Neval ELGÖRMÜŞ, Güner SÖYLETİR
The Injector - 2026;5(2):86-96
Objective: Extended-spectrum beta-lactamases (ESBLs) and acquired AmpC beta-lactamases complicate the detection and interpretation of antimicrobial resistance. This study provides a historical characterization of selected clinical Escherichia coli (E. coli) and Klebsiella pneumoniae (K. pneumoniae) isolates collected during 2004-2006. Methods: From 4,478 E. coli and 1,302 Klebsiella spp. isolates, 107 isolates (59 E. coli and 48 K. pneumoniae) were selected on the basis of reduced susceptibility or resistance to cefoxitin and amoxicillin/clavulanate, together with reduced susceptibility to aztreonam or at least one third-generation cephalosporin. Phenotypic assays, including three-dimensional testing (TDT) with and without cloxacillin treatment, were compared with targeted PCR. Results: At least one of the investigated TEM, SHV, or CTX-M gene families was detected in 105/107 isolates (98.1%); family-level TEM or SHV detection does not independently establish an ESBL variant. Acquired AmpC-family genes were detected in seven isolates (6.5%), including six E. coli (10.2%) and one K. pneumoniae (2.1%). Standard TDT showed 100% sensitivity and 70% specificity against the targeted PCR results. With cloxacillin treatment, the reported sensitivity was 42.9% and specificity was 93.3%. Conclusion: These findings document resistance gene families and phenotypic-genotypic discordance in a resistance-enriched historical collection. They cannot estimate contemporary prevalence, establish temporal trends, or guide current empirical therapy. Contemporary isolate collections and validated molecular confirmation are needed to assess present-day epidemiology and diagnostic performance.
