Conjugative (horizontally transmissible) plasmids are autonomous replicators, whose ‘self-interests’ do not necessarily overlap with those of their hosts. This situation causes plasmids and bacteria to sometimes experience differing selection pressures. Escherichia coli plasmid pB15 contains genes for resistance to several antibiotics, including tetracycline. When plasmid-bearing cells were experimentally evolved in the laboratory, changes in resistance level in the unselected tetracycline marker coincided with changes in plasmid rates of vertical versus horizontal transmission. Here we used minimum-inhibitory assays that measure resistance levels as quantitative traits to determine phenotypic correlations among plasmid characters and to estimate divergence among plasmid lineages. Results suggested that plasmid-level evolution led to formation of two phenotypically-dissimilar groups: virulent (highly infectious) and avirulent (weakly infectious) plasmids. In contrast, measures of carbon-source utilization, and fitness assays relative to a common competitor revealed that bacterial hosts generally converged in phenotypic performance, despite divergence among their associated plasmids. Preliminary sequence analyses suggested that divergence in plasmid conjugation was due to altered configurations of a shufflon region (a site-specific recombination system), where genetic rearrangements affect conjugative ability. Furthermore, we proposed that correlated resistance and transmission in pB15 derivatives were caused by a tetracycline-resistance transposon inserted into a transfer operon, allowing transcription from its promoter to simultaneously affect both plasmid resistance and transmission.

Etest TetracyclineKBDD Tetracycline and NeomycinConjugation ratesGlucose and Maltose Fitness