Integrons and Antibiotic Resistance in Phylogenetic Group B2 Escherichia coli

It has been reported that Escherichia coli B2 phylogenetic group strains are more susceptible to antibiotics, especially to quinolones, and tend to carry less integrons than other phylogenetic groups in commensal environments. To gain a better understanding of the relationships between antibiotic resistance, integrons, and phylogenetic groups in an environment with high antibiotic selective pressure, we compared these characteristics in three selected groups of urinary tract infection E. coli isolated in a university hospital (G1, G2, and G3). The isolates were fully susceptible to antibiotics, resistant to amoxicillin and cotrimoxazol, or resistant to amoxicillin, cotrimoxazol, and nalidixic acid in the G1, G2, and G3 group, respectively. The prevalence of B2 isolates was significantly lower in the most resistant G3 group (22.6%) than in susceptible G1 (57.8%, p < 0.001) and G2 groups (50%, p < 0.01). In contrast, the prevalence of B2 isolates was not significantly different between G1 and G2 groups. The prevalence of integrons was nil in G1 isolates but very high in G2 (94.3%) and G3 (87.5%) isolates, and integrons were equally distributed among the phylogenetic groups. We propose a step-by-step mechanism for the emergence of antibiotic resistance in E. coli. Under very low selective pressure, resistance emerges without integrons. When the antibiotic pressure increases, quinolone and integron-mediated resistance occurs outside phylogenetic group B2. With strong antibiotic selective pressure, integrons are highly prevalent and widespread regardless of the phylogenetic group.