TRANSPORT OF A BIOCONTROL PSEUDOMONAS-FLUORESCENS THROUGH 2.5-M DEEP OUTDOOR LYSIMETERS AND SURVIVAL IN THE EFFLUENT WATER

Application of wild-type or genetically-modified bacteria to the soil environment entails the risk of dissemination of these organisms to th e groundwater. To measure vertical transport of bacteria under natural climatic conditions, Pseudomonas fluorescens strain CHAO was released together with bromide as a mobile tracer at the surface of large outd oor lysimeters. Two experiments, one starting in autumn 1993 and the o ther in spring 1994 were performed. Shortly after a heavy rainfall in late spring 1993, the released bacteria were detected for the first ti me in effluent water from the 2.5-m-deep lysimeters in both experiment s, i.e. 210 d and 21 d, respectively, after inoculation. Only a 10(-9) to 10(-6) fraction of the inoculum was recovered as culturable cells in the effluent water, but a larger fraction of the CHAO cells was in a non-culturable state as detected with immunofluorescence microscopy. As much as 50% of the mobile tracer percolated through the lysimeters , indicating that, compared with bromide, bacterial cells were retaine d in soil. In the second part of this study, persistence of CHAO in gr oundwater microcosms consisting of lysimeter effluent water was studie d for 380 d. Survival of the inoculant as culturable cells was better under anaerobic than under aerobic conditions. However, a large fracti on of the cells became non-culturable in both cases. When the experime nt was performed with filter-sterilized effluent water, the total coun t of introduced bacteria did not decline with time. In conclusion, the biocontrol strain was transported in low numbers to a potential groun dwater level under natural climatic conditions, but could persist for an extended period in groundwater microcosms. (C) 1998 Elsevier Scienc e Ltd. All rights reserved.