Rapid Method for the Construction of Salmonella enterica Serovar Typhimurium Vaccine Carrier Strains

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Abstract
Salmonella enterica serovar Typhimurium is a versatile organism for the generation of live recombinant vaccines for mucosal immunization. Various strategies have been devised for the stable and efficient expression of heterologous antigens by attenuated S. enterica strains, but these methods often require complex manipulations. Use of phage λ Red recombinase has recently been devised for gene replacements in Escherichia coli and S. enterica after introduction of PCR products. Based on this method, we have developed an approach that allows the integration of recombinant expression cassettes for heterologous antigens in a single step. The recombinant construct is integrated into the chromosome and is devoid of any selective marker such as antibiotic resistance. We observed the stable expression of model antigens without selective pressure. In addition, the method allows the simultaneous generation of attenuating mutations by gene deletions. The novel “knock-in” approach allows the rapid and efficient construction of recombinant Salmonella strains as vaccine carriers.