{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,10,21]],"date-time":"2026-10-21T13:42:02Z","timestamp":1792590122328,"version":"4.5.0"},"reference-count":23,"publisher":"American Society for Microbiology","license":[{"start":{"date-parts":[[2026,10,21]],"date-time":"2026-10-21T00:00:00Z","timestamp":1792540800000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"},{"start":{"date-parts":[[2026,10,21]],"date-time":"2026-10-21T00:00:00Z","timestamp":1792540800000},"content-version":"tdm","delay-in-days":0,"URL":"https:\/\/journals.asm.org\/non-commercial-tdm-license"}],"funder":[{"name":"Privatdozenten-Stiftung of the University of Zurich, Zurich, Switzerland","award":["PD-Stiftung [no specific number]"],"award-info":[{"award-number":["PD-Stiftung [no specific number]"]}]}],"content-domain":{"domain":["journals.asm.org"],"crossmark-restriction":true},"short-container-title":["Microbiol Spectr"],"abstract":"<jats:title>ABSTRACT<\/jats:title>\n                  <jats:sec>\n                    <jats:title\/>\n                    <jats:p>\n                      Accurate discrimination between extended-spectrum \u03b2-lactamase (ESBL) production and hyperproduction of the intrinsic OXY-\u03b2-lactamase (hyperOXY) in the\n                      <jats:italic toggle=\"yes\">Klebsiella oxytoca<\/jats:italic>\n                      complex remains challenging, as both mechanisms can generate overlapping resistance phenotypes and lead to unnecessary confirmatory testing or misclassification. We analyzed a collection of 123 whole-genome-sequenced clinical\n                      <jats:italic toggle=\"yes\">K. oxytoca<\/jats:italic>\n                      complex isolates (2009\u20132024), including 33 ESBL producers and multiple species-specific\n                      <jats:italic toggle=\"yes\">bla<\/jats:italic>\n                      <jats:sub>OXY<\/jats:sub>\n                      variants, to develop a phenotypic strategy for distinguishing these resistance mechanisms using disc diffusion data. We performed antimicrobial susceptibility testing according to EUCAST guidelines and correlated phenotypic findings with genomic resistance determinants. HyperOXY isolates harbored classical activating mutations in\n                      <jats:italic toggle=\"yes\">bla<\/jats:italic>\n                      <jats:sub>OXY<\/jats:sub>\n                      promoter regions and displayed a characteristic resistance pattern marked by high-level resistance to amoxicillin\u2013clavulanic acid and piperacillin\u2013tazobactam, resistance to ceftriaxone and cefpodoxime, preserved or borderline susceptibility to ceftazidime, and variable reductions in cefepime susceptibility. In contrast, ESBL-producing isolates showed reproducible clavulanate synergy and distinct inhibition zone distributions for piperacillin\u2013tazobactam, cefpodoxime, and gentamicin. A cefpodoxime inhibition zone diameter &lt;17 mm identified all ESBL-producing isolates (sensitivity 100%) but showed limited specificity (44%). Combining cefpodoxime &lt;17 mm with additional phenotypic criteria allowed confident ESBL attribution without additional testing (specificity 100%), albeit at reduced sensitivity (72%). EUCAST-recommended combination disc ESBL confirmatory testing using cefotaxime or cefepime with clavulanic acid demonstrated the highest diagnostic performance (sensitivity\/specificity \u2265 96%). Thirty-three isolates showed ambiguous phenotypes requiring confirmatory testing. Overall, interpretative reading of routine disc diffusion patterns can reliably classify characteristic hyperOXY and ESBL phenotypes and restrict additional confirmatory testing to ambiguous isolates, thereby improving diagnostic efficiency while maintaining high accuracy.\n                    <\/jats:p>\n                    <jats:sec>\n                      <jats:title>IMPORTANCE<\/jats:title>\n                      <jats:p>\n                        Members of the\n                        <jats:italic toggle=\"yes\">K. oxytoca<\/jats:italic>\n                        complex possess an intrinsic OXY \u03b2-lactamase that can be overexpressed and generate resistance patterns closely resembling those of extended-spectrum \u03b2-lactamase (ESBL) producers. This phenotypic overlap complicates routine antimicrobial susceptibility testing and frequently leads to unnecessary ESBL confirmatory testing or misclassification, with potential consequences for patient management and infection control. Using a large, genomically characterized strain collection, this study defines reproducible disc diffusion patterns that distinguish hyperOXY-mediated resistance from true ESBL production and evaluates screening and confirmatory strategies based on routinely tested antibiotics. By providing practical, phenotype-based guidance aligned with EUCAST methodology, our findings help clinical laboratories improve diagnostic accuracy, streamline workflows, and avoid ESBL-misidentification in\n                        <jats:italic toggle=\"yes\">K. oxytoca<\/jats:italic>\n                        complex isolates.\n                      <\/jats:p>\n                    <\/jats:sec>\n                  <\/jats:sec>","DOI":"10.1128\/spectrum.01327-26","type":"journal-article","created":{"date-parts":[[2026,10,21]],"date-time":"2026-10-21T13:01:27Z","timestamp":1792587687000},"update-policy":"https:\/\/doi.org\/10.1128\/asmj-crossmark-policy-page","source":"Crossref","is-referenced-by-count":0,"title":["Improved disc diffusion-based detection of extended spectrum \u03b2-lactamase production in the\n                    <i>Klebsiella oxytoca<\/i>\n                    complex"],"prefix":"10.1128","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-4516-5952","authenticated-orcid":true,"given":"Stefano","family":"Mancini","sequence":"first","affiliation":[{"id":[{"id":"https:\/\/ror.org\/02crff812","id-type":"ROR","asserted-by":"publisher"}],"name":"Institute of Medical Microbiology, University of Zurich","place":["Zurich, Switzerland"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Florian","family":"Tribelhorn","sequence":"additional","affiliation":[{"id":[{"id":"https:\/\/ror.org\/02crff812","id-type":"ROR","asserted-by":"publisher"}],"name":"Institute of Medical Microbiology, University of Zurich","place":["Zurich, Switzerland"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0003-3435-4723","authenticated-orcid":true,"given":"Tim","family":"Roloff","sequence":"additional","affiliation":[{"id":[{"id":"https:\/\/ror.org\/02crff812","id-type":"ROR","asserted-by":"publisher"}],"name":"Institute of Medical Microbiology, University of Zurich","place":["Zurich, Switzerland"]},{"id":[{"id":"https:\/\/ror.org\/002n09z45","id-type":"ROR","asserted-by":"publisher"}],"name":"SIB Swiss Institute of Bioinformatics","place":["Lausanne, Switzerland"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-6082-5114","authenticated-orcid":true,"given":"Helena M. B.","family":"Seth-Smith","sequence":"additional","affiliation":[{"id":[{"id":"https:\/\/ror.org\/02crff812","id-type":"ROR","asserted-by":"publisher"}],"name":"Institute of Medical Microbiology, University of Zurich","place":["Zurich, Switzerland"]},{"id":[{"id":"https:\/\/ror.org\/002n09z45","id-type":"ROR","asserted-by":"publisher"}],"name":"SIB Swiss Institute of Bioinformatics","place":["Lausanne, Switzerland"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Natalia","family":"Kolesnik-Goldmann","sequence":"additional","affiliation":[{"id":[{"id":"https:\/\/ror.org\/02crff812","id-type":"ROR","asserted-by":"publisher"}],"name":"Institute of Medical Microbiology, University of Zurich","place":["Zurich, Switzerland"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Ramon M.","family":"Eichenberger","sequence":"additional","affiliation":[{"id":[{"id":"https:\/\/ror.org\/05pmsvm27","id-type":"ROR","asserted-by":"publisher"}],"name":"Medical Micro- & Molecular Biology, Institute of Chemistry and Biotechnology, Zurich University of Applied Sciences (ZHAW)","place":["W\u00e4denswil, Switzerland"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Chantal","family":"Quiblier","sequence":"additional","affiliation":[{"id":[{"id":"https:\/\/ror.org\/02crff812","id-type":"ROR","asserted-by":"publisher"}],"name":"Institute of Medical Microbiology, University of Zurich","place":["Zurich, Switzerland"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Vladimira","family":"Hinic","sequence":"additional","affiliation":[{"id":[{"id":"https:\/\/ror.org\/02crff812","id-type":"ROR","asserted-by":"publisher"}],"name":"Institute of Medical Microbiology, University of Zurich","place":["Zurich, Switzerland"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Adrian","family":"Egli","sequence":"additional","affiliation":[{"id":[{"id":"https:\/\/ror.org\/02crff812","id-type":"ROR","asserted-by":"publisher"}],"name":"Institute of Medical Microbiology, University of Zurich","place":["Zurich, Switzerland"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-1761-0812","authenticated-orcid":true,"given":"Oliver","family":"Nolte","sequence":"additional","affiliation":[{"id":[{"id":"https:\/\/ror.org\/02crff812","id-type":"ROR","asserted-by":"publisher"}],"name":"Institute of Medical Microbiology, University of Zurich","place":["Zurich, Switzerland"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"235","published-online":{"date-parts":[[2026,10,21]]},"reference":[{"key":"e_1_3_4_2_2","doi-asserted-by":"publisher","DOI":"10.1128\/CMR.00006-21"},{"key":"e_1_3_4_3_2","doi-asserted-by":"publisher","DOI":"10.1128\/AAC.39.6.1365"},{"key":"e_1_3_4_4_2","doi-asserted-by":"publisher","DOI":"10.1128\/AAC.47.12.3739-3742.2003"},{"key":"e_1_3_4_5_2","doi-asserted-by":"publisher","DOI":"10.1093\/jac\/dkv002"},{"key":"e_1_3_4_6_2","doi-asserted-by":"publisher","DOI":"10.1093\/jac\/dkh112"},{"key":"e_1_3_4_7_2","doi-asserted-by":"publisher","DOI":"10.1001\/jama.2018.12163"},{"key":"e_1_3_4_8_2","doi-asserted-by":"publisher","DOI":"10.1093\/ofid\/ofw132"},{"key":"e_1_3_4_9_2","doi-asserted-by":"publisher","DOI":"10.1128\/JCM.01980-19"},{"key":"e_1_3_4_10_2","doi-asserted-by":"publisher","DOI":"10.1007\/s00284-021-02402-2"},{"key":"e_1_3_4_11_2","doi-asserted-by":"publisher","DOI":"10.1128\/AEM.01435-19"},{"key":"e_1_3_4_12_2","doi-asserted-by":"publisher","DOI":"10.1128\/jcm.00128-25"},{"key":"e_1_3_4_13_2","doi-asserted-by":"publisher","DOI":"10.1128\/jcm.01706-22"},{"key":"e_1_3_4_14_2","doi-asserted-by":"publisher","DOI":"10.2174\/0118742858461926260713092245"},{"key":"e_1_3_4_15_2","unstructured":"EUCAST: European Committee on Antimicrobial Susceptibility Testing. 2017. EUCAST guidelines for detection of resistance mechanisms and specific resistances of clinical and\/or epidemiological importance. Available from: http:\/\/www.eucast.org\/resistance_mechanisms\/"},{"key":"e_1_3_4_16_2","unstructured":"Testing ECoAS. 2025. EUCAST disk diffusion method for antimicrobial susceptibility testing. EUCAST V\u00e4xj\u00f6 Sweden. https:\/\/www.eucast.org\/fileadmin\/src\/media\/PDFs\/EUCAST_files\/Disk_test_documents\/2025_manuals\/Manual_v_2013.2020_EUCAST_Disk_Test_2025.pdf."},{"key":"e_1_3_4_17_2","doi-asserted-by":"publisher","DOI":"10.1016\/j.ijantimicag.2020.106019"},{"key":"e_1_3_4_18_2","doi-asserted-by":"publisher","DOI":"10.1128\/JCM.00404-11"},{"key":"e_1_3_4_19_2","doi-asserted-by":"publisher","DOI":"10.1097\/INF.0000000000004588"},{"key":"e_1_3_4_20_2","doi-asserted-by":"publisher","DOI":"10.1093\/bioinformatics\/btu170"},{"key":"e_1_3_4_21_2","doi-asserted-by":"publisher","DOI":"10.1371\/journal.pcbi.1005595"},{"key":"e_1_3_4_22_2","doi-asserted-by":"publisher","DOI":"10.1038\/s41598-021-91456-0"},{"key":"e_1_3_4_23_2","doi-asserted-by":"publisher","DOI":"10.1099\/mgen.0.001413"},{"key":"e_1_3_4_24_2","doi-asserted-by":"publisher","DOI":"10.1128\/AAC.33.1.63"}],"container-title":["Microbiology Spectrum"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/journals.asm.org\/doi\/pdf\/10.1128\/spectrum.01327-26","content-type":"application\/pdf","content-version":"vor","intended-application":"text-mining"},{"URL":"https:\/\/journals.asm.org\/doi\/pdf\/10.1128\/spectrum.01327-26","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2026,10,21]],"date-time":"2026-10-21T13:01:29Z","timestamp":1792587689000},"score":1,"resource":{"primary":{"URL":"https:\/\/journals.asm.org\/doi\/10.1128\/spectrum.01327-26"}},"subtitle":[],"editor":[{"given":"Haifang","family":"Zhang","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"editor"}]}],"short-title":[],"issued":{"date-parts":[[2026,10,21]]},"references-count":23,"alternative-id":["10.1128\/spectrum.01327-26"],"URL":"https:\/\/doi.org\/10.1128\/spectrum.01327-26","relation":{},"ISSN":["2165-0497"],"issn-type":[{"value":"2165-0497","type":"electronic"}],"subject":[],"published":{"date-parts":[[2026,10,21]]},"assertion":[{"value":"2026-05-08","order":0,"name":"received","label":"Received","group":{"name":"publication_history","label":"Publication History"}},{"value":"2026-10-06","order":2,"name":"accepted","label":"Accepted","group":{"name":"publication_history","label":"Publication History"}},{"value":"2026-10-21","order":3,"name":"published","label":"Published","group":{"name":"publication_history","label":"Publication History"}}],"article-number":"e01327-26"}}