INTERACTIONS BETWEEN A GENETICALLY MARKED PSEUDOMONAS-FLUORESCENS STRAIN AND BACTERIOPHAGE PHI-R2F IN SOIL - EFFECTS OF NUTRIENTS, ALGINATEENCAPSULATION, AND THE WHEAT RHIZOSPHERE

The introduction of bacteriophages could potentially be used as a cont rol method to limit the population size of engineered bacteria that ha ve been introduced into soil. Hence, the ability of a species-specific phage, Phi R2f, to infect and lyse its host, a Pseudomonas fluorescen s R2f transposon Tn5 derivative, in soil, was studied. Control experim ents in liquid media revealed that productive lysis of host cells by p hage Phi R2f occurred when cells were freely suspended, whereas cells present in alginate beads resisted lysis. The presence of nutrients en hanced the degree of lysis as well as the production of phage progeny, both with the suspended cells and with cells escaped from the alginat e beads. Experiments in which host cells and phage Phi R2f were introd uced into two soils of different texture revealed that host cells were primarily lysed in the presence of added nutrients, and phage reached highest titres in these nutrient-amended soils. Encapsulation of the host cells in alginate beads inhibited lysis by the phage in soil. Pop ulations of free host cells introduced into soil that colonized the rh izosphere of wheat were not substantially lysed by phage Phi R2f. Howe ver, P. fluorescens R2f populations colonizing the rhizosphere after i ntroduction in alginate beads were reduced in size by a factor of 1,00 0. Cells migrating from the alginate beads towards the roots may have been in a state of enhanced metabolic activity, allowing for phage Phi R2f infection and cell lysis.