Nitrate Broth is a growth media used to carryout the biochemical test ”nitrate reductase test”.The purpose of nitrate reduction test is to detect nitrate reduction by some bacteria. The ability to reduce nitrate by the enzyme Nitrate reductase found in some bacteria is valuable for differentiating and identifying various types of bacteria especially those of the family of Enterobacteriaceae
Principle of Nitrate reduction Test
Members of Enterobacteriaceae characteristically reduce nitrate to nitrite which reacts with sulfanilic acid and N,N-dimethyl-1-naphthylamine to produce a red color.
This reaction is known as Griess reaction. Therefore if an organism grows rapidly and actively reduces nitrate, the test should be performed after an early incubation period since the nitrite may be further reduced to nitrogen.
Nitrate conversion to nitrite
nitrate reduction test medium composition
The following ingredients are used for the composition Nitrate Broth
Ingredients
Grams / Litre
Peptic digest of animal tissue
5.000
Meat extract
3.000
Potassium nitrate
1.000
Sodium chloride
30.000
Final pH at temperature 25°C
7.0±0.2
How to Prepare Nitrate Broth
Start by suspending 39 grams in 1000 ml distilled water
Heat if necessary in order to dissolve the medium completely.
Now dispense in tubes then sterilize by autoclaving at 15 lbs pressure (121°C) for 15 minutes.
Appearance of Medium after preparation
It appears light amber coloured clear to slightly opalescent solution forms in tubes.
Uninoculated Nitrate broth
Reagents for Nitrate Reduction Test
Reagents
How to Prepare
Sulfanilic Acid
Dissolving 8 grams of sulfanilic acid in 1 litre 5 N acetic acid
Alpha-Naphthylamine reagent
Achieve by dissolve 5 grams of alpha-naphthylamine in 1 litre 5 N acetic acid
Inoculate the broth with a 18 to 24 hour old pure culture.
Then incubate for 24-48 hours at temperature 20-24 °C aerobically
After incubation of the broth,add about 5 drops of a-naphthylamine and sulfanilic acid to the medium and shake gently in order to mix reagents..
If there is no color development in the broth after addition of a-naphthylamine and sulfanilic acid then add a small amount of zinc dust to to confirm the absence of nitrate in the medium
Result and interpretation
Positive Test
Observe the formation of a pink or red color in the medium within 1-2 minutes following the addition of a-naphthylamine and sulfanilic acid. or no color development within 5-10 minutes after adding zinc dust.
Negative Test
There is no pink or red color development within 1-2 minutes following the addition of a-naphthylamine and sulfanilic acid or red color development within 5-10 minutes after adding zinc dust..
Cultural response to Nitrate reduction test
Culture observation and Nitrate reduction reaction
Cultural Response
Nitrate Reduction
Bacillus cereus
Positive reaction, red colour developed within 1-2 minutes
Enterobacter aerogenes
Positive reaction, red colour developed within 1-2 minutes
Escherichia coli
Positive reaction, red colour developed within 1-2 minutes
Salmonella Typhimurium
Positive reaction, red colour developed within 1-2 minutes
Some Precaution to Take into Consideration
Always make sure to kindly watch for the color change as the color may fade quickly.
Before final determination of results are made, be sure to add a small amount of zinc dust. Too much zinc dust can a quick reduction of the nitrate resulting in a false negative reaction.
Also the nitrate reduction test is not a confirmatory test hence complete bacteria identification should include the morphology, gram reaction, biochemical and serological tests.
Strongly reducing bacteria may exhibit a brown precipitate.
