Nuclease-producing bacteria in soil cultivated with herbicide resistant transgenic white poplars

This study was carried out using soil cultivated, under greenhouse conditions, with transgenic white poplars expressing the bar gene for tolerance to the Basta (R) herbicide. The occurrence of extracellular nucleolytic activity was monitored in soil samples collected at four different times over a 26-month period. The fraction of nuclease producing bacteria (NPB) ranged from 62.5 to 100% of the total culturable bacterial population. The DNA-methyl green plate assay allowed to distinguish five groups of bacteria showing increasing levels of extracellular DNase activity. The NPB isolates were classified by 16S rDNA sequence analysis as members of the Bacillus, Brevibacillus, Microbacterium, Pseudomonas and Stenotrophomonas genera. For each genus, NPB isolates were cultured in liquid medium and the nucleolytic activity during different growth phases was monitored. Production of extracellular nucleases was observed only during the mid-exponential growth phase of the Brevibacillus, Microbacterium and Stenotrophomonas isolates, while no activity was evidenced for isolates classified within the Bacillus and Pseudomonas genera.