Nutrient deprivation and the subsequent survival of biocontrol Pseudomonasfluorescens CHA0 in soil

The effects of deprivation of multiple nutrients or of selected single nutr ients (C, S, N, or P) for 7 d in vitro on the subsequent persistence of the biocontrol agent Pseudomonas fluorescens CHA0 in natural soil were investi gated. Experiments were carried out with the spontaneous rifampicin-resista nt mutant CHA0-Rif and the strain was monitored in vitro and in soil using colony counts ton plates containing rifampicin), Kogure's direct viable cou nts and total cell counts (by immunofluorescence microscopy). Single nutrie nt or multiple nutrient deprivation in vitro did not affect the colony-form ing ability of CHA0-Rif cells. However, cell length of the strain was small er in all nutrient deprivation treatments but one (P deprivation) when comp ared with cells from log-phase cultures. Once introduced into soil, CHA0-Ri f cells from log-phase cultures persisted up to 14 d as culturable cells, a nd their population numbers (10(8) CFU (g soil)(-1)) had not declined. The strain was recovered at lower cell numbers at subsequent samplings, regardl ess of the method used for cell counts, and at 48 d about 90% of the cells had lost both their ability to respond to Kogure's viability test and to fo rm a colony on plate. Cells of CHA0-Rif deprived of a single nutrient persi sted similarly to log-phase cells once introduced into soil. In contrast, d eprivation of CHA0-Rif cells for multiple nutrients prior to their introduc tion into soil resulted in the early occurrence (i.e. within hours of soil inoculation) of cells that had lost their colony-forming ability and that d id not respond to Kogure's viability test. This suggests that the lack of a single nutrient (other than C, S, N or P) or deprivation of a combination of several nutrients, under in vitro conditions, had a negative effect on t he subsequent survival of CHA0-Rif in soil. (C) 1999 Elsevier Science Ltd. All rights reserved.