Nitrate reduction

Sample type:

1) A pure culture of a suspect gram negative.

2) Oxidase-positive diplococcus (Neisseria spp. or M. catarrhalis) on chocolate agar incubated in a carbon dioxide-enriched atmosphere at 35C to 36.5C for 18 to 24 h.

Uses:

1) Identifying species of Neisseriaand separating them from Moraxella and Kingella The nitrate reduction test is a critical test for differentiating between N. gonorrhoeae and K. denitrificans, particularly when strains of K. denitrificans appear to be gram-negative diplococci in stained smears.

2) It is thus used to differentiate members of Enterobacteriaceae that produce enzyme nitrate reductase from Gram negative bacteria that do not produce the enzyme nitrate reductase.

3) It is used in differentiating Mycobacterium.

Nitrate reduction

Precautions:

1) Tests should only be performed with inoculum harvested from 24 h cultures.

2) Nitrate medium should be inoculated within 30 min. of removing culture from the incubator; prolonged exposure of the culture at room temperature may result in diminished enzyme activity.

3) Due to the possible presence of nitrite in the culture media, a low nitrite media such as Nitrate Agar or Nitrate Broth should be used for the nitrate reduction test.

Interfering Factors:

Excess zinc dust has been reported to cause false-positive nitrite reduction reactions due to complete reduction of previously unreduced nitrate to ammonia.

Pre analytical errors:

1) The test may be performed in a medium that does not support growth of the organisms if the inoculum is sufficiently dense that preformed enzymes can exhaust the existing oxygen supply and reduce the nitrate at a faster rate than that at which the oxygen diffuses into the medium

Corrective action:

1) To avoid false-negative nitrite reduction reactions, negative nitrite reactions must be verified by the addition small amount of zinc dust to the medium.