[en] Escherichia coli bacterial species is subdivided into several strains that are pathogenic for man and animals, on the basis of their specific properties and factors which are responsible for their pathogenic characters. The pathogenic strains are classically subdivided into strains with intestinal tropism (enterotoxigenic, enteropathogenic, entero-haemorrhagic, 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 serie of review articles is to present the current knowledge on specific properties and factors of extraintestinal strains : adhesins and colonisation factors, transmucosal transfer and survival in blood and internal organs, toxicity. The fourth manuscript will deal with the invasive strains themselves, focusing on the necrotoxigenic strains. This second manuscript presents the current knowledge on the transmucosal transfer, resistance to the bactericidal activity of the complement, resistance to phagocytosis and production of colicins, that were, for several years, considered as an important specific property of invasive strains of E coli.