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BACKGROUND: Pulsed-field gel electrophoresis (PFGE) is a laboratory technique that generates a DNA “fingerprint” for a bacterial isolate. Isolates with patterns that are indistinguishable from one another are considered to be the same strain. Scientists and epidemiologists are then able to link cases to each other and to environmental samples to identify outbreaks and their sources. Although this technique traditionally has been performed on isolates associated with enteric illnesses, PFGE subtyping also has been applied to healthcare-associated infection (HAI) clusters to provide information about organism relatedness, or lack thereof. In 2013, the Kentucky Division of Laboratory Services performed PFGE subtyping on HAI isolates from 6 outbreaks. This technique was particularly useful during an investigation of an outbreak of carbapenem-resistant Enterobacteriaceae (CRE) (Klebsiella pneumoniae) in a long-term acute care hospital in Kentucky.
METHODS: Klebsiella pneumoniae isolates were identified by the referring facility as CREs using standard laboratory definitions and testing methods. Once received at the Kentucky Division of Laboratory Services, pure bacterial isolates were plugged in molten agarose and digested in situ using the restriction enzyme XbaI. Plugs were loaded into 1% agarose gels and bands were separated in a CHEF Mapper electrophoresis system. Data were analyzed using the BioNumerics software package. The degree of similarity among fingerprints was calculated using the Dice Coefficient and cluster analysis was performed with the unweighted pair group method.
RESULTS: During May 2013, 32 separate cases of CRE infection were identified by the referring laboratory with collection dates ranging from August 2012 through May 2013. A total of 42 specimens were sent to the Kentucky Division of Laboratory Services. Of those, 21 representative specimens were selected for PFGE analysis. Twelve PFGE patterns were identified during analysis. The most common PFGE pattern was considered the outbreak strain (pattern 1). The remaining isolates were assigned pattern names in sequential order. Nine isolates had pattern 1, two isolates had pattern 2, and the remaining isolates each had a distinct pattern.
CONCLUSIONS: PFGE subtyping identified 12 distinct patterns among the CRE isolates. Pattern 1 and pattern 2 indicated that horizontal transmission was occurring within the facility. Facility leadership was reticent to acknowledge the presence of an outbreak. Utilizing these results in conjunction with onsite inspection and epidemiologic investigation convinced leadership an outbreak was indeed occurring. This outbreak demonstrated that PFGE subtyping can augment the epidemiologic investigation of HAI outbreaks.