Nonfimbrial Adhesin Mutants Reveal Divergent Escherichia coli O157:H7 Adherence Mechanisms on Human and Cattle Epithelial Cells

Abstract

Shiga toxin-producing, enterohemorrhagic Escherichia coli (EHEC) serotype O157:H7 is a major foodborne pathogen causing symptoms ranging from simple intestinal discomfort to bloody diarrhea and life-threatening hemolytic uremic syndrome in humans. Cattle can be asymptomatically colonized by O157:H7 predominantly at the rectoanal junction (RAJ). Colonization of the RAJ is highly associated with the shedding of O157:H7 in bovine feces. Supershedding (SS) is a phenomenon that has been reported in some cattle that shed more than 104 colony-forming units of O57:H7 per gram of feces, 100–1000 times more or greater than normal shedders. The unique bovine RAJ cell adherence model revealed that O157:H7 employs a LEE-independent mechanism of attachment to one of the RAJ cell types, the squamous epithelial (RSE) cells. Nine nonfimbrial adhesins were selected to determine their role in the characteristic hyperadherent phenotype of SS O157 on bovine RSE cells, in comparison with human HEp-2 cells. A number of single nucleotide polymorphisms (SNPs) were found amongst these nonfimbrial adhesins across a number of SS isolates. In human cells, deletion of yfaL reduced the adherence of both EDL933 and SS17. However, deletion of eae resulted in a significant loss of adherence in SS17 whereas deletion of wzzB and iha in EDL933 resulted in the same loss of adherence to HEp-2 cells. On RSE cells, none of these nonfimbrial deletion mutants were able to alter the adherence phenotype of SS17. In EDL933, deletion of cah resulted in mitigated adherence. Surprisingly, four nonfimbrial adhesin gene deletions were actually able to confer the hyperadherent phenotype on RSE cells. Overall, this study reveals that the contribution of nonfimbrial adhesins to the adherence mechanisms and functions of O157:H7 is both strain and host cell type dependent as well as indicates a possible role of these nonfimbrial adhesins in the SS phenotype exhibited on RSE cells.

Escherichia coli 0157:H7 virulence factors and the ruminant reservoir

“…Transmission and persistence of E. coli O157:H7 in cattle herds depends on a small percentage of cattle called ‘supershedders’. These animals shed 100 to 1000 times more E. coli O157:H7 than average shedders [139 ▪ ,150]. Composition of GIT commensals differs between supershedders and nonshedders, suggesting that the RAJ microbiota may be influenced by or affected by E. coli O157:H7 [135,151–154].…”

“…The bacteria predominately inhabit the colon and recto-anal junction mucosa (RAJ) [135,136]. RAJ colonization is highly associated with E. coli O157:H7 in bovine feces and fecal concentration correlates with herd prevalence and long-term carriage [137,138,139 ▪ ]. Strategies to control bovine E. coli O157:H7 shedding to prevent and reduce transmission in a herd includes: vaccination with E. coli O157:H7 variants (LEE-negative and Stx-negative strains), protein supplements in feed, bacteriophage thearapies, prebiotic and probiotic treatments, farm managment practices, and feed containing CRISPR-cas9-engineered phage or conjugative plasmids [58,140–145].…”

“…RAJ colonization is mediated by LEE-dependent and LEE-independent adherence factors [139 ▪ ,163–164]. Expression of nearly all LEE genes is induced by rectal digesta and intimin-Tir interactions promotes E. coli O157:H7 colonization in cattle [155,158,165].…”

Recent Genetic Changes Affecting Enterohemorrhagic Escherichia coli Causing Recurrent Outbreaks

“…Adhesins are found on the surface of bacteria and mediate adhesion to host cells, abiotic surfaces, and/or other bacterial cells, and can promote biofilm formation. Adhesins are involved in EHEC adherence to the cells of the RAJ ( 13 ). The adhesin mutations described above may alter their adhesive properties, enabling colonization of the RAJ or otherwise accounting for the recurrence of outbreaks.…”

“…Truncation may even be necessary for this binding ( 15 ). In any case, inactivation of adhesin genes can promote adhesion to cells of the RAJ ( 13 ).…”