[en] Escherichia coli bacterial species is subdivided into several pathogen strains in man and animals, on the basis of their specific properties and factors which are responsible for their virulence. The pathogenic strains are classically subdivided into strains with intestinal tropism (enterotoxigenic, enteropathogenic, enterohaemmorrhagic, verotoxigenic and enteroinvasive) and with extraintestinal tropism (uropathogenic and invasive). Invasive strains cause septicaemia and/or bacteraemia with localisations in different internal organs (systemic infections). If specific virulence properties and factors of strains with intestinal tropism are quite well known and described, those of strains with extraintestinal tropism are much less characterised, especially in animals. The purpose of this series of review articles is to present the current knowledge on specific properties and factors of extraintestinal strains. The first three manuscripts reviewed the characteristics of adhesins and colonisation factors, transmucosal transfer, survival in blood and internal organs, and toxicity. After the presentation of the animal and human invasive E. coli, this fourth manuscript reviews the current knowledge on the necrotoxigenic strains and finishes with a discussion on the meaning of the bacterial virulence in general.