The current outbreak of Listeria monocytogenes linked to imported Frescolina brand ricotta salata cheese has several distinct PFGE patterns. Pritzker Olsen’s client, John McKissick, is one of 14 confirmed cases of listeriosis linked to the cheese or to products cross-contaminated by the cheese. The bacteria that caused Mr. McKissick’s illness is part of the outbreak strain.
Pulsed-field gel electrophoresis, or PFGE, is the gold standard of bacterial DNA analysis. The technique separates DNA molecules by placing the bacteria in an agarose gel solution. Voltage is switched among three directions through the gel. This separates the DNA in a forward migration. The different lengths of DNA move through the gel at different rates. Larger pieces move more slowly than small pieces. Over time, the bands separate more and establish a distinct banding pattern. You can see the movement occurring in this demonstration from Davidson College.
The three main PFGE patterns associated with this particular outbreak are: GX6A16.0408/GX6A12.0096, GX6A16.0268/GX6A12.2297, and GX6A16.0068/GX6A12.0096. The patterns are all closely related.
After scientists have made the bacterial fingerprint, they share pictures of it with PulseNet, which keeps an electronic database of the fingerprints. PulseNet was created in 1993, after the Jack-in-the-Box E. coli 0157:H7 outbreak occurred. When PulseNet finds a match, they make a family tree called a dendogram. That helps researchers and epidemiologists find the source of a foodborne illness outbreak.
In fact, lab scientists perform regular searches on the databases, looking for matching clusters. This surveillance can help scientists and epidemiologists determine that an outbreak is occurring, identifying the outbreak in hours, instead of days. It can also help link sporadic cases and identifies related cases.