Blackwell Publishing Journal Backfiles 1879-2005
Abstract: The possibilities of mobilizing recombinant DNA (rDNA) by strains from activated sludge have been studied. A mixture of ten bacterial strains isolated from activated sludge was grown on a support in a continuously fed fixed-bed reactor or in suspension in a sequenced-batch reactor with biomass recycling. After reaching steady state, the bacteria in both types of reactors were used as the recipient population for evaluating the dissemination of the non-conjugative and non-mobilizable recombinant plasmid pCE328. The dissemination of this plasmid, derived from pBR328 (tra−, mob−, oriT−), was studied in comparison with that of pCE325 (pBR325 derivative tra−, mob−, oriT+), and that of the natural broad host range conjugative plasmid R388 (IncW). Initially the mobilization properties of pBR type recombinant plasmids between two strains of Escherichia coli were determined in the fixed-bed reactor. Only the mobilization of the recombinant plasmid bearing a transfer origin (pCE325) was observed. The survival of E. coli strains bearing either the non-transferable plasmid pCE328 or the conjugative plasmid R388 and added to the activated sludge reactors was measured in the effluent of both types of reactors. In the fixed-bed reactor effluent, the donor populations remained stable after three days of operation and reached 103 to 105 colony forming units (cfu) ml−1, whereas they decreased in the sequenced-batch reactors: donors bearing the pCE328 plasmid decreased by 2 log units in 20 days, whereas those bearing the R388 plasmid decreased by less than 1 log unit. Mobilization of the pCE328 plasmid could not be detected in any case. Transfer of the conjugative plasmid R388, used as a positive propagation control, was detected in strains arising from activated sludge in both types of reactors. Nevertheless, establishment of transconjugants was observed only in the fixed biomass at the rate of 10−7 transconjugants per recipient.
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