LARGE PSEUDOMONAS PHAGES ISOLATED FROM BARLEY RHIZOSPHERE

Five bacteriophages infecting common fluorescent pseudomonads (Pseudom onas fluorescens and Pseudomonas putida) were isolated from barley rhi zosphere soil. Morphological and molecular characteristics of the phag es are described together with selected phage-host interactions. All p hages belonged to the Myoviridae family with isometrical heads on cont ractile tails; 4 of them were unusually large and had complex protein and DNA profiles. The large phages had estimated genome sizes of 200 k b or more. Restriction enzyme analyses and DNA-DNA hybridizations show ed that all isolates represented different phage species. None of the isolates were observed to establish lysogeny with the main host strain , P. putida MM1. The large phages multiplied slowly on their hosts, pr oducing very small plaques; one-step growth experiments with one of th e large phages (Psp 4) hence demonstrated a long latent period (2.5 h) and a very small burst size (10 particles). One of the large phages ( Psp 3) was abundant in the rhizosphere (approx. 10(4) pfu g(-1) soil) and had a particularly broad host range which extended to both fluores cent (Pseudomonas aeruginosa, P. fluorescens, P. putida and Pseudomona s chlororaphis) and non-fluorescent (Pseudomonas stutzeri) Pseudomonas spp. occurring in soil. The ecological importance of the large Pseudo monas phages must be further studied, but their slow multiplication ra tes suggested a possible mechanism of balanced phage-host co-existence in the rhizosphere.