There are few published reports of the use of cephamycins (for example, cefoxitin and cephamycin) in the treatment of ESBL producers (55, 285, 374). In one of these reports, selec- tion of porin resistant mutants occurred during therapy, result- ing in cefoxitin resistance and relapse of infection. In addition, combined cephamycin and carbapenem resistance in Klebsiella pneumoniae has been observed in the setting of widespread cephamycin use in response to an outbreak of infection with ESBL-producing organisms (55). Therefore, we are reluctant to recommend cephamycins as first-line therapy for ESBL- producing organisms, despite their good in vitro activity.þ-Lactam/þ-lactamase inhibitor combinations are subject to rising MICs as inoculum rises (392). Additionally, it is well known that ESBL-producing organisms may continue to har- bor parent enyzmes (for example, SHV-1 or TEM-1). Hyper- production of these non-ESBL-producing þ-lactamases (4) or the combination of þ-lactamase production and porin loss can also lead to a reduction in activity of þ-lactamase inhibitors. Some animal studies have shown þ-lactam/þ-lactamase inhib- itor combinations to be less effective than carbapenems against ESBL-producing organisms (187). Stochastic modeling sug- gests that some dosing regimens of piperacillin-tazobactam may not have a high probability of achieving pharmacokinetic/ pharmacodynamic targets which have previously been corre- lated with clinical success (11). Published clinical experience with þ-lactam/þ-lactamase inhibitor combinations in the treat- ment of serious infections due to ESBL producers is limited to just a few patients (65, 95, 124, 293, 299, 310), and for this reason we do not regard þ-lactam/þ-lactamase inhibitor com- binations as suitable first-line therapy for serious infections with ESBL-producing organisms.
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