SURVIVAL STRATEGIES OF PLASMID-CARRIER AND PLASMIDLESS ESCHERICHIA-COLI STRAINS UNDER ILLUMINATED AND NONILLUMINATED CONDITIONS, IN A FRESH-WATER ECOSYSTEM

A comparative study, in illuminated and non-illuminated systems, was made to determine the survival strategies of plasmid-carrier and plasmidless bacteria in sterile river water. Two strains of Escherichia coli from river water were selected: one plasmidless, EC1, and one antibiotic-resistant strain, EC7, which showed plasmid bands. By matings with EC7 as donor and E. coli K12 strain J62 as recipient, transconjugants were generated, the J62(7) strain, which showed both antibiotic resistance and plasmid bands. Ethidium bromide curing of the EC7 strain generated the EC7(2) strain which showed a partial loss of resistance and a reorganization of plasmid bands. Under non-illuminated conditions the total number of cells detected by direct count and the number of culturable cells (injured and non-injured cells) remained practically constant throughout the period of incubation. In the illuminated systems, however, the number of cfu decreased in four of the five strains studied. The greatest decreases are those of the J62 strain, followed by those of the J62(7), EC1, EC7(2) and EC7 strains. Differences in survival strategies as a consequence of the presence or absence of plasmids are discussed.