Investigation of Tigecycline Resistance Mechanisms in Escherichia coli and Klebsiella pneumoniae Isolates From Hospitalized Patients in Selected Tehran Hospitals During 2022–2023

Background Escherichia coli and Klebsiella pneumoniae are recognized as significant opportunistic pathogens responsible for a substantial proportion of hospital‐acquired infections. The growing prevalence of multidrug resistance in isolates of these pathogens represents a critical global health concern. Tigecycline, a broad‐spectrum antibiotic, has demonstrated efficacy in the treatment of carbapenem‐resistant Enterobacteriaceae. The present study is aimed at investigating the mechanisms underlying tigecycline resistance in clinical isolates of E. coli and K. pneumoniae. Methods This study included 103 clinical isolates of E. coli and K. pneumoniae obtained from hospitalized patients in selected Tehran hospitals during 2022–2023. Isolates were identified and confirmed through standard microbiological phenotypic techniques. Antimicrobial susceptibility testing was conducted using the disk diffusion method and minimum inhibitory concentration (MIC) determination following Clinical and Laboratory Standards Institute (CLSI) guidelines. Polymerase chain reaction (PCR) and sequencing were employed to detect resistance genes ( acrR, acrB, oqxA, oqxB, marA, tet(X), and ramA ) and identify mutations. Gene expression analysis for acrB, ramA, and marA was performed using real‐time PCR. Tigecycline‐resistant isolates were subjected to MLST for molecular typing. Results Among the 103 isolates, 58 were identified as E. coli and 45 as K. pneumoniae . The lowest resistance rates were observed for doripenem (18.4%, 19/103) and tigecycline (0.97%, 1/103). Notably, an additional 15 isolates (14.6%) exhibited intermediate susceptibility to tigecycline. Among the 16 tigecycline nonsusceptible isolates (1 resistant and 15 intermediate; 4 E. coli and 12 K. pneumoniae ), all harbored the acrR , acrB , oqxA , and marA genes, whereas 93.7% (15/16) carried the ramA and oqxB genes. None of the isolates contained the tet(X) gene. The MIC range for these isolates was 0.5–8 μ g/mL. One tigecycline‐resistant K. pneumoniae isolate (K9) exhibited a mutation in the acrB gene. Expression analysis revealed significant upregulation of marA , followed by acrB and ramA . MLST analysis identified isolate K9 as belonging to sequence type ST268. Conclusion In this study, it was found that resistance to tigecycline was low (0.97%), but 14.6% of the samples were intermediate, indicating an increase in resistance to this antibiotic. Tigecycline can still be considered a suitable option for carbapenem‐resistant isolates, but its administration requires careful consideration.