Piperacillin-tazobactam resistance in <i>Klebsiella pneumoniae</i> is often associated with IS <i>26-</i> mediated <i>bla</i> _SHV-1 amplification in a widespread <i>Klebsiella</i> -adapted plasmid

Piperacillin-tazobactam (TZP) resistance in <i>Klebsiella pneumoniae</i> involves diverse mechanisms with unclear prevalence and phenotypic impact. To elucidate these mechanisms, we analyzed <i>K. pneumoniae</i> clinical isolates resistant to TZP but susceptible to cefotaxime and cefepime. Among 53 isolates, 14 were further studied by MIC testing for TZP, amoxicillin-clavulanic acid (AMC), and ceftazidime (CAZ). Short-read sequencing was performed for all 14 isolates and long-read sequencing for two. Core-genome MLST showed that all were unrelated. Two had a <i>bla</i>_OXA-1 gene, one also carrying an <i>ompK35</i> porin gene mutation; two others had the same mutation in the promoter of the chromosomal copy of <i>bla</i>_SHV usually associated with overexpression. In the remaining 10, resistance correlated with plasmid-borne <i>bla</i>_SHV-1 copies. Nine isolates carried <i>bla</i>_SHV-1v1 in the same IS<i>26</i> pseudocompound transposon (PTn), corresponding to PTnSHV-L and located on a conserved IncFIB(K)_1_Kpn3 plasmid in eight. The tenth isolate carried PTnSHV-L with a distinct <i>bla</i>_SHV-1 variant on both an IncHI1B_1_pNDM-MAR plasmid and a high-copy-number Col-type plasmid. Read depth analysis confirmed that <i>bla</i>_SHV copy number correlated with TZP, AMC, and CAZ MICs. Large-scale database screening identified related IncFIB(K)_1_Kpn3 plasmids, strongly associated with <i>K. pneumoniae</i> and frequently carrying a PTnSHV-L marker. Analysis of a <i>K. pneumoniae</i> genome data set confirmed the frequent co-occurrence of this plasmid and the PTnSHV-L marker in strains with multiple <i>bla</i>_SHV copies. These findings suggest the emergence of an epidemic plasmid adapted to <i>K. pneumoniae</i> and driving TZP resistance through <i>bla</i>_SHV-1 amplification, underscoring the need for genomic surveillance to detect amplification-based resistance overlooked by standard phenotypic or PCR assays.