Are hospital environments an underestimated source for Gram-negative infections in critically ill patients? A non-multi-drug-resistant Klebsiella pneumoniae outbreak in an Irish intensive care unit

<i>Klebsiella pneumoniae</i> is a major cause of healthcare-associated infections. Although colonisation with Gram-negative bacteria in hospitalised patients is well recognised, the relative contributions of patient-to-patient versus environment-to-patient transmission remain unclear. Most outbreak investigations focus on multi-drug-resistant (MDR) strains. This study investigates a non-MDR <i>K. pneumoniae</i> outbreak in an intensive care unit (ICU) to examine the role of the hospital environment in nosocomial transmission. Clinical isolates of Klebsiella spp. were obtained from six patients between January 2021 and June 2021. Environmental swabs were collected from handwashing sinks in patient and preparation rooms, 3 months apart. Whole-genome sequencing (WGS) assessed singlenucleotide polymorphism (SNP)-based relatedness using literature-informed thresholds. WGS identified five distinct clusters of genetically related K. pneumoniae isolates, linking clinical and environmental sources. The closest relationships (2-5 SNPs) were observed between patient and sink isolates within the same room, consistent with recent transmission or a shared source. Additional clusters (5-23 SNPs) involved isolates from sinks in different rooms, indicating environmental persistence and potential inter-room dissemination. Two <i>Klebsiella varicola</i> subspecies variicola bloodstream isolates from spatially distinct patients differed by only 2 SNPs, forming an additional cluster consistent with a common clonal lineage. Following enhanced daily sink disinfection and staff education, no further clinical acquisitions were identified. WGS demonstrated genetic relatedness between nonMDR Klebsiella spp. strains and ICU environmental isolates, underscoring the role of environmental reservoirs in transmitting antimicrobial-susceptible Gram-negative organisms and the importance of targeted surveillance beyond MDR settings.