In vitro interaction between cefixime and amoxicillin-clavulanate against extended-spectrum-beta-lactamase-producing Escherichia coli causing urinary tract infection.

We read with interest the article by Campbell et al. (4) suggesting that the combination of clavulanate and oral expanded-spectrum cephalosporins (OESCs) could be an interesting therapeutic option for urinary tract infection (UTI) due to Escherichia coli producing extended-spectrum beta-lactamase (ESBL), for which no or few oral antibiotic compounds are available (4). However, among the OESCs tested in that study, cefdinir and cefpodoxime may not be the best options. Indeed, among the OESCs, cefixime has the lowest MICs and the best pharmacokinetic/pharmacodynamic parameters for E. coli (9, 11). Even if the serum concentrations of all OESCs are low (Cmax, 1.5 to 4.5 g/ml), the serum concentrations of free cefixime are estimated to be greater than the E. coli MIC90 for more than 50% of the time (11). Furthermore, two randomized trials of oral cefixime versus sequential intravenous-oral antibiotic treatment for acute pyelonephritis in children supported the use of this compound for febrile UTI (2, 10). Here we tested in vitro the combination of cefixime and amoxicillin-clavulanate (AC) to assess whether this combination could be useful for UTI due to ESBLproducing E. coli. The isolates tested in this study were 64 previously characterized E. coli strains harboring ESBL obtained from 2009 to 2011. Antimicrobial susceptibility was determined by the disk diffusion method and interpreted according to the guidelines of the Clinical and Laboratory Standards Institute (5). Isolates were screened for ESBL production by the double-disk synergy test with an extended-spectrum cephalosporin (cefotaxime, ceftazidime) and AC (7). Multiplex PCR was used to characterize -lactamase genes, including blaCTX-M, blaSHV, blaTEM, and blaOXA-1, by previously described methods and primers and then sequencing (6). The MICs of cefixime and AC were determined by the Etest method (AB bioMérieux, Solna, Sweden). The Etest has also been used to evaluate the activity of the antimicrobial combination of cefixime and AC: for the evaluation of synergy, one AC strip is placed on an agar plate for 1 h and then removed and a second cefixime strip is placed on top of the gradient of the first agent (1, 3, 12). The MIC of the combination was interpreted as the value at which the inhibition zone intersected the scale on the Etest strip. (Fig. 1). Synergy was evaluated by calculating the fractional inhibitory concentration index (1). Table 1 presents the distribution of MIC50s and MIC90s of cefixime and AC and their combination according to ESBL, TEM-1, and OXA-1 genes. Among the 64 ESBL-producing E. coli isolates, 85% carried CTX-M genes and 15% carried TEM-52 or SHV-12 ESBL genes. With the combination, synergy was obtained for 60 out of 64 strains, additivity was obtained for 2 strains, and indifference was obtained for 2 strains. Results did not differ when cefixime was placed first and AC second. Based on the European Committee on Antibiotic Susceptibility Testing breakpoint of 1 g/ l, 93% of the ESBL-producing E. coli strains were resistant to cefixime alone (8). In contrast, the combination was active against more than 90% of these strains. The MIC50 of the combination was 0.25 g/ml, which was comparable to the MIC50 for non-ESBL-producing E. coli (11). Our study shows that AC can protect the activity of cefixime in vitro and the in vitro Etest can easily detect the synergy of these agents, which is practical for routine use in clinical laboratories. A simple approach would be to determine the MIC of the combina-