Influence of penicillin/amoxicillin non-susceptibility on the activity of third-generation cephalosporins against Streptococcus pneumoniae
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To study the influence of penicillin/amoxicillin non-susceptibility on the activity of third-generation cephalosporins, 430 consecutive penicillin non-susceptible Streptococcus pneumoniae 2007 isolates received in the Spanish Reference Pneumococcal Laboratory were tested. For comparative purposes, 625 penicillin-susceptible 2007 isolates were also tested. Susceptibility was determined by agar dilution using Mueller-Hinton agar supplemented with 5% sheep blood. Penicillin-susceptible strains were susceptible to amoxicillin, cefotaxime and ceftriaxone, 99.8% to cefpodoxime and 99.5% to cefdinir, and were inhibited by 0.12 μg/ml of cefditoren and 4 μg/ml of cefixime. Penicillin-intermediate strains were susceptible to cefotaxime and ceftriaxone, with <50% susceptibility to cefdinir and cefpodoxime. The MIC50 and MIC90 values of cefditoren were 0.25 μg/ml and 0.5 μg/ml, respectively, whereas cefixime exhibited only marginal activity (MIC90=16 μg/ml). Penicillin-resistant strains were resistant to cefdinir and cefpodoxime, with 74.8% and 94.1% susceptibility to cefotaxime and ceftriaxone, respectively. Cefditoren MIC50/MIC90 (0.5/1 μg/ml) were lower than cefotaxime and ceftriaxone. Among amoxicillin non-susceptible strains, susceptibility to cefdinir and cefpodoxime was <10%, and susceptibility to cefotaxime decreased from 87.9% in the intermediate category to 63.0% in the resistant group. Cefditoren MIC50/MIC90 (0.5/1 μg/ml) were lower than cefotaxime. In conclusion, the activity of cefixime, cefdinir and cefpodoxime was highly affected by penicillin/amoxicillin non-susceptibility, while parenteral third-generation cephalosporins exhibited higher intrinsic activity (MIC90=1 μg/ml for penicillin-resistant and 2 μg/ml for amoxicillin-resistant strains). Cefditoren exhibited one-dilution lower MIC90 values for these strains, even against those of the most troublesome serotypes.
KeywordsAmoxicillin Ceftriaxone Cefotaxime Cefixime Cefpodoxime
This study was supported by an unrestricted grant from GlaxoSmithKline S.A., Tres Cantos, Madrid, Spain.
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