Abstract
BackgroundEscherichia coli Nissle 1917 (EcN) is a probiotic used in the treatment of intestinal diseases. Although it is considered safe, EcN is closely related to the uropathogenic E. coli strain CFT073 and contains many of its predicted virulence elements. Thus, it is relevant to assess whether virulence-associated genes are functional in EcN. One of these genes encodes the secreted autotransporter toxin (Sat), a member of the serine protease autotransporters of Enterobacteriaceae (SPATEs) that are secreted following the type V autotransporter pathway. Sat is highly prevalent in certain E. coli pathogenic groups responsible for urinary and intestinal infections. In these pathogens Sat promotes cytotoxic effects in several lines of undifferentiated epithelial cells, but not in differentiated Caco-2 cells.ResultsHere we provide evidence that sat is expressed by EcN during the colonization of mouse intestine. The EcN protein is secreted as an active serine protease, with its 107 kDa-passenger domain released into the medium as a soluble protein. Expression of recombinant EcN Sat protein in strain HB101 increases paracellular permeability to mannitol in polarized Caco-2 monolayers. This effect, also reported for the Sat protein of diffusely adherent E. coli, is not observed when this protein is expressed in the EcN background. In addition, we show that EcN supernatants confer protection against Sat-mediated effects on paracellular permeability, thus indicating that other secreted EcN factors are able to prevent barrier disruption caused by pathogen-related factors. Sat is not required for intestinal colonization, but the EcNsat::cat mutant outcompetes wild-type EcN in the streptomycin-treated mouse model. Analysis of the presence of sat in 29 strains of the ECOR collection isolated from stools of healthy humans shows 34.8 % positives, with high prevalence of strains of the phylogenetic groups D and B2, related with extra-intestinal infections.ConclusionsSat does not act as a virulence factor in EcN. The role of Sat in intestinal pathogenesis relies on other genetic determinants responsible for the bacterial pathotype.Electronic supplementary materialThe online version of this article (doi:10.1186/s12866-015-0591-5) contains supplementary material, which is available to authorized users.
Highlights
Escherichia coli Nissle 1917 (EcN) is a probiotic used in the treatment of intestinal diseases
secreted autotransporter toxin (Sat) is autotransported and its passenger domain is released into the medium in EcN cultures Previous studies by our group identified Sat-specific peptides in the proteome of Outer membrane vesicles (OMVs) isolated from EcN cultures grown in Luria-Bertani broth (LB) [33]
To examine whether EcN Sat is secreted, supernatants of LB cultures were processed and analyzed by Western blot using antibodies anti-Sat obtained against a peptide of the passenger domain
Summary
Escherichia coli Nissle 1917 (EcN) is a probiotic used in the treatment of intestinal diseases. EcN is furnished with a large repertoire of fitness factors that promote its competitiveness, which probably explains its success as a probiotic Among these fitness factors there are microcins, iron uptake systems, adhesins and proteases that contribute to the colonization of the human gut [10,11,12]. The borderline between virulence and fitness factor is in some cases diffuse, as virulence depends on factors that increase fitness during colonization of specific host niches One of these factors is the secreted autotransporter toxin Sat, encoded in EcN by a gene located in the genomic island II. Sat belongs to the subfamily of serine protease autotransporters of Enterobacteriaceae (SPATEs) This family is composed of extracellular proteases with diverse functions, normally associated with virulence of Gramnegative pathogens, which are secreted by the type Va secretion pathway [14, 15]. Class I includes proteins with cytotoxic activity and class II comprises non-cytotoxic proteins with roles in colonization and immunomodulation [16, 17]
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