Introduction. Gram-negative bloodstream infections (GNBSIs) carry a significant global health burden. Escherichia coli and Klebsiella pneumoniae are the two most common causes of healthcare-associated GNBSI, which may arise from gastrointestinal tract (GIT) colonization.Gap Statement. We do not fully understand how GNBSIs arise from GIT colonization.Aim. To understand E. coli and K. pneumoniae genomic and phenotypic adaptations that underpin transition from GIT colonization to invasive bloodstream infection.Methodology. This study identified 'linked' faecal and blood isolates from children with healthcare-associated GNBSI caused by E. coli and K. pneumoniae. Linked pairs were compared for antimicrobial resistance and biofilm formation and underwent comparative genomic analysis via whole-genome sequencing, comparative average nucleotide identity and core genome single nucleotide polymorphism (SNP) analysis.Results. Five isolate pairs (three E. coli, two K. pneumoniae) showed high relatedness, supporting the GIT origin of bloodstream infection. Isolates within pairs had identical virulence genes, whereas phenotypic assays revealed changes in antimicrobial susceptibility, with one pair undergoing changes in resistance gene profiles and increased biofilm formation in four out of five isolates.Conclusion. This study provides insight into within-host evolution from gastrointestinal colonization to bloodstream invasion in Gram-negative pathogens. Convergence on metabolic adaptation and biofilm formation suggests that these traits may be advantageous in healthcare-associated GNBSI. Further studies involving larger cohorts alongside functional validation of mutations are needed to better understand GNBSI pathogenesis.
Keywords: Escherichia coli; Klebsiella; antimicrobial resistance (AMR); biofilm; bloodstream infection; plasmids.