Figure 1.

Fitness of plasmid-carrying strains. (a) Fitness (±s.e.) of PAO1 carrying one and two plasmids, relative to the plasmid-free ancestor, P. aeruginosa PAO1. The horizontal dotted red line represents the fitness of PAO1. Vertical dotted black lines separate each pair of strains carrying a large plasmid alone and combined with the small plasmid pNI105. (b) Positive epistasis between large and small plasmids. Comparison of the relative fitness of P. aeruginosa PAO1 carrying large and both large and small plasmids. The blue line represents the expected fitness if the fitness cost produced by large and small plasmids is multiplicative. The red line represents the expected fitness if the small plasmid (pNI105) produces no extra cost. Error bars represent standard error. The points above the blue line represent associations of plasmids showing positive epistasis.