R-plasmid effects on bacterial multiplication and survival.

The multiplication of Escherichia coli C containing either the plasmid R46 or its non-selftransmissible derivative was studied in the presence or absence of the isogenic R- parent strain. Neither plasmid conferred any detectable effect on the host's ability to multiply. Similarly under conditions of prolonged incubation neither plasmid conferred a disadvantage on its host when the bacteria were grown in pure culture. However, when the incubation of mixed R+/R- cultures was prolonged, the possession of either R-plasmid resulted in small but reproducible differences which favoured the R- strain.