Screening Streptomycin Resistant Mutations from Gamma Ray Irradiated Bacillus subtilisB5 for Selection of Potential Mutants with high Production of Protease
The suspensions of Bacillus subtilisB5, a rather high protease production strain, in
logarithmic growth phase were irradiated under gamma Cobalt-60 source at Hanoi Irradiation
Center. After treatment, the irradiated cells were intermediately cultured in the nutrient agar plates
supplemented without and with 20µg/ml streptomycin for screening. The radiation effects on their
viability and mutant frequency were studied with radiation dose. The results showed that its
survival rate was reduced with the dose as biphasicfunction. The cells irradiated at dose higher
than 1200 Gy do not form colony in the medium containing streptomycin though they could
survive in nutrient agar. Therefore, potential streptomycin resistance mutations were collected as
survivals from the cells irradiated with radiation dose ranging from 100 to 1000 Gy. Within this
dose range, mutation frequency of Bacillus subtilisB5 increased with the rising dose. The greatest
mutation frequency was determined as 1.61×10
-3
obtained by irradiation at 1000 Gy, and the
smallest as 3.09×10
-6
at 100 Gy. The enzyme activities of 361 screened colonies from all irradiated
samples were investigated in casein agar, and the results revealed 25 colonies having protease
activity higher than parent strain.
