Immunization of pigs to prevent disease in humans:: Construction and protective efficacy of a Salmonella enterica serovar Typhimurium live negative-marker vaccine

Zoonotic infections caused by Salmonella enterica serovar Typhimurium pose a constant threat to consumer health, with the pig being a particularly major source of multidrug-resistant isolates. Vaccination, as a promising approach to reduce colonization and shedding, has been scarcely used, as it interferes with current control programs relying on serology as a means of herd classification. In order to overcome this problem, we set out to develop a negative-marker vaccine allowing the differentiation of infected from vaccinated animals (DIVA). Applying an immunoproteomic approach with two-dimensional gel electrophoresis, Western blot, and quadrupole time-of-flight tandem mass spectrometry, we identified the OmpD protein as a suitable negative marker. Using allelic exchange, we generated an isogenic mutant of the licensed live vaccine strain Salmoporc and showed that virulence of Salmoporc and that of the mutant strain, Salmoporc Delta ompD, were indistinguishable in BALB/c mice. In a pig infection experiment including two oral immunizations with Salmoporc Delta ompD and challenge with a multiresistant S. enterica serovar Typhimurium DT104 clinical isolate, we confirmed the protective efficacy of Salmoporc Delta ompD in pigs, showing a significant reduction of both clinical symptoms and colonization of lymph nodes and intestinal tract. OmpD immunogenic epitopes were determined by peptide spot array analyses. Upon testing of several 9-mer peptides, each including an immunogenic epitope, one peptide (positions F-100 to Y-108) that facilitated the detection of infected animals independent of their vaccination status (DIVA function) was identified. The approach described overcomes the problems currently limiting the use of bacterial live vaccines and holds considerable potential for future developments in the field.