USE OF TN5 MUTANTS TO ASSESS THE ROLE OF THE DISSIMILATORY NITRITE REDUCTASE IN THE COMPETITIVE ABILITIES OF 2 PSEUDOMONAS STRAINS IN SOIL

We examined the influence of soil aeration state and plant root presen ce on the comparative survival of wild-type bacteria and isogenic Tn5 (Nir(-)) mutants lacking the ability to synthesize nitrite reductase, Two denitrifying Pseudomonas strains with different nitrite reductase types were used, Enumeration of bacteria in sterile and nonsterile soi ls was based on differential antibiotic resistance, The validity of th e bacterial models studied (i.e., equal growth of wild-type and mutant bacteria under aerobic conditions and significantly better growth of wild-type bacteria under denitrifying conditions) was verified in pure -culture studies, In sterile soil, both strains survived better under aerobic than under anaerobic conditions, The lower efficiency of denit rification than O-2 respiration in supporting bacterial growth explain ed this result, and the physical heterogeneity of soil did not strongl y modify the results obtained in pure-culture studies. In nonsterile s oil, one of the Pseudomonas strains survived better under anaerobic co nditions while the other competed equally with the indigenous soil mic roflora under aerobic and anaerobic conditions, However, when the Nir( -)-to-total inoculant ratios (wild type plus Nir(-) mutant) were analy zed, it appeared that the presence of nitrite reductase conferred on b oth Pseudomonas strains a competitive advantage for anaerobic environm ent or rhizosphere colonization, This is the first attempt to demonstr ate with isogenic nondenitrifying mutants that denitrification can con tribute to the persistence and distribution of bacteria in fluctuating soil environments.