Degradation of 3-phenoxybenzoic acid in soil by Pseudomonas pseudoalcaligenes POB310(pPOB) and two modified Pseudomonas strains

Pseudomonas pseudoalcaligenes POB310(pPOB) and Pseudomonas sp. strains B13- D5(pD30.9) and B13-ST1 (pPOB) were introduced into soil microcosms containi ng 3-phenoxybenzoic acid (3-POB) in order to evaluate and compare bacterial survival, degradation of 3-POB, and transfer of plasmids to a recipient ba cterium. Strain POB310 was isolated for its ability to use 3-POB as a growt h substrate; degradation is initiated by POB-dioxygenase, an enzyme encoded on pPOB. Strain B13-D5 contains pD30.9, a cloning vector harboring the gen es encoding POB-dioxygenase; strain B13-ST1 contains pPOB. Degradation of 3 -POB in soil by strain POB310 was incomplete, and bacterial densities decre ased even under the most favorable conditions (100 ppm of 3-POB, supplement ation with P and N, and soil water-holding capacity of 90%). Strains B13-D5 and B13-ST1 degraded 3-POB (10 to 100 ppm) to concentrations of <50 ppb wi th concomitant increases in density from 10(6) to 10(8) CFU/g (dry weight) of soil. Thus, in contrast to strain POB310, the modified strains had the f ollowing two features that are important for in situ bioremediation: surviv al in soil and growth concurrent with removal of an environmental contamina nt. Strains B13-D5 and B13-ST1 also completely degraded 3-POB when the inoc ulum was only 30 CFU/g (dry weight) of soil. This suggests that in situ bio remediation may be effected, in some cases, with low densities of introduce d bacteria. In pure culture, transfer of pPOB from strains POB310 and B13-S T1 to Pseudomonas sp. strain B13 occurred at frequencies of 5 x 10(-7) and 10(-1) transconjugant per donor, respectively. Transfer of pPOB from strain B13-ST1 to strain B13 was observed in autoclaved soil but not in nonautocl aved soil; formation of transconjugant bacteria was more rapid in soil cont aining clay and organic matter than in sandy soil. Transfer of pPOB from st rain POB310 to strain B13 in soil was never observed.