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.