Presence of antibiotics in the medium where recombinant bacteria are growing makes sure that the bacteria retain the plasmid containing the antibiotic resistant gene (as well as the gene of interest). In the absence of antibiotics, the pressure will be eliminated and the bacteria will not maintain the copy number of the plasmid. Hence, the plasmid will be lost (so as the gene of interest). So, in such a bioreactor the quantity of the desired product will be greatly reduced.