Maine CDC is investigating a cluster of 7 shiga toxin positive E. coli O157:H7 (STEC) cases in Cumberland and York counties that occurred among residents over the past month (case onset dates of April 17 to May 17). This is double the usual number of STEC cases reported this time of year (n=3). The median age of cases was 26 years (age range 14 years to 65 years). As of May 28, 6 of the 7 cases have been confirmed shiga positive E. coli O157:H7 by the Health and Environmental Testing Laboratory (HETL). Of these, 4 cases match by Pulse-Field Gel Electrophoresis (PFGE) and are considered part of a national cluster. At this time, the investigation is ongoing although we have not identified any common venues, events or foods based on case interviews.
What is PFGE?
When a sample is taken from either a person, piece of meat or poultry that is contaminated with a dangerous form of bacteria, such as E. coli O157:H7, listeria, or campylobacter, it can be cultured to obtain and identify the bacterial isolate. If a person consumes some of the contaminated meat or poultry, 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 E. coli O157:H7 as well as strains of listeria and campylobacter, and other 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.
It will be interesting to see if Maine, the CDC and other States’ Health Departments will be able to link the illnesses in Maine to other states and to a possible source.