Fig. 1: Design of the evolution experiment.
In a previous study, serial coevolution of two bacterial genotypes of Escherichia coli, which were auxotrophic for the two amino acids tryptophan (ΔtrpB, red cells) or tyrosine (ΔtyrA, blue cells), had resulted in the evolution of obligate mutualistic cooperation between both cell types27. One of these mutualistic consortia was used in the current study. The two strains were either grown together in coculture (minimal medium) or in individual monocultures (minimal medium + required amino acid, 100 µM each, red circle, blue triangle). Initial populations, which were sensitive to the four different antibiotics ampicillin, kanamycin, chloramphenicol and tetracycline, were serially propagated for 15 transfers, during which the concentration of these four antibiotics was gradually increased.
