TheEscherichia coliO157:H7 Carbon Starvation-Inducible Lipoprotein Slp Contributes to Initial AdherenceIn Vitrovia the Human Polymeric Immunoglobulin Receptor

Abstract

AbstractEscherichia coliO157:H7 is the most well-studied serotype of enterohemorrhagicE. coli(EHEC) class ofE. coliintestinal pathogens, and is responsible for many outbreaks of serious food-borne illness worldwide each year. Adherence mechanisms are a critical component of its pathogenesis, persistence in natural reservoirs, and environmental contamination.E. coliO157:H7 has a highly effective virulence operon, the Locus of Enterocyte Effacement (LEE), and its encoded intimate adherence mechanism is well characterized. However, factors involved in the preceding initial attachment are not well understood. In this study, we propose a mechanism of initial adherence used byE. coliO157:H7in vitro. We describe a bacterial protein not previously reported to be involved in adherence, Slp, and its interactions with the human host protein polymeric immunoglobulin receptor (pIgR). The human pIgR has previously been shown to act as an adherence receptor for some mucosal pathogens, and is highly expressed in the intestine. Following observation of significant colocalization betweenE. coliO157:H7 and pIgR location on Caco-2 cells, a co-immunoprecipitation (Co-IP) assay using a human recombinant Fc-tagged pIgR protein led to the identification of this protein. Disruption of Slp expression inE. coliO157:H7, through deletion of its encoding geneslp, produced a significant adherence deficiency to Caco-2 cells at early time points associated with initial adherence. Plasmid complementation ofslpfully restored the wild-type phenotype. Furthermore, immunofluorescence microscopy revealed evidence that this interaction is specific to the pathogenic strains ofE. colitested, and not the nonpathogenic strain E. coli K12. Additionally, deletion ofslpresulted in the absence of the corresponding protein band in further Co-IP assays, while the plasmid-encodedslpcomplementation of the deletion mutant strain restored the wild-type pattern. These data support the proposal that Slp directly contributes to initial adherence, with the pIgR protein as its proposed receptor.Author summaryEscherichia coliO157:H7 and other enterohemorrhagicE. coli(EHEC) are responsible for tens of thousands of cases of food-borne illness in the United States each year.E. coliO157:H7 has a particularly effective intimate adherence mechanism. However, the mechanisms of initial adherence, which facilitate attachment and virulence prior to the engagement of intimate adherence, are not well understood. In this study, we describe an initial adherence interaction between theE. coliO157:H7 Slp and the human polymeric immunoglobulin receptor (pIgR) expressed by the human colonic epithelial cell line Caco-2. The relationship was first demonstrated as a significant colocalization between the locations ofE. coliO157:H7 bacterial cells and pIgR protein using immunofluorescence microscopy. TheE. coliO157:H7 Slp protein was identified, and disruption of theslpgene resulted in a severe adherence deficiency to Caco-2 cells during initial adherence. This effect was reversed upon complementation of the Δslpstrain with a plasmid-encodedslpgene, and the constitutive over-expression ofslpresulted in hyper-adherence exceeding that of the wild-typeE. coliO157:H7. These data support the proposition that Slp directly contributes to initial adherence, with the pIgR protein as its proposed receptor.