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Counting viable azotobacter chroococcum in vertisols ii. comparison of media



Counting viable azotobacter chroococcum in vertisols ii. comparison of media



Plant and Soil 117(1): 17-30



Counts of Azotobacter on Brown's medium containing 0.5% glucose sterilized separately from phosphates were greater than on some 20 other published media and modifications. Brown's medium gave considerably greater counts than Rovira's combined carbon medium, but pigmentation of A. chroococcum allowing ready identification was more developed on the latter. Addition of components of Rovira's medium to improve pigmentation in Brown's medium was only partly successful. With Ca acetate + Na citrate as C sources, percentage pigmented colonies was increased from 0 to 85% but colony counts were lower than with glucose. Sucrose as carbon source resulted in larger colonies but lower counts. In an experiment with nine published media plus eleven modifications comprising carbon sources of glucose, sucrose, arabinose, dextrin, mannitol, acetate, citrate, and benzoate in various mineral bases. Brown's medium gave greatest counts. Counts on other media were increased to comparable levels by substituting separately sterilized 0.5% glucose for original carbon sources. Other media like Rovira's, Parker's and Aleem's had greater percentages of pigmented colonies with Aleem's benzoate medium having the most intensely pigmented colonies. Mineral composition had little effect on colony counts. Density of oligonitrophilic bacteria on the various media had little effect on counts of Azotobacter. Multiple regression analyses were used to relate Azotobacter colony counts and characteristics to 22 organic and inorganic constituents of media. Counts were positively related to glucose sterilized separately and negatively related to sucrose. Colony diameter was positively related to sucrose. Percentage of pigmented colonies was positively related to Ca, glucose and benzoate and negatively related to Ca/Na ratio and S. Non-pigmented Azotobacter colonies on Brown's medium became pigmented when replica-plated to other media indicating all were A. chroococcum. Brown's medium is recommended for counting Azotobacter in vertisols with replica plating to a chromogenic medium, e.g., Rovira's, for identification of A. chroococcum if required.

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