reductase is produced by bacteria involved in denitrification is correct.

Answer 5) The statement if False

Bacteria which are positive for nitrite production remains colorless after addition of reagent A. However addition of reagent B change the color of the medium to RED. Hence, the statement is incorrect or FALSE.