Induction of human beta-defensin 2 by the probiotic Escherichia coli Nissle 1917 is mediated through flagellin

Human beta-defensin 2 (hBD-2) is an inducible antimicrobial peptide synthesized by the epithelium to counteract bacterial adherence and invasion. Proinflammatory cytokines, as well as certain bacterial strains, have been identified as potent endogenous inducers. Recently, we have found that hBD-2 induction by probiotic Escherichia coli Nissle 1917 was mediated through NF-kappaB- and AP-1-dependent pathways. The aim of the present study was to identify the responsible bacterial factor. E. coli Nissle 1917 culture supernatant was found to be more potent than the pellet, indicating a soluble or shed factor. Chemical analysis demonstrated the factor to be heat resistant and proteinase digestible. Several E. coli Nissle 1917 deletion mutants were constructed and tested for their ability to induce hBD-2 expression in Caco-2 cells. Deletion mutants for flagellin specifically exhibited an impaired immunostimulatory capacity. Reinsertion of the flagellin gene restored the induction capacity to normal levels. Isolated flagellin from E. coli Nissle 1917 and from Salmonella enterica serovar Enteritidis induced hBD-2 mRNA significantly in contrast to the flagellin of the apathogenic E. coli strain ATCC 25922. H1 flagellin antiserum abrogated hBD-2 expression induced by flagellin as well as E. coli Nissle 1917 supernatant, confirming that flagellin is the major stimulatory factor of E. coli Nissle 1917.

Both α-haemolysin determinants contribute to full virulence of uropathogenic Escherichia coli strain 536

Uropathogenic Escherichia coli strain 536 possesses two intact copies of the alpha-haemolysin determinant localised on distinct pathogenicity islands. The coding regions of the two hlyCABD operons are conserved; however, upstream sequences are entirely dissimilar. Consequently, expression of the encoded toxin molecules in vitro is highly different. On the other hand, the contribution of the individual determinants to the strain's virulence is the same. Isogenic mutants lacking individual hly determinants have a similar increase in LD50 value in a mouse model of urinary tract infection. Mouse lung toxicity as well as in vitro assays reveals a significant decrease in acute cytotoxicity of both mutants in comparison to the parent wild-type strain; however, the two hly mutants do not significantly differ from each other in these respects. Single channel recordings show no difference in electrophysiological characteristics of the pores formed by the individual HlyA molecules on synthetic planar lipid membranes. Nor do the paralogues have any target cell preference in an in vitro cytotoxicity assay. Our data suggest that the two hly paralogues encode identical toxin functions; however, due to different regulation of expression, they participate at distinct stages of the infectious process. Interestingly, the unrelated uropathogenic E. coli strain J96 shares the same two hly alleles, suggesting that acquisition of the two paralogues accorded a selective evolutionary advantage.

The probiotic Escherichia coli strain Nissle 1917 interferes with invasion of human intestinal epithelial cells by different enteroinvasive bacterial pathogens

The probiotic Escherichia coli strain Nissle 1917 (Mutaflor) of serotype O6:K5:H1 was reported to protect gnotobiotic piglets from infection with Salmonella enterica serovar Typhimurium. An important virulence property of Salmonella is invasion of host epithelial cells. Therefore, we tested for interference of E. coli strain Nissle 1917 with Salmonella invasion of INT407 cells. Simultaneous administration of E. coli strain Nissle 1917 and Salmonella resulted in up to 70% reduction of Salmonella invasion efficiency. Furthermore, invasion of Yersinia enterocolitica, Shigella flexneri, Legionella pneumophila and even of Listeria monocytogenes were inhibited by the probiotic E. coli strain Nissle 1917 without affecting the viability of the invasive bacteria. The observed inhibition of invasion was not due to the production of microcins by the Nissle 1917 strain because its isogenic microcin-negative mutant SK22D was as effective as the parent strain. Reduced invasion rates were also achieved if strain Nissle 1917 was separated from the invasive bacteria as well as from the INT407 monolayer by a membrane non-permeable for bacteria. We conclude E. coli Nissle 1917 to interfere with bacterial invasion of INT407 cells via a secreted component and not relying on direct physical contact with either the invasive bacteria or the epithelial cells.