Deleterious Impact of a Virulent Bacteriophage on Survival and Biocontrol Activity of Pseudomonas fluorescens Strain CHA0 in Natural Soil

Abstract

Many biotic and abiotic factors affect the persistence and activity of beneficial pseudomonads introduced into soil to suppress plant diseases. One such factor may be the presence of virulent bacteriophages that decimate the population of the introduced bacteria, thereby reducing their beneficial effect. We have isolated a lytic bacteriophage (phi)GP100) that specifically infects the biocontrol bacterium Pseudomonas fluorescens CHA0 and some closely related Pseudomonas strains. phiGP100 was found to be a double-stranded-DNA phage with an icosahedral head, a stubby tail, and a genome size of approximately 50 kb. Replication of phiGP100 was negatively affected at temperatures higher than 25 degrees C. phiGP100 had a negative impact on the population size and the biocontrol activity of P. fluorescens strain CHA0-Rif (a rifampicin-resistant variant of CHA0) in natural soil microcosms. In the presence of phiGP100, the population size of strain CHA0-Rif in soil and on cucumber roots was reduced more than 100-fold. As a consequence, the bacterium's capacity to protect cucumber against a root disease caused by the pathogenic oomycete Pythium ultimum was entirely abolished. In contrast, the phage affected neither root colonization and nor the disease suppressive effect of a phiDGP100-resistant variant of strain CHA0-Rif. To our knowledge, this study is the first to illustrate the potential of phages to impair biocontrol performance of beneficial bacteria released into the natural soil environment.

Metagenomics: Application of Genomics to Uncultured Microorganisms

“…Historically, genetics has provided the most convincing evidence for traits contributing to microbial fitness. Classic mutant analysis has revealed genes required for microbial competition (9,15,20,27,82,98,100,140,143), antagonism (40,51,77,117,130), and mutualism (10,54,86,92). Mutant analyses have provided the greatest advances in knowledge because screening mutants containing random mutations for effects on fitness has led to the identification of genes that would not have been predicted to play a role in microbial competition or mutualism.…”

Bacterial Subfamily of LuxR Regulators That Respond to Plant Compounds

“…Arina) were surface sterilized (18 min in 5% NaClO [vol/vol]), rinsed with sterile water, and germinated on soft agar (Oxoid technical agar at 8.5 g/liter for 90 h at 20°C in the dark). For the biocontrol assays, 200-ml Erlenmeyer flasks with wide openings were partially filled with 60 g of natural sandy loam soil from Eschikon, Switzerland (23). When appropriate, the soil was artificially infested with 2.5 g (cucumber) or 18.75 g (wheat) of a 5-day-old millet-seed inoculum of Pythium ultimum isolate 67-1/kg (23).…”

“…For the biocontrol assays, 200-ml Erlenmeyer flasks with wide openings were partially filled with 60 g of natural sandy loam soil from Eschikon, Switzerland (23). When appropriate, the soil was artificially infested with 2.5 g (cucumber) or 18.75 g (wheat) of a 5-day-old millet-seed inoculum of Pythium ultimum isolate 67-1/kg (23). Three sterile-grown seedlings were then placed in each flask.…”

Sampling Natural Viral Communities from Soil for Culture-Independent Analyses

“…Furthermore, the dynamics of the phage-host population tended toward equilibrium, with bacteria outnumbering phages. More recently, Keel et al (30) pointed out the overlooked potential of phage activity in soil when they found that the efficacy of the widely used biocontrol bacterium Pseudomonas fluorescens CHA0 was completely eradicated by an epidemic of autochthonous phage in the rhizosphere of cucumbers.…”