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.
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 beta-lactamase (ESBL) and AmpC beta-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 bla(FOX-5) gene, while the other two isolates harbored the bla(ACT-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 beta-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.
Coudron
Philip E
PE
Pathology and Laboratory Medicine Service/113, McGuire Veterans Affairs Medical Center, Richmond, Virginia 23249-0001, USA. Philip.Coudron@med.va.gov
Hanson
Nancy D
ND
Climo
Michael W
MW
D001426:Bacterial Proteins; D006801:Humans; D007711:Klebsiella pneumoniae / Q000201:enzymology* / Q000302:isolation & purification; D010641:Phenotype; D010957:Plasmids / Q000235:genetics; D001618:beta-Lactamases / Q000097:blood / Q000235:genetics / Q000378:metabolism*
D001426:Bacterial Proteins; C444464:beta-lactamase FOX-5; C048795:AmpC beta-lactamases; D001618:beta-Lactamases