Is Escherichia coli invading tubuloepithelial cells?

The adhesion of Escherichia coli to host epithelium cells is the very first step of urinary tract infections followed by the internalization of the bacteria into these cells. These steps are influenced by several surface antigens or products of the pathogen, e.g. fimbriae or adhesins, K antigen, and hemolysin. The bacterial adherence and the internalization of several isogenic E. coli O18 strains differing in the expression of K5 antigen, hemolysin, and type of fimbriae were measured by using a permanent line of porcine tubuloepithelial cells (LLC-PK1). Strains with K5 antigen were reduced in their adherence and internalizability as compared to the K-negative strains. The expression of hemolysin by these strains lead to an increase of adherence and internalization. The internalization of bacteria is influenced mainly by their adherence to the epithelial cells. Thus, the engorgement of attached bacteria is rather a kind of endocytosis than an invasion of bacteria.