In the experiment attached to this question, there are two different strains of E coli which are resistance to different antibiotics. E coli 1 has a gene that is extracted from chromosomal DNA and which code for resistance to streptomycin. Ecoli 2 has a gene that is extracted from plasmid DNA and which code for resistance to ampicillin.
When these two strains of E coli were mixed together, they produce a new stain of E coli which is resistance to both ampicillin and streptomycin. Because of this, the new strain was able to grow on all the plates which contain these antibiotics.