Characterization of Indigenous Azotobacter Isolates from Sorghum Rhizosphere

Abstract

This study was conducted at the College of Agricultural Studies – Sudan University of science and technology field. Soil samples were collected from the rhizosphere of sorghum (0-15 cm), with the objectives of isolating Azotobacter spp. from Sudanese soils, physiological, morphological and biochemical characterization of isolates, were taken to identify them, to look into the effect of pH, temperature and different salt concentrations on the growth of the isolates and test their resistance to different streptomycin antibiotic concentrations. The obtained results showed that the colonial morphology of the isolates was as follows: The size of colonies ranged from 1-5 mm diameter; the shape was circular; pigmentation was creamy beige white, with similar color and hue as the agar; elevation was raised; edge was smooth, surface was smooth, glossy, viscous, musky in odor and negative reaction with gram stain represented by the red color of cells. The cells of isolated bacteria were oval rods and big in size. There were a vegetative cells and there was an evidence of cysts formation in the mature cells under the microscope after a long incubation period. These observations exactly matched with the characters of Azotobacter spp reported in the literature. The results of the biochemical tests of the isolated bacteria showed a positive reaction with mannitol, indole, methyl red, citrate utilization, urease and catalase test. These results indicated that the isolated bacteria were Azotobacter spp. The growth rate of Azotobacter varied according to salts concentrations. Maximum growth occured at 0.2% and 0.4% NaCl. III Medium growth occurred at 0.6% and 0.8% NaCl concentration, and at 1%, minimum growth was attained. The effect of pH on Azotobacter growth varied according to the pH ranges. The results showed that no growth of Azotobacter spp. was observed at a pH value of 5.Minimum growth occurred at pH 6, and the optimum growth rate was reported at pH 7. No growth was reported at pH 10. This clearly illustrated the effect of alkalinity and acidity on the growth of Azotobacter spp. The best growth occurred at the neutral pH value of 7. The results revealed that Azotobacter spp. did not grow when the incubation temperature was lowered to 10˚C. Growth, however, occurred when the incubation temperature was adjusted at 20˚C, although growth was generally weak to moderate. The highest growth occurred when the temperature was adjusted at 30˚C. No growth was reported at 40˚C incubation temperature, or when the incubation temperature was raised to 50˚C. The antibiotic resistance tests revealed that the isolated Azotobacter spp. tolerated the streptomycin antibiotic up to a level of 5μ/ml concentration.