Molecular characterization of colistin resistance genes in a high-risk ST101/KPC-2 clone of Klebsiella pneumoniae in a University Hospital of Split, Croatia

Klebsiella pneumoniae carbapenemase-producing K. pneumoniae (KPC-KP) has become a major concern worldwide due to multidrug resistance and the ability to spread locally and globally. Infections caused by KPC-KP are great challenge in the healthcare systems because these are associated with longer hospitalization and high mortality. The emergence of colistin resistance has significantly reduced already limited treatment options. This study describes the molecular background of colistin-resistant KPC-KP isolates in the largest hospital in southern Croatia. Thirty-four non-duplicate colistin-resistant KPC-KP isolates were collected during routine work from April 2019 to January 2020 and from February to May 2021. Antimicrobial susceptibility was determined using disk diffusion, broth microdilution, and the gradient strip method. Carbapenemase was detected with an immunochromatographic test. Identification of bla(KPC) and mcr genes or mutations in pmrA, pmrB, mgrB, phoP, and phoQ genes were performed by polymerase chain reaction (PCR) and positive products were sequenced. Pulsed-field gel electrophoresis (PFGE) and multilocus sequence typing (MLST) were used for epidemiological analysis. All isolates were multidrug-resistant, with colistin minimum inhibitory concentrations (MICs) from 4 to >16 mg/L, and all harbored bla(KPC-2) and had a single point mutation in the mgrB gene resulting in a premature stop codon, with the exception of one isolate with four point mutations corresponding to stop codons. All isolates were negative for mcr genes. PFGE analysis identified a single genetic cluster, and MLST revealed that all isolates belonged to sequence type 101 (ST101). These results show emergence of the high-risk ST101/KPC-2 clone of K. pneumoniae in Croatia as well as appearance of colistin resistance due to mutations in the mgrB gene. Molecular analysis of epidemiology and possible resistance mechanisms are important to develop further strategies to combat such threats.