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Multistate Outbreak of Salmonella Typhimurium Infections Linked to Peanut Butter, 2008–2009

Posted January 29, 2009

This outbreak appears to be over. However, Salmonella is an important cause of human illness in the United States. More information about Salmonella, and steps people can take to reduce their risk of infection, can be found on the CDC Salmonella Web Page.

How the PFGE Process Works

PFGE fingerprintFinding bacteria in a sample is important for understanding the cause of illness.  When microbiologists find disease-causing bacteria, they send them to a laboratory for DNA fingerprinting.  Laboratories use high-tech equipment to make the DNA fingerprints.  Creating a DNA fingerprint is a way for scientists to learn exactly which type of bacteria is making a person sick. Bacteria have unique fingerprints just like people do.  Each type of bacteria has unique DNA which makes up a pattern of bands called a fingerprint.

The fingerprints that scientists use to identify bacteria are called PFGE patterns.  PFGE stands for Pulsed-Field Gel Electrophoresis.  Scientists find bacterial fingerprints by cutting the bacteria’s DNA into tiny pieces and then placing them on a gel, which is a flat slab of gelatin.  When scientists send electricity through the gel, the DNA pieces separate.  Small pieces of DNA get carried farther down the gel than bigger pieces.  This process creates a banding pattern or “fingerprint”.  After scientists at the public health laboratories have made a bacterial fingerprint, they share pictures of it with the PulseNet team.  The PulseNet team is an important part of disease detection.  They match up bacterial fingerprints from different states.  This helps the team know if a person is getting sick from the same foodborne bacteria as someone else.

Once the PulseNet team has found DNA fingerprints that look the same, they make a dendrogram, or bacteria family tree.  Bacteria that are closely related sometimes come from the same place.  The dendogram lets PulseNet find all the bacteria that are closely related using their PFGE fingerprints.  This helps the PulseNet team work with epidemiologists to find the source of an outbreak.

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