Fourier-transform infrared spectroscopy reliably differentiates Klebsiella pneumoniae outbreak isolates.

International, multidrug-resistant (MDR) high-risk clonal Klebsiella pneumoniae lineages are a frequent cause of outbreaks, posing a serious public health threat. Tracking, understanding and containing such outbreaks remain a challenge for infection control. In this study, we evaluated Fourier-transform infrared (FTIR) spectroscopy - a method previously shown to enable bacterial typing at various intraspecies levels - as a tool to type 53 MDR K. pneumoniae isolates that had been geno- and phenotypically characterized and collected during an outbreak in North-Eastern Germany in 2019. Infrared (IR) spectra were acquired from dried spots of bacterial suspensions in ethanol solution, with three independent cultures of each isolate. FTIR spectroscopy exploratory data analysis, performed by Hierarchical Cluster Analysis (HCA) and Principal Component Analysis (PCA), revealed six distinct clusters. These showed perfect concordance with K locus typing (Adjusted Rand index [AR]: 100 %), very high concordance with sequence type (ST) (AR: 99.1 %), and high but lower concordance with the O locus (AR: 90.6 %). All MDR outbreak isolates clustered tightly together, indicating strong spectral similarity and confirming their clonal relatedness. These findings demonstrate that FTIR spectroscopy could serve as a rapid, first-line screening tool to support early detection of cross-transmission events involving clonally related K. pneumoniae strains.