A 45-year-old man with diabetes and recurrent urinary tract infections presents with fever, flank pain, and dysuria. Urine culture yields a lactose-fermenting, oxidase-negative organism. Gram stain shows Gram-negative rods. The organism is positive for indole, methyl red, and negative for Voges-Proskauer. Which is the most appropriate next investigation to confirm uropathogenic E. coli (UPEC) and guide antibiotic selection?