Antibiotic resistance is an increasing global public health concern that is limiting our ability to treat bacterial infections. Current problems of antibiotic resistance result from a reduced development of truly novel antibiotics over that past several decades. Yet, during this time bacteria have continued to evolve resistance in response to treatment. A long term solution to the antibiotic resistance problem must involve both improved prudence in antibiotic consumption as well as development of novel types of antibiotics. Yet, such efforts will take political will and time to develop. In the meantime, we must use the antibiotics we have at our disposal in the most efficient way possible. Recently, it has been discovered that evolution of antibiotic resistance in bacteria can be associated with increased sensitivity towards other antibiotics. This is referred to as collateral sensitivity. The current project is focused on elucidating the collateral sensitivities of antibiotic resistant bacteria belonging to the important human pathogens E.coli and P.aeruginosa. Through a thorough characterisation of the molecular mechanisms and evolutionary trajectories that lead to collateral sensitivity and antibiotic resistance we shall develop strategies to rationally treat these infections. These results will serve to improve the use of the current arsenal of drugs for the benefit of patients suffering from multidrug resistant infections.