Although antibacterial therapy has an impact on human intestinal flora and the emergence of resistant bacteria, its role in the amplification of antimicrobial resistance and the quantitative exposure-effect relationship is not clear. An observational prospective study was conducted to determine whether and how ceftriaxone exposure is related to amplification of resistance in non-intensive care unit (non-ICU) patients. Serial stool samples from 122 extended-spectrum -lactamase-positive (ESBL) hospitalized patients were analyzed by quantitative real-time PCR to quantify the resistant gene blaCTX-M. Drug exposure was calculated for each patient by using a population pharmacokinetic model. Multi- and univariate regression and classification regression tree (CART) analyses were used to explore relationships between measures of exposure and amplification of blaCTX-M genes. Amplification of blaCTX-M was observed in 0% (0/11) of patients with no treatment and 33% (20/61) of patients treated with ceftriaxone. Stepwise regression analysis showed a significant association between amplification of blaCTX-M and the plasma area under the concentration-time curve from 0 to 24 h for the unbound fraction of the drug (fAUC0–24), the maximum concentration of drug in serum for the unbound fraction of the drug (fCmax), and the duration of ceftriaxone therapy. Using CART analysis, amplification of blaCTX-M was observed in 11/16 (69%) patients treated for 14 days and in 9/40 (23%) patients treated for 14 days (P 0.0019). In the latter group, amplification was observed in 5/7 (71%) patients with an fAUC0–24 of 222 mg · h/liter and in 4/33 (12%) patients with lower drug exposures (P 0.0033). A similar association was found for an fCmax of 30 mg/liter (63% versus 13%, P 0.0079). A significant association was found between the amplification of blaCTX-M resistance genes and exposure to ceftriaxone. Both duration of treatment and degree of ceftriaxone exposure have a significant impact on the amplification of resistance genes. (The project described in this paper has been registered at under identifier NCT01208519.)

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Keywords Ceftriaxone, Pharmacodynamics, Pharmacokinetics, Resistance
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Journal Antimicrobial Agents and Chemotherapy
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Meletiadis, J, Turlej-Rogacka, A. (A.), Lerner, A. (A.), Adler, A. (A.), Taconelli, E. (E.), & Mouton, J.W. (2017). Amplification of antimicrobial resistance in gut flora of patients treated with ceftriaxone. Antimicrobial Agents and Chemotherapy, 61(11). doi:10.1128/AAC.00473-17