Gene transfer and plasmid instability within pilot-scale sewage filter beds and the invertebrates that live in them.

The environmental plasmid pQKH6 was transferred conjugatively between strains of Pseudomonas putida at mean frequencies of up to 8.4x10(-4) within pilot-scale sewage filter beds. This frequency was 10-fold higher than that reported previously for this environment and was probably due to seasonal temperature changes. Many (45%) of the plasmids isolated subsequently from the filter beds had restriction fragment length polymorphism (RFLP) profiles that differed from that expected for pQKH6. RFLP analysis revealed structural rearrangements occurring within a particularly restriction-site-rich region of the plasmid. Although no evidence was obtained showing the indigenous invertebrate populations within the filter beds to influence the rate of gene transfer, pQKH6 was transferred with frequencies of up to 1.6x10(-2) within the guts of the filter-bed-dwelling Sylvicola fenestralis larvae during laboratory experiments. This transfer was strongly influenced by donor to recipient ratios. Laboratory experiments also showed that Serratia fonticola survived better within invertebrate guts than P. putida. This evidence, along with experiments showing that S. fonticola could participate in pQKH6 transfer within filter-bed biofilm, identify this bacterium as a better model than P. putida for examining the effect of invertebrates on gene transfer.

Seasonal population dynamics and interactions of competing bacteriophages and their host in the rhizosphere.

We describe two prolonged bacteriophage blooms within sugar beet rhizospheres ensuing from an artificial increase in numbers of an indigenous soil bacterium. Further, we provide evidence of in situ competition between these phages. This is the first in situ demonstration of such microbial interactions in soil. To achieve this, sugar beet seeds were inoculated with Serratia liquefaciens CP6RS or its lysogen, CP6RS-ly-phi 1. These were sown, along with uninoculated seeds, in 36 field plots arranged in a randomized Latin square. The plots were then sampled regularly over 194 days, and the plants were assayed for the released bacteria and any infectious phages. Both the lysogen and nonlysogen forms of CP6RS survived equally well in situ, contradicting earlier work suggesting lysogens have a competitive disadvantage in nature. A Podoviridae phage, identified as phi CP6-4, flourished on the nonlysogen-inoculated plants in contrast to those plants inoculated with the lysogen. Conversely, the Siphoviridae phage phi CP6-1 (used to construct the released lysogen) was isolated abundantly from the lysogen-treated plants but almost never on the nonlysogen-inoculated plants. The uninoculated plants also harbored some phi CP6-1 phage up to day 137, yet hardly any phi CP6-4 phages were found, and this was consistent with previous years. We show that the different temporal and spatial distributions of these two physiologically distinct phages can be explained by application of optimal foraging theory to phage ecology. This is the first time that such in situ evidence has been provided in support of this theoretical model.