Plasmid transfer between strains of Pseudomonas putida, and their survival, within a pilot scale percolating-filter sewage treatment system

The ability of non-indigenous bacteria to survive and conjugate within a fully functioning pilot-scale percolating filter bed, with a substantial biofilm and a diverse invertebrate community, was investigated. Pseudomonas putida UWC8, harbouring the conjugative plasmid pQKH6, isolated from river epilithon, and P. putida UWC9 were introduced and survived in a culturable state for up to 145 days. During this time plasmid transfer, with transfer frequencies of up to 6.54 × 10−5 cfu per recipient, could be detected. The macro-invertebrate community was also investigated. The invertebrate grazers had no detectable effect upon the densities of the introduced bacteria.