The molecular characteristics of E.coli and Klebsiella spp.with CLSI ESBL-screening test positive but cefepime-susceptible / 中华检验医学杂志

ABSTRACT

Objective To study dIe ESBLs and plasmid-mediated AmpC enzymes in E.Coli and Klebsiella spp. with CLSI ESBL-screening test positive,confirmation test negative but cefepime susceptible.Methods Antimierobial susceptibility testing were performed by Kirby-Bauer(K-B)method.The genes encoding ESBLs and plasmid-mediated AmpC enzymes were detected by PCR Transfer of ESBLs or plagmid-mediated AmpC resistance was studied by conjugation experiments.The homology of donor (E.coli),recipient(E.coli J53)and their transconjugants were analyzed by ERIC-PCR DNA fingerprints of E.coli and Klebsiella pneumoniae were analyzed by PFGE as recommended bv PulseNet protocoL Results Of 18 isolates from Huashan Hospital,11 were E.coli.6 were Klebsiella pneumoniae and 1 was Klebsiella oxytoca.Antimicrobial susceptibility testing indicated all of 18 isolates were positive on the CLSI ESBL screening test but negative on the confirmation test.and all of isolates were susceptible to cefepime(a zoneof-inhibition diameter of≥18 mm wag considered to indicate susceptible).PCR results indicated that 9 of the 11 E.coli isolates predued CMY-2 AmpC enzyme.TEM,SHV,CTX-M,PER,VEB or SFO type β-lactamages were not identified.Of 6 Klebsiella pneumoniae isolates.5 were DHA-1 AmpC-producing strains.4 of the 5 DHA-1 AmpC-producing strains were coexistence of broad-speetrumβ-lactamaae or extended-spectrumβ-lactamase.including two producing SHV-11 and two producing CTX-M-14 and SHV-62 type ESBL respectively.One Klebsiella oxytoca wag also DHA-1 AmpC producing strain.Conjugation experiments indicated that both ESBLs and AmpC enzymes could be transfefred from donor to recipient.PFGE indicated that the DNA fingerprints of K.pneumoniae were difierent but seven CMY-2 AmpC-producing E.coli isolates from general surgieal ward were similar.Concluslons The main mechanism of antibiotic resistance in CLSI ESBLs-screening test-positive but eefepime.susceptible E.coli and KIebsiellaspp.is production of plagmid-mediated AmpC enzymes.Some strains produce both AmpC enzyme and ESBLs.Such strains should be reported as resistant to cefepime.The results suggest that laboratories should routinely conduct research on the ESBLs and plnsmid.mediated AmpC enzymes in Enterobacteriaceae in order to report antimicrobial susceptibility testing results more correcdy.