What would happen when you grow a recombinant in a bioreactor but forget to add antibiotic to the medium in which the recombinant is growing.

In the absence of antibiotic, there will be no pressure on recombinants to retain the plasmid (containing the gene of your interest). Since, intaining a high copy number of plasmids is a metabolic burden to the microbial ells, will thus tend to lose the plasmid.