Public health officials in Massachusetts have confirmed that a listeriosis outbreak was caused by the consumption of milk purchased from Whittier Farms. They used Pulsed Field Gel Electrophoresis (PFGE) to obtain the "genetic fingerprint" of the strain of Listeria that was isolated from case-patients and milk provided by the dairy for testing.
PFGE is a technique used to separate the DNA of a bacterial isolate into its component parts. It operates by causing alternating electric fields to run the bacteria’s DNA through a flat gel matrix of agarose, a polysaccharide obtained from agar. The pattern of bands of the DNA fragments — or “fingerprints” — in the gel after exposure to the electrical current is unique for each strain and sub-type of bacteria. By performing this procedure, scientists can identify hundreds of strains of Listeria, E. coli O157:H7, and and other pathogenic bacteria.
PFGE patterns of bacteria isolated from products can be compared and matched to PFGE patterns of bacteria isolated from people suffering illness after consuming contaminated products. When PFGE patterns of bacteria isolated from foods and human samples match, they, along with solid epidemiological work, provide proof that the contaminated product was the source of a person’s illness.
The Boston Globe reported on the outbreak investigation today:
"The pattern is very unique," said Dr. Alfred DeMaria, the state’s director of communicable disease control. "It means there’s an outbreak here. There’s no question there’s an outbreak. And it implies that the dairy is the common source."
It is exceedingly rare, disease investigators said, to discover that cases of listeria are caused by germs with identical genetic profiles. Instead, each infection tends to be the result of a slightly different form of the bacterium.
In 19 other cases in 2007 in Massachusetts, each infection was caused by a germ with a distinctive fingerprint. Similarly, there were no genetic matches among 99 listeriosis cases in the previous five years.
That, specialists said, is why it was so telling that the samples from the dairy, the patient’s refrigerator, and the four patients all matched.
According to the article, the investigation is now focusing on the packaging process as the potential source of contamination.
More information on the outbreak investigation is available on the MDH website.