Introduction:
The purpose of this study was to determine the relationship between the composition and drug susceptibility of the bacterial flora of urinary tract infections (UTI) and the presence of a congenital urinary tract malformation in children.

Material and methods:
The study included analysis of 515 urine cultures obtained from patients hospitalized at a tertiary referral hospital over a 24-month period. Drug susceptibility of Escherichia coli strains, which are the leading uropathogen in UTI, to antibiotics of the group comprising penicillins, carbapenems, aminoglycosides, fluoroquinolones, and nitrofurantoin and trimethoprim-sulfamethoxazole was determined using the automated Vitek 2 Compact method, the plate-diffusion method and E-tests. The abundance of strains producing an extended-spectrum β-lactamase (ESBL) type resistance mechanism was also analyzed.

Results:
The distribution of cases for Escherichia coli (n = 228) was comparable (p = 0.134) for patients with (40.10%) and without urinary tract defects (46.86%). Comparing the numbers of etiological agents in this group of patients, statistically significant differences were found for infections caused by yeast-like fungi – Candida spp. (p = 0.011) and Pseudomonas aeruginosa (p = 0.002). In terms of Escherichia coli antibiotic resistance, statistically significant differences in their effectiveness were observed for all cephalosporins analyzed, as well as for nitrofurantoin. No such effect was noted for other antibiotics. An extended-spectrum β-lactamase type resistance mechanism was present in Escherichia coli strains isolated from patients with a urinary tract defect – 10.13% of cases, vs. only 2.01% of cases for patients without a malformation (p = 0.016).

Conclusions:
The study showed that the presence of congenital anomalies of the kidney and urinary tract (CAKUT) in children predisposes to Pseudomonas aeruginosa infections, but does not affect the frequency of isolation of Escherichia coli or other strains of bacteria causing urinary tract infections. The presence of CAKUT increases the risk of infection with bacteria with lower sensitivity to the most commonly used first-line antibiotics. Moreover, it increases the risk of Escherichia coli strains with β-lactamase-producing extended substrate spectrum (ESBL+).