Some of the organisms (in most cases bacteria) responsible are often found to be normal intestinal or fecal flora, such as E. coli and klebsiella, or mycoplasma found in the reproductive system.
Enrofloxacin is generally the best first-line antibiotic for UTIs in rats due to its broad activity and urinary tract penetration. However, choosing the best antibiotic depends on the causative organism, site of infection, and antibiotic sensitivity. Antibiotics like Trimethoprim-sulfamethoxazole, Doxycycline can also be used. And please note there is nothing universally best, antibiotic should be situation, target and dosage specific.