Efficacy of amoxycillin–clavulanate in an experimental model of murine pneumonia caused by AmpC-non-hyperproducing clinical isolates of Escherichia coli resistant to cefoxitin

ABSTRACTThe algorithms included in most automated systems used for antimicrobial susceptibility testing (e.g., Vitek 2) consider that Escherichia coli isolates resistant to cefoxitin are AmpC-hyperproducers and, consequently, resistant also to amoxycillin–clavulanate. However, a recent study revealed that 30% of E. coli clinical isolates resistant to cefoxitin remained susceptible in vitro to amoxycillin–clavulanate. The aim of the present study was to evaluate the in-vivo efficacy of amoxycillin–clavulanate in the treatment of an experimental model of pneumonia, using two clonally related isolates (with identical repetitive extragenic palindromic sequence (REP)-PCR patterns) of AmpC-non-hyperproducing and OmpF-lacking E. coli (Ec985 and Ec571) that were resistant to cefoxitin and susceptible to cefotaxime and amoxycillin–clavulanate. MICs were determined using a microdilution technique, and in-vitro bactericidal activity was tested using time-kill assays. The in-vivo efficacy of amoxycillin, amoxycillin–clavulanate and cefotaxime against both isolates was tested in a murine pneumonia model using immunocompetent C57BL/6 mice. Ec571 (a TEM-1/2 producer) was resistant to amoxycillin, whereas Ec985 (a TEM-1/2 non-producer) was susceptible. Amoxycillin, amoxycillin–clavulanate and cefotaxime were bactericidal for Ec985, and amoxycillin–clavulanate and cefotaxime were bactericidal for Ec571 at different concentrations and time-points, as determined using time-kill assays. Treatment with amoxycillin, amoxycillin–clavulanate and cefotaxime reduced the bacterial lung concentration of Ec985 compared with non-treated controls (p <0.05), whereas amoxycillin–clavulanate and cefotaxime showed efficacy against Ec571 when compared with the control and amoxycillin groups (p <0.05). Regardless of the exact underlying mechanism(s) of resistance, amoxycillin–clavulanate was effective in the experimental murine model in the treatment of pneumonia caused by AmpC-non-hyperproducing strains of E. coli resistant to cefoxitin.