Development of Pseudomonas aeruginosa resistance to norfloxacin during therapy

Two elderly patients diagnosed with Pseudomonas aeruginosa urinary tract infections were treated with oral norfloxacin in the recommended dose of 400 mg q12h. Initially, antimicrobial susceptibility data indicated the organisms were sensitive to norfloxacin. Six to eight days into therapy urine cultures became positive for P. aeruginosa once again; this time, however, susceptibility reports indicated the organisms were now resistant to norfloxacin. Since cross-resistance among norfloxacin, other quinolones, and cephalosporins can occur, we recommend repeated urine cultures during and after norfloxacin therapy in elderly patients with complicated P. aeruginosa urinary tract infections.