The E. coli bacteria is composed of several different strains. Scientists give them a name according to the identity of their surface antigens and from the moment they have been described. These names resemble for example O104:H4 or O157:H7. But what is the significance of these letters and numbers?

The E. coli bacteria is made up of a structure protected by two membranes. On one of these the antigen known as O is present, protected by another strain, the K antigen (which is rarely mentioned today). Finally, the bacteria can carry another antigen, the H antigen, which is a flagellum that allows E. coli to move around. In the case of the K antigen, the protective layer can mean Kaufmann, from the name of its discoverer in the 1940s, or “Kapsel” (capsule). The O antigen signifies “Ohne Kapsel” (without capsule), because it can only be detected if the capsule does not exist or is destroyed. The antigen H comes from “Hauch”. “The reason for the designation of this flagellated antigen is independent of the E. coli species. Also the fact of understanding that the H antigen allows the bacteria to move around is sufficient.” Explains Professor Jacques Mainil. Around 200 O antigens, 70 K antigens and 60 H antigens have been described. The different combinations of these three types of antigens form the different strains of the E. coli bacteria which will have general and pathogenic properties that are different from each other.

With regard to the figures present beside the letters O and H (and also K when they exist), these signify that these antigens have been the ones described umpteen times. For example, the O157:H7 strain is made up of the 157^th O antigen and the 7^th H antigen described. This pathogenic strain was observed for the first time at the end of the 1970s. The O104:H4 strain is therefore composed of an O antigen which was the 104^th described, and a H antigen which was the 4th described. However, while these antigens were discovered before those of the O157:H7 strain, the pathogenic strain O104:H4 was only observed in 2011, during the outbreak in Germany.