In order to select efficient and competitive glufosinate-degrading bacteria, two soils which had been treated with glufosinate annually for more than 5 yr were screened. Three strains tolerant to this herbicide were identified by 16S rDNA analysis as Burkholderia sacchari, Serratia marcescens, and Pseudomonas psychrotolerans. In addition, a moderately tolerant strain, P. citronellolis, was isolated from a soil which had received glufosinate treatment for only 6 mo. In culture medium containing high concentration of glufosinate, the former three strains showed significant ability to degrade this glutamine synthetase inhibitor, suggesting that glufosinate-degrading bacteria would be readily found in soils after a long-term induction or selection. A subsequent biodegradation experiment showed that 30 and 50% of glufosinate was degraded 7 and 21 d after treatment (DAT), respectively, in sterilized soils inoculated with the above-mentioned three tolerant strains. While more than 30% of the glufosinate in nonsterilized soils was degraded 7 DAT by the indigenous edaphic microbes, inoculation with the three selected strains enhanced glufosinate degradation to nearly 50%. A study on the competition from edaphic microorganisms in soils by polymerase chain reaction denaturing gradient gel electrophoresis (PCR-DGGE) analysis revealed that within 21 d after inoculation (DAI), the propagation of B. sacchari and P. psychrotolerans was not affected, whereas that of the less tolerant P. citronellolis was inhibited. This observation suggests that a long-term herbicide exposure is a promotive factor in generating bacterial strains having high degradation efficiency and showing vigorous propagation under the competition pressure arising from indigenous microbes.
Nomenclature: Glufosinate; Burkholderia sacchari; Serratia marcescens; Pseudomonas psychrotolerans; Pseudomonas citronellolis