Annals of Clinical Microbiology, The official Journal of the Korean Society of Clinical Microbiology


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Differences in Antimicrobial Resistance Phenotypes by the Group of CTX-M Extended-Spectrum β-Lactamase

Original article

Annals of Clinical Microbiology (Ann Clin Microbiol) 2019 March, Volume 22, Issue 1, Pages 1-8.

Differences in Antimicrobial Resistance Phenotypes by the Group of CTX-M Extended-Spectrum β-Lactamase

Bareum Gwon1,2, Eun-Jeong Yoon2, Dokyun Kim2, Hyukmin Lee2, Jong Hee Shin3, Jeong Hwan Shin4, Kyeong Seob Shin5, Young Ah Kim6, Young Uh7, Hyun Soo Kim8, Young Ree Kim9, Seok Hoon Jeong2

1Department of Clinical Pathology, Sangji University College of Science, Wonju, 2Department of Laboratory Medicine and Research Institute of Bacterial Resistance, Yonsei University College of Medicine, Seoul, 3Department of Laboratory Medicine, Chonnam National University School of Medicine, Gwangju, 4Department of Laboratory Medicine, Inje University Busan Paik Hospital, Busan, 5Department of Laboratory Medicine, College of Medicine, Chungbuk National University, Cheongju, 6Department of Laboratory Medicine, National Health Insurance Service Ilsan Hospital, Goyang, 7Department of Laboratory Medicine, Wonju Severance Christian Hospital, Yonsei University Wonju College of Medicine, Wonju, 8Department of Laboratory Medicine, Hallym University College of Medicine, Hwaseong, 9Department of Laboratory Medicine, School of Medicine, Jeju National University, Jeju, Korea


Background: Escherichia coli and Klebsiella pneumoniae clinical isolates producing CTX-M extended- spectrum β-lactamases (ESBLs) were assessed for antimicrobial resistance phenotypes varied by group of enzymes.

Methods: A total of 1,338 blood isolates, including 959 E. coli and 379 K. pneumoniae, were studied. All the strains were collected between January and July 2017 from eight general hospitals in South Korea. The species were identified by matrix-assisted laser desorption ionization-time of flight mass spectrometry. Antimicrobial susceptibilities were determined by disk diffusion methods and ESBL phenotypes by double-disk synergy tests using disks containing cefotaxime, ceftazidime, cefepime, aztreonam, and clavulanic acid (CA). The genes for β-lactamases were identified by PCR and sequencing.

Results: Of total microbes, 31.6% (303/959) E. coli and 24.0% (91/379) K. pneumoniae were resistant to cefotaxime and 28.1% (269/959) E. coli and 20.1% (76/379) K. pneumoniae were CTX-M-type ESBL producers. Among the detected CTX-M ESBLs, 58.0% (156/269) in E. coli and 86.8% (66/76) in K. pneumoniae belonged to group 1, 46.8% (126/269) in E. coli and 14.5% (11/76) in K. pneumoniae were group 9. Ten E. coli and one K. pneumoniae isolates co-produced both groups of CTX-M ESBL. The group 1 CTX-M producers had a higher level of resistance to cefotaxime, ceftazidime, cefepime, and aztreonam and exhibited stronger synergistic activities when combined with CA compared to group 9.

Conclusion: ESBL phenotypes differ by CTX-M ESBL group and phenotype testing with drugs including 4th generation cephalosporins and monobactams is critical for screening CTX-M-producers with better sensitivity. (Ann Clin Microbiol 2019;22:1-8)


CTX-M, Escherichia coli, Extended-spectrum β-lactamase, Klebsiella pneumoniae