Because there are so many different strains of E. coli, microbiologists classify it into more than 170 serogroups. Within each serogroup, there are one or more serotypes. For example, O126:H and O126:H27 represent two serotypes of E. coli, with the O126 signifying the particular serogroup to which these serotypes belong. E. coli O157:H7 was identified for the first time at the U. S. Centers for Disease Control (CDC) in 1975. However, it was not until seven years later, in 1982, that E. coli O157:H7 was conclusively determined to be a cause of enteric disease. Specifically, in 1982, following outbreaks of foodborne illness that involved several cases of bloody diarrhea, E. coli O157: H7 was firmly associated with hemorrhagic colitis.4 As a result of this association, E. coli O157: H7 was designated as an enterohemorrhagic E. coli, or EHEC.
Historically, most procedures that were used to detect fecal coliforms or generic E. coli in foods used methods that did not detect the presence of E. coli O157:H7. It was not until the Jack in the Box E. coli O157:H7 outbreak occurred, in January 1993, that the importance of testing specifically for E. coli O157:H7 was truly understood. Infection with E. coli O157:H7 is usually confirmed by detecting the bacterium in the stool of the infected individual. Until recently, however, most laboratories did not routinely test stools for the presence of E. coli O157:H7. Now, most hospitals and physicians know to test for this particular bacterium, especially if the potentially infected individual has bloody diarrhea. Still, it remains a good idea to specifically request that you or your child’s stool specimen be tested with Sorbitol MacConkey (SMAC) Agar for the presence of E. coli O157:H7.
In addition, E. coli O157:H7 is now commonly “fingerprinted”. When a sample is taken from either a 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 “fingerprint” of the bacteria can then be compared and matched up to the “fingerprint” of persons who consumed the contaminated product. When DNA “fingerprints” match, it, along with solid epidemiological work, is proof that the contaminated produce was the source of the illness.
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. See also, http://www.fsis-pfge.org.