Escherichia coli 0157:H7
Name: Darlene H.
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
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
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
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.
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Update: June 2012