Occurrence of Extended-Spectrum and AmpC Beta-Lactamases in Bloodstream Isolates of Klebsiella pneumoniae: Isolates Harbor Plasmid-Mediated FOX-5 and ACT-1 AmpC Beta-Lactamases

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Abstract
We tested 190 Klebsiella pneumoniae bloodstream isolates recovered from 189 patients in 30 U.S. hospitals in 23 states to determine the occurrence of extended-spectrum β-lactamase (ESBL) and AmpC β-lactamase producers. Based on growth inhibition by clavulanic acid by disk and MIC test methods, 18 (9.5%) of the isolates produced ESBLs. Although the disk diffusion method with standard breakpoints identified 28 cefoxitin-nonsusceptible isolates, only 5 (18%) of these were confirmed as AmpC producers. Of two AmpC confirmatory tests, the three-dimensional extract test was easier to perform than was the double-disk approximation test using a novel inhibitor, Syn2190. Three of the five AmpC producers carried the blaFOX-5 gene, while the other two isolates harbored the blaACT-1 gene. All AmpC genes were transferable. In vitro susceptibility testing with standard inocula showed that all five AmpC-producing strains were susceptible to cefepime, imipenem, and ertapenem but that with a high inoculum, more of these strains were susceptible to the carbapenems than to cefepime. All but 1 of 14 screen-positive AmpC nonproducers (and ESBL nonproducers) were susceptible to ceftriaxone and cefepime at the standard inoculum as were 6 of 6 isolates that were randomly selected and tested with a high inoculum. These results indicate that (i) a significant number of K. pneumoniae bloodstream isolates harbor ESBL or AmpC β-lactamases, (ii) confirmatory tests are necessary to identify true AmpC producers, and (iii) in vitro, carbapenems are active against AmpC-producing strains of K. pneumoniae.

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