KOH (potassium hydroxide) test for Gram negativity in bacterial blight pathogen of Pomegranate




Kale, Prashant B.


Plant pathogenic bacteria are mostly Gram negative in nature and the correct identification of the bacterium is of prime importance in developing management or control measures. Wide classification of pathogenic bacterium is of hence importance and different method available including Gram’s staining, KOH-String test. Out of which KOH-string test have rapid outcomes, thought for confirmation Gram staining useful.


Gram negative bacterial cell wall dissolves with KOH 3 percent concentration whereas not in Gram positive cell walls. Dissolved cell wall releases the intercellular viscous or emulsified material out and it forms string.


1. Microscope glass slide

2. KOH 3% solution

3. Pure culture of isolated from diseased Pomegranate

4.Nichrome loops etc.


1.Isolate pure cultures form bacterial blight infected pomegranate.

2.Grow sufficient solid culture to get loop-full mass of colony.

3.Take loop-full mass of pure culture on glass slide and mix with 3 percent KOH by loop.

4.Formation of string with the loop is a record of Gram negativity.

5.While no formation of string is a record of Gram positivity of culture.

6.This provides a fast selection of target pathogen form infected tissue and also time saving.

7.This method found as good phenotypic bacterial distribution techniques in the study.

Literature references

1. Halebian, S., B. Harris, S. M. Finegold, and R. D. Rolfei. 1981. Rapid method that aids in distinguishing Gram-positive from Gram-negative anaerobic bacteria. Journal of Clinical Microbiology, pp. 444-448.

2. Suslow, T. V., M. N. Schroth, and M. Isaka. 1982. Application of a rapid method for Gram differentiation of plant pathogenic and saprophytic bacteria without staining. Phytopathology 72: 917-918.

Leave a Reply