Molecular ecology of Pseudomonas aeruginosa bacteriophages in a freshwater environment

The distribution, molecular characteristics, and host-specific activities of Pseudomonas aeruginosa bacteriophages indigenous to a freshwater habitat were investigated. The occurrence of particulate phage and phage genomic sequences in bacterial populations of environmental samples were evaluated by hybridization reactions. Phage LLPP5, an avirulent P. aeruginosa-specific virus characterized by a contractile tail, an octahedral head, and a double stranded DNA genome of approximately 78 kilobases, was isolated from the water column of Lake Louden. A variety of phage particles specific for P. aeruginosa were recovered from the same habitat by enrichment with an indigenous host strain. The interaction of phage LLPP5 and the heterogenous phage population with the indigenous host strain, P. aeruginosa LLPA10 was studied in lakewater microcosms over a 45 day incubation period. The transduction of heavy metal resistance, antibiotic resistance and auxotrophic requirement between laboratory strains of P. aeruginosa and the lakewater isolate, by means of phage LLPP5 or the heterogenous phage population was investigated.

The distribution of P. aeruginosa phage particles in lakewater fluctuate seasonally with the host population. When present, the detectable phage density is about 4.8 x 10^-2 per ml of lakewater. DNA sequences that are homologous to phage genomes appear to be common in the bacterial population, but their occurrence may not necessarily indicate a wide distribution of lysogenized bacteria. The development of a phage-carrier state in bacteria hosts seems to be enhanced by competition with the natural microbial community. The stability of phage-host interactions is influenced by prevailing multiplicity of infection. However, phage did not reduce host populations below a threshold density of 10⁴ cfu per ml. The transduction of mercury resistance genes was demonstrated for phage LLPP5 at a frequency of 1.67 to 3.89 per 10⁷ plaque forming units (pfu). The transduction of chromosomally encoded streptomycin resistance was observed at a frequency of 2.5 per 10⁸ pfu. The transduction of plasmid encoded antibiotic resistance (pRms149, 55 kb) was not observed for phage LLPP5. However, the heterogenous phage population from lakewater transduced carbenicillin resistance genes that was carried on this plasmid at a frequency of 1.5 per 10⁷ pfu.