Genetic analysis of a plasmid-encoded, host genotype-specific enhancement of bacterial fitness.

In the absence of antibiotics, carriage of pACYC184 reduces the competitive fitness of an Escherichia coli B genotype that was not previously selected for plasmid carriage, relative to that of an isogenic plasmid-free competitor. However, a host genotype propagated with the plasmid for 500 generations evolved an unexpected competitive advantage from plasmid carriage, relative to its own isogenic plasmid-free segregant. We manipulated the pACYC184 genome in order to identify the plasmid-encoded function that was required for the enhancement of the coevolved host genotype's competitive fitness. Inactivation of the plasmid-encoded tetracycline resistance gene, by deletion of either the promoter region or the entire gene, eliminated the beneficial effect of plasmid carriage for the coevolved host. This beneficial effect for the coevolved host was also manifest with pBR322, which contains a tetracycline resistance gene identical to that of pACYC184 but is otherwise heterologous.