Pharmacological principles in the treatment of urinary tract infections

The ideal antibacterial drug for urinary tract infections should eliminate most urinary pathogens but not alter the colonic flora. Levels in the urine can be attained with the beta-lactam and aminoglycoside antibiotics which are up to several powers of 10 higher than the minimal inhibitory concentration values of the causative organisms. At these levels even penicillin G eliminates many Gram-negative urinary pathogens. Trimethoprim has a lesser propensity to select resistant organisms than most other antimicrobials. It occurs in vaginal secretions and reduces the number of Enterobacteriaceae surrounding the urethral orifice, thereby diminishing the chance of an ascending reinfection, and it is often effective in the treatment of bacterial prostatitis as it reaches therapeutic concentrations in prostatic secretions. Tetracycline therapy, however, carries a substantial chance of bacterial resistance at the next reinfection of the urinary tract, as these antibiotics produce almost uniform resistance in Escherichia coli in the faecal flora. Failure to respond to single-dose therapy actually implies the presence of antibody-coated bacteria in the urine, which in turn implies an upper tract or prostatic infection. This is an easy means of indicating which patients require further evaluation by intravenous pyelography or cystoscopy.