In a hospital, an outbreak of carbapenem-resistant Klebsiella pneumoniae occurs in the ICU. Molecular typing shows all isolates share an identical pulsed-field gel electrophoresis (PFGE) pattern. This indicates:

• A Clonal outbreak — all cases derived from the same source strain with person-to-person transmission ✓
• B Independent emergence of the same resistance mutation in multiple patients
• C Horizontal gene transfer of resistance plasmid between different Klebsiella strains
• D Contamination of the PFGE analysis

Explanation

PFGE (pulsed-field gel electrophoresis) is the gold standard for epidemiological typing of nosocomial outbreaks. Identical or highly similar PFGE banding patterns (< 3 band differences) indicate that isolates are clonally related — derived from the same progenitor strain. This confirms a clonal outbreak with person-to-person transmission (or common source) rather than independent emergence. Horizontal gene transfer of a plasmid between different strains would produce isolates with different chromosomal PFGE patterns (since PFGE types the chromosome) but shared plasmid-borne resistance genes. Whole-genome sequencing has since surpassed PFGE in resolution.

Reference: Ananthanarayan & Paniker's Textbook of Microbiology, 11th ed.

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Written and medically reviewed by the StethoPrep medical team.