Kovac’s reagent – Knowledge and References

Kovac's reagent

Estimation of Bacterial Numbers by Indirect Methods

Published in Maria Csuros, Csaba Csuros, Klara Ver, Microbiological Examination of Water and Wastewater, 2018

The classic test calls for p-dimethylamino-benzaldehyde as the indicator (Kovacs reagent). Broth for the indole test is commercially available. Positive result is visible as the red to pink ring around the top of the tube where the reagent floats after incubation at least for 24 h. In the negative test, no change in the reagent’s color is seen. Prepare 1 percent indole solution—1 g para-dimethyl-amino-cinnamaldehyde, dissolved in 10 percent v/v hydrochloric acid (10 ml conc HC1 and 90 ml laboratory pure water). Store in a dark bottle in the refrigerator.Saturate a qualitative filter paper (Whatman No. 1 is fine) in the bottom of a petri dish with the reagent prepared in step 1.Using a wooden stick or loop, rub a portion of the colony on the filter paper. Rapid development of a blue color indicates a positive test. Most indole positive organisms turn blue within 30 s. An indole test is shown in Figure 14.3.

Biostimulation of Petroleum-Contaminated Soils with Synthetic and Natural Sources of NPK Fertilizer

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Journal Information

Published in Soil and Sediment Contamination: An International Journal, 2023

Seyed Mohsen Dehnavi, Gholamhossein Ebrahimipour

Indole test. The pure bacterial culture was firstly inoculated in a sterilized test tube containing 4 ml of tryptophan broth for about 18–24 hours and then it was incubated at 37°C for 24–48 hours. The appearance of bright red and yellow colors after the addition of 0.5 mL of Kovac’s reagent to the incubated bacterial culture was an indication of positive and negative results, respectively (Cheesbrough 1981).

Kovac's reagent

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Estimation of Bacterial Numbers by Indirect Methods

Biostimulation of Petroleum-Contaminated Soils with Synthetic and Natural Sources of NPK Fertilizer