Girgis M.G.Z.; M.S . Sharaf and Heba M.A. Khalil( 2007). Differentiation Between Bradyrhizobium Isolates By Colony Type,
Exopolysaccharides And Streptomycin Resistance
Journal of Applied Sciences Research, 3(12): 1997-2008, 2007
Abstract: Twenty seven Bradyrhizobium isolates were collected from fresh young nodules developed on soybean seedlings from six different locations in Egypt. After isolation, two types of colony morphology were observed: a “wet” type with flatter and entire edge associated with gum production, and“dry” small colonies, circular with entire margin. Unusual little dark-brown colour was noted in the external appearance of collected soybean nodules from Wadi El-Natroon (moderately saline soil). The dark-nodule isolates had a dry colony one type. Isolate derived from “wet” colonies produced an acidic reaction and 4 were non-acid producers, while the "dry" ones was weak acidic. Six representative isolates were selected to examine their growth characteristics and ability to produce exo-polysaccharides (EPS). Five isolates formed “wet” flat colonies while one isolate (designated as Sh13) formed “dry” colony. The EPS variation in production and viscosity was found amongst the “wet” isolate Sa4 which compared to Sh13 “dry” one. The intrinsic antibiotic resistance (IAR) pattern of isolates to antibiotics inhibiting protein synthesis, i.e.
kanamycin, chloramphenicol and streptomycin was applied for screening the isolates using antibiotic discs. At low concentration antibiotics (30μg disc-1) of isolates namely, Wn9, Is18 and Sa4 were naturally resistant, while the others showed less resistance. Therefore, further evaluation of the biomass yield of all isolates up to increased concentrations of streptomycin was conducted. With 50μg ml-1 of streptomycin, no growth was detected for “dry” isolate Sh13, while the others showed good yield up to 100 μg ml-1. By increasing the concentration to 200 μg ml-1, most of the isolates showed intermediary yield, while the “wet” isolate Sa4 showed continuous resistance up to 400μg ml-1 where it gave slight growth. No growth was detected with the 500μg ml-1 concentration for all tested isolates. The “wet” isolate resistant to 400 streptomycin and producing high amount of EPS (R.Sa4 str. ) as well as “dry” isolate sensitive to 30 streptomycin and producing low amount of EPS (S.Sh13 str. ), were used in mixed inoculation experiment of soybean seedlings up in a modified Gibson system. The isolate-specific characteristics remained stable after infection of soybean and re-isolation from nodules. However, 31% of the isolation attempts contained both isolates, while 23 and 47% of the nodules were found to contain a single isolates of either streptomycin resistant isolate or sensitive isolate respectively.
Keywords: (Soybean, Bradyrhizobium, colony type, streptomycin resistance, exo-polysaccharides, competition. )
