CAN PHAGE DEFENSE MAINTAIN COLICIN PLASMIDS IN ESCHERICHIA-COLI

We examined the role of plasmid-based phage defence in maintaining pla smids, using colicin plasmids in Escherichia coli as a model system. E xperimental data indicated that the possession of a colicin plasmid ca n confer limited protection against bacteriophages. A continuous cultu re model, using these experimental values, indicated that the observed limited protection alone could selectively maintain colicin plasmids, without requiring a competitive advantage due to colicinogeny. Phage defence might explain the current maintenance of colicin plasmids, giv en the naturally occurring high levels of resistance to colicins. This model also suggests that many plasmids might be maintained in natural populations, in part, by phage resistance, including 'cryptic' plasmi ds for which no phenotype is known.