Experiment

Figure1 Increase of the initial population density enhances system resistance to antibiotics.
In a culture where ampicillin and kanamycin were both added, it required the expresses of the two resistant genes for a strain’s survival. While each one of the strains could constitutively express one resistant gene, they needed the inducers supplied by the other strain to express the other gene required to resist the antibiotics.

As is shown in the images, the population of the cells at high concentration of antibiotics increased dramatically when amplifying the inoculation concentration of the cells, which demonstrates that the initial cell density will affect the robustness of the coculture.

Figure2 the relevant cell density against the inoculating cell density

The initial concentration of ampicillin and kanamycin in the culture determined the least inoculating population of the EG and ER cells.

Figure3 the relevant cell density in varied concentrations of antibiotics.

In different ranges of antibiotic concentrations the system showed quite different behaviors.
In the region(indicated by the red rings) where the concentration of both antibiotics is low, it shows no sign of mutualism between the two strains, instead competition was presented in terms that they fought with each other for food and space. In the region with medium concentrations of antibiotics, which is illustrated by the green rings, the survival of one strain is the prerequisite for that of the other, thus obligatory symbiosis is reformed. In the regions of high concentrations of antibiotics (illustrated by blue rings), no strains will survive whether in monoculture or coculture.