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Comparison of the serum sensitivity and antibiotic susceptibility of uropathogenic Escherichia coli strains isolated from different diagnostic groups

Objective: To determine and compare the serum sensitivity, antigenic structure, expression of adhesins and antibiotic susceptibility of uropathogenic Escherichia coli strains isolated from patients with urinary tract infection and asymptomatic bacteriuria. Significance: The bactericidal activity of serum caused by complement system is an important defence mechanism protecting the host organism against infection. The capacity to resist bactericidal activity of normal human serum contributes to the virulence of many gram-negative pathogens. Study design: Ex vivo study Setting: University hospital for infectious diseases Study population: Patients with urinary tract infection or asymptomatic bacteriuria Methods: The capacity of Escherichia coli to resist the bactericidal action of serum was examined in 85 clinical isolates obtained from patients with acute pyelonephritis (n=23), acute cystitis (n=22), chronic pyelonephritis (n=22) and asymptomatic bacteriuria (n=18). Serum sensitivity was also examined in relation to the serogroup specificity and expression of the different adhesins of the strains. Bacterial susceptibility to serum killing was measured by assessing regrowth after incubation in serum according to Schiller and Hatch method, and susceptibility to antimicrobial agents was determined by Kirby-Bauer disk diffusion method. The adhesins of E. coli were determined by hemagglutination and inhibition of hemagglutiation, and serotyping was performed on glass slides and confirmed using a mechanized microtechnique. Results: The significant correlation between serum resistance of uropathogenic strains of E. coli and expression of P-fimbriae and O6 serogroup was observed. P-fimbriated strains were more frequently resistant to tetracycline and carbenicillin than strains in which P-fimbriae were not detected, and they all were resistant to amoxicillin, in contrast with non-P-fimbriated strains (p<0.01). The incidence of serum-resistant E. coli strains was significantly higher in strains isolated from urine of patients with acute pyelonephritis, as compared to strains isolated in other diagnostic groups (p<0.01), which is in accordance with higher virulence and invasive potential of these strains. Conclusion: The observed relationship between P-fimbriae and amoxicillin resistance among strains in which different serogroups were detected suggests the possibility of horizontal gene transfer of these properties.

Urinary tract infections; Escherichia coli; serum sensitivity; antibiotic susceptibility

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