In vitro activity of ceftazidime+NXL104 against Pseudomonas aeruginosa and other non-fermenters

NXL104 potentiates ceftazidime and ceftaroline against Enterobacteriaceae with extended-spectrum, AmpC, KPC and OXA beta-lactamases. We examined whether similar potentiation was obtained against non-fermenters, which are less permeable than Enterobacteriaceae and have more potent efflux. MICs of ceftazidime+NXL104 (with NXL104 at 4 mg/L) and comparators were determined by CLSI agar dilution for: (i) Pseudomonas aeruginosa AmpC mutants and extended-spectrum beta-lactamase (ESBL)-producing transconjugants; (ii) clinical P. aeruginosa isolates with AmpC enzymes, ESBLs or up-regulated efflux; (iii) P. aeruginosa and Burkholderia cepacia complex isolates from cystic fibrosis patients; and (iv) Acinetobacter baumannii with OXA carbapenemases, which also compromise ceftazidime. NXL104 reversed AmpC-mediated ceftazidime resistance in P. aeruginosa, reducing MICs for fully derepressed mutants and isolates to < 8 mg/L. NXL104 also reversed ceftazidime resistance caused by the ESBL PER-1, but not that due to OXA ESBLs or VEB-1 enzyme. Efflux-mediated resistance was unaffected. Resistance to ceftazidime in isolates of P. aeruginosa and the B. cepacia complex from patients with cystic fibrosis was variably overcome, generally to greater effect for B. cepacia. NXL104 had little effect on MICs of ceftazidime for A. baumannii isolates with OXA carbapenemases. The potentiation of ceftazidime against many beta-lactamase-producing P. aeruginosa and B. cepacia complex strains confirms that NXL104 penetrates these organisms. The utility of the combination against these pathogens will depend on the local prevalence of strains with beta-lactamase- versus efflux-mediated resistance. The lack of potentiation against A. baumannii may reflect failure of NXL104 to penetrate these bacteria to inhibit relevant (OXA-23, -40, -51 and -58) carbapenemases.