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Name: Darlene H.
Status: Other
Age: 40s
Location: N/A
Country: N/A
Date: February 2003

Escherichia coli 0157:H7 Can you explain the significance of the numerical code after the name? I was teaching a class on the topic of "food safety" to a group of management personnel in the meat processing industry -- and the question was raised. I am assuming that the numbers refer to classification, but I was hoping for some more specific information.

The number refers to a strain of E. coli. Just as all dogs are the same species, but different breeds, you can have bacteria that are the same species, but with enough genetic differences to qualify as different strains. The human gut is filled with E. coli, but it is harmless, even beneficial. But E. coli 0157 encodes a protein that causes cell damage and cell death in humans. A difference in one protein between two strains of E. coli can make a big difference!

Paul Mahoney, PhD

Yes, it is a way of further subclassifying a species. Within any population there is variation. Those variations can be selected for either artificially or naturally depending on the environment. Think of the species Canis familiaris, or the domestic dog. They are all the same species but over time we have selectively bred many different varieties of dog. Some are calm and gentle, like the golden retriever. Some are nasty and fierce like the pit bull. Well, within bacteria the same thing can happen. We all carry E. coli in our intestines, hence the name. We have a very nice relationship with them-we provide them with a nice dark, moist place and send food by them all day long. They in turn provide us with vitamins and also help to further digest our food. (They also produce gas in the process, but I digress...) Anyway, think of most E. coli as the golden retriever version of bacteria. There are lab tests to distinguish one strain of bacteria from another. In each part of the organism, there are distinct "markers" that are slightly different for each. Organisms that have flagella will have different types of proteins in them that can be distinguished. These are designated as "H" proteins. There are also proteins in the outer membrane called "O" proteins.

So E. coli O157:H7 is a particular strain. Bacteria reproduce asexually but can recombine their genes through other means, one of them being conjugation where plasmids are traded back and forth. It is believed that this particular strain got its nasty genes through this process. While they are detrimental to us, these genes help the bacteria to survive better in their environment.


The letter-number combination is called the 'serotype' of the E. coli strain. The O157 (letter 'oh') is a designation for the LPS type of the E. coli. Simply said, each strain of E. coli cells has its own type of lipopolysaccharides (LPS), a coating of sugar molecules bound to fat molecules. These coats are immunogenic, which means one can raise antibodies that specifically recognize that type of LPS only. Scientists have thus developed a whole series of antisera that each react with a specific LPS type. In this case the strain reacts to O157. The H7 is also a specific antiserum reaction, but this time the antibodies recognize specific epitopes (antibody-binding sites) on the flagella of the bacteria. Theoretically one could have a strain with a different O number combined with H7, or an O157 with another H number.

The combination of O:157 and H7 normally indicates the presence of this particular strain, which is highly virulent. The virulence has nothing to do with its O or H antigens per se. These are only easily recognized markers. The virulence of O157:H7 is so high because it carries a combination of virulence genes, some of which are present on a plasmid.

Trudy Wassenaar

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