Epidemiologic features of urinary infections due to enterobacteriaceae resistant to nalidixic acid and trimethoprim.

For a period of one year we identified all urinary isolates of Enterobacteriaceae resistant to either nalidixic acid (NA) or trimethoprim (TMP). Host and organism characteristics associated with the occurrence of 68 NA and 61 TMP-resistant isolates were compared with 61 matched antimicrobial-susceptible controls. Minimum inhibitory concentrations to NA and TMP were carried out on all isolates, Escherichia coli isolates were biotyped and TMP-sulfamethoxazole (SMX) synergy studies were performed on TMP-resistant isolates. Study patients were reviewed with regard to age, renal function, presence of structural urinary tract abnormality, history of antimicrobial treatment and persistence of study strains within the urinary tract. Resistance to both drugs was associated with prior treatment with the relevant antimicrobial and with underlying urinary tract abnormality. No association was noted between resistance and patient age or renal function. Once resistant organisms infected an abnormal urinary tract, they were able to persist almost indefinitely. Patients with persistent urinary infections associated with structural urinary tract abnormality account for most NA or TMP-resistant infections in our hospital and constitute an identifiable group in whom the therapeutic usefulness of these drugs is limited.