Mode of Action
Tetracyclines (there are many) enter the cell by diffusion through OmpF porins. It inhibits growth by preventing codon-anticodon interactions during translation.
There are a number of resistance mechanisms, most require tetracycline antiporters. The most commonly used is the tetA(C) gene. The gene product of tetA(C) forms a pore in the inner membrane and exports one tet molecule for every proton imported. Molecular Cloning states that tet resistance has a number of adverse affects on the cell, chief among those being a reduction in growth rate and viability.
Using a plasmid with copy number 15-20, 20ug/ml tet has proved a successful selector. The reduction in growth rate, described above was significant.