MECHANISMS OF INCREASED SHIGA TOXIN PRODUCTION BY COMMENSAL E. COLI

Project summary/abstract Escherichia coli O157:H7 is a foodborne pathogen most commonly transmitted to humans whenfecal material from cattle comes into contact with food, and the food is subsequently uncooked orundercooked. After colonization of the gastrointestinal tract, the organism can elaborate a toxin calledShiga toxin (Stx), which is necessary for the severe symptoms observed in some patients such ashaemolytic uremic syndrome. Previous genetic studies identified several features common to highlyvirulent E. coli O157:H7 strains. For example, such strains often carry the Stx2a allelic variant, andinvariably have a type III secretion system called the LEE. Genome analysis has also supported thehypothesis that highly virulence strains fall within defined phylogenetic lineages. More recently, studiesfrom our group and others began to mechanistically describe how members of the gut microbiota mayincrease Stx2a production. This proposal builds on our preliminary data that suggests there are at leastthree mechanisms by which non-pathogenic E. coli increase Stx2a production by E. coli O157:H7 during co-culture: 1) through infection of the non-pathogenic strain by the stx2a-converting bacteriophage, describedpreviously by others; 2) through secretion of a small molecule that triggers Stx2a activation; 3) through anundefined mechanism that does not appear to involve the phage or secreted molecules. Here, we proposein Aim 1 complementary genetic and biochemical approaches to identify the molecule secreted by a humancommensal isolate designated 0.1229. Our published work demonstrated that toxin amplification drivenby mechanism #1 could be recapitulated in a germ-free mouse model, and we will similarly test thehypothesis here that co-infection of mice with 0.1229 and E. coli O157:H7 results in greater virulence.Preliminary data indicates the secreted molecule has the characteristics of a microcin ? it is small (<10kDa), heat resistant, and susceptible to Proteinase K. It was previously reported that E. coli fromphylogroup B2, which is the most common lineage of human fecal E. coli isolates in the United States andseveral other countries including Japan and Australia, encode microcins and bacteriocins more frequentlythan isolates from other phylogroups. Therefore in Aim 2, we will leverage Penn State?s E. coli ReferenceCenter collection to test the hypothesis that phylogroup B2 isolates increase toxin production by E. coliO157:H7 more commonly than non-B2 isolates. Together, these two aims are expected to identify a new mechanism by which commensal E. colicould potentiate E. coli O157:H7 disease, and continue the development of tools for the identification of gutbacteria that impact Stx2a production. These data could be used to understand differences in diseaseseverity between individuals, how diet-induced changes in the microbiota affect disease outcome, and todevelop probiotic therapies for reducing toxin production by E. coli O157:H7 during an infection.1