Survival and cell culturability of biocontrol Pseudomonas fluorescens CHAOin the rhizosphere of cucumber grown in two soils of contrasting fertilitystatus

The effect of cucumber roots on survival patterns of the biocontrol soil in oculant Pseudomonas fluorescens CHA0-Rif was assessed for 22 days in two no n-sterile soils, using a combination of total immunofluorescence cell count s, Kogure's direct viable counts and colony counts on plates containing rif ampicin. In Eschikon soil (high fertility status for cucumber), CHA0-Rif pe rsisted as culturable cells in bulk soil and in the rhizosphere, but colony counts were lower than viable counts and total cell counts inside root tis sues. The occurrence of viable but non-culturable (VBNC) cells inside root tissues (5 log cells g(-1) root) was unlikely to have resulted from the hyd rogen peroxide treatment used to disinfect the root surface, as hydrogen pe roxide caused the death of CHA0-Rif cells in vitro. In Siglistorf soil (low fertility status for cucumber), the inoculant was found mostly as non-cult urable cells. Colony counts and viable counts of CHA0-Rif were similar, bot h in bulk soil and inside root tissues, whereas in the rhizosphere viable c ounts exceeded colony counts at the last two samplings (giving about 7 log VBNC cells g-l). In conclusion, soil type had a significant influence on th e occurrence of VBNC cells of CHA0-Rif, although these cells were found in root-associated habitats (i.e. rhizosphere and root tissues) and not in bul k soil.