On the final day of the experiment they took one colony from each sample and sequenced the whole genome to see what genetic changes had occurred. Despite the fact that both chloramphenicol and doxycycline target the ribosome, no mutation to the ribosome was seen. This is not surprising as the ribosome is an important piece of cellular equipment and not to be tampered with! Instead, all the mutations in these bacteria were involved in transport and membrane proteins. The bacteria were acquiring mutations that allowed them to shuttle the antibiotic out of the cell. Not only that but the shuttling mechanism wasn’t hugely specific either; bacteria with chloramphenicol resistance were also resistant to doxycycline.
For trimethoprim on the other hand, all the mutations were found in the region of the protein that the antibiotic affects (the synthesis pathway of an important metabolic enzyme). Trimethoprim resistance therefore was not transferable, bacteria resistant to trimethoprim were unable to survive when challenged with chloramphenicol or doxycycline.
Much more via Discrete steps to antibiotic resistance | Lab Rat, Scientific American Blog Network.
