Colonization of cattle by non-O157 Shiga Toxin-producing Escherichia coli serotypes
Asper, David Jose
Shiga toxin-producing E. coli (STEC) is an important food- and water-borne pathogen of humans, causing Hemorrhagic Colitis and Haemolytic Uremic Syndrome. Colonization of both cattle and human hosts is mediated through the action of effector molecules secreted via a type III secretion system (T3SS), which forms attaching and effacing lesions (A/E). The necessary effectors which form A/E by manipulation of host signalling and actin nucleation are present on a pathogenicity island called the Locus of Enterocyte Effacement (LEE). It has been reported that vaccination of cattle with Type III-secreted proteins (T3SPs) from STEC O157 resulted in decreased shedding. In order to extend this to non-O157 STEC serotypes, we examined the serological cross-reactivity of T3SPs of serotypes O26:H11, O103:H2, O111:NM and O157:H7. Groups of cattle were vaccinated with T3SPs produced from each of the serotypes and the magnitude and specificity of the responses were measured resulting in limited cross reactivity. Overall, results suggest that vaccination of cattle with T3SPs as a means of reducing the risk of STEC transmission to humans will induce protection that is serotype specific. To pursue the possibility of a cross-protective vaccine, we investigated the protective properties of a chimeric Tir protein against STEC serotypes. Several studies have reported that Tir is highly immunogenic and capable of producing high antibody titers. Potter and colleagues also demonstrated that the vaccination of cattle with ∆tir STEC O157 strain did not protect as well as the wildtype strain. We constructed thirty-mer peptides to the entire STEC O157 Tir protein, as well as to the intimin binding domain of the Tir protein from STEC serotype O26, O103 and O111. Using sera raised against STEC O157 and non-O157 T3SPs, we identified a number of immunogenic peptides containing epitopes unique to a particular serotype. Two different chimeric Tir proteins were constructed containing the STEC O157 Tir protein fused with six STEC non-O157 peptides with or without the Leukotoxin produced by Mannheimia haemolytica. However, the vaccination of mice with the chimeric protein did not protect against challenge with STEC O157 or STEC O111. These results suggest that to achieve cross protection against STEC serotypes using a recombinant protein vaccine, other immunogenic and protective antigens must also be included. In order to identify other immunogenic and cross-protective antigens we cloned and expressed the genes coding for 66 effectors and purified each as histidine-tagged proteins. These included 37 LEE-encoded proteins and 29 non-LEE effectors. The serological response against each protein was measured by Western blot analysis and an enzyme-linked immunosorbent assay (ELISA) using sera from rabbits immunized with T3SPs from four STEC serotypes, experimentally infected cattle and human sera from 6 HUS patients. A total of 20 proteins were recognized by at least one of the STEC T3SP- vaccinated rabbits using Western blots. Sera from experimentally infected cattle and HUS patients were tested using an ELISA against each of the proteins. Tir, EspB, EspD, EspA and NleA were recognized by the majority of the samples tested. Overall, proteins such as Tir, EspB, EspD, NleA and EspA were highly immunogenic for both vaccinated and naturally infected subjects. Based on the above results, two different mixtures of secreted proteins (5 proteins and 9 proteins) were used to vaccinate mice and test the level of shedding following challenge with STEC O157. Overall, the cocktail vaccine containing 9 immunogenic effectors including Tir, EspB, EspD, NleA and EspA was capable of reducing shedding as effectively as the current STEC T3SPs vaccine, Econiche®.
DegreeDoctor of Philosophy (Ph.D.)
Copyright DateSeptember 2009
Shiga Toxin-producing Escherichia coli
Type III Secretion System