I was watching again this morning the Vice President and Secretaries Vilsack and Sebelius talking about the Food Safety Working Group as I was responding to emails from people concerned about yet another recall of a staple food product. This time the Plainview Milk Products Cooperative Salmonella Recall.
According to the FDA, Plain is voluntarily recalling instant nonfat dried milk, whey protein, fruit stabilizers, and gums (thickening agents) that it has manufactured over the past two years, because they might be contaminated with Salmonella. Plainview has stopped production of these products and has notified its customers of the recall. During an investigation of the Plainview facility, FDA found that some of the equipment was contaminated with Salmonella. At this time, the Centers for Disease Control and Prevention has not linked any human illnesses to potentially contaminated products from the Plainview facility. But how do they know that? Given that FDA, USDA and/or CDC presumably has the Salmonella positive milk product test result? Has a Genetic Fingerprint (PFGE) been done? Has it been compared to ill people in the United States? Have those ill people been linked to the consumption of Plainview Milk Products?
So, how does PFGE work? When a sample is taken from either a piece of meat or poultry that is contaminated with a dangerous form of bacteria, such as Salmonella, it can be cultured to obtain and identify the bacterial isolate. If a person consumes some of the contaminated product, and becomes infected as a result, a stool sample can then be cultured to obtain and identify the bacterial isolate. These bacterial isolates are then broken down into their various component parts creating a DNA "fingerprint".
The process of obtaining the DNA fingerprint is called Pulse Field Gel Electrophoresis, or PFGE. This technique is used to separate the DNA of the bacterial isolate into its component parts. It operates by causing alternating electric fields to run the 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 Salmonella as well as strains of pathogenic bacteria. The PFGE pattern of the bacteria can then be compared and matched up to the PFGE pattern of the strain of infected persons who consumed the contaminated product. When PFGE patterns match, they, along with solid epidemiological work, are proof that the contaminated product was the source of a person’s illness.
So, where is the PFGE?