This study is being conducted to evaluate the safety, efficacy (clinical and microbiological), pharmacokinetics/pharmacodynamics of GSK2251052 and to assess whether it would be a suitable antibiotic for the treatment for febrile lower cUTI and pyelonephritis(complicated and uncomplicated). GSK2251052 will be compared to imipenem-cilastatin, which is an antibiotic commonly used to treat serious cUTI infections. GSK2251052 has a spectrum of microbiological activity that includes pathogens responsible for cUTI.