Why did the mix of E. coli 1 and E. coli 2 grow on the S and A petri dish when neither strain grew alone?

The growth of E. coli 1 and E. coli 2 on the S and A petri dish can be attributed to metabolic cooperation. One strain may produce a metabolite that the other strain can utilize, leading to growth that neither strain could achieve alone. This mutualistic interaction can enhance the overall fitness of the mixed population.