Provided herein are Salmonella enteritidis 13A strains and compositions comprising these strains. Also provided are methods of enhancing an immune response against Influenza A and methods of reducing morbidity associated with an Influenza A infection. Methods of enhancing an immune response to a vaccine vector by expressing a polypeptide of CD154 capable of binding CD40 are also disclosed. Methods of developing a bacterial vaccine vector are disclosed. Methods of generating scarless site-specific mutations in a bacterium are also disclosed.