Potentialization of β-lactams with colistin: in case of extended spectrum β-lactamase producing Escherichia coli strains isolated from children with urinary infections

Five strains producing extended-spectrum β-lactamases (ESBL) bacteria, identified as Escherichia coli, were isolated from children with urinary infections hospitalized at Roubaix hospital in the north of France. The DNA genotypes of these non-nosocomial isolates were determined by Random Amplified Polymorphic DNA (RAPD) method. Further, their DNA plasmids content revealed the presence of two distinct plasmids for S1, S2, S3 and one plasmid for S4 and S5. The antibacterial susceptibility of these ESBL bacteria was tested mainly against antibiotics of β-lactams family. The ESBL producing bacteria were resistant to ticarcillin and cefotaxime but the combination of these antibiotics with colistin has dropped the MIC of ticarcillin below its breakpoint (isolates S2, S3 and S4), and has almost reached the breakpoint for cefotaxime (isolate S2). Thus, kill curves analyses carried out with only isolates S1 and S2, strengthened the bactericidal activity of the combinations of colistin-ticarcillin and colistin-cefotaxime against ESBL E. coli. Indeed, reduction of 3 log10 colony count were observed after 24 h of incubation.