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.