Levels of expression and immunogenicity of attenuated salmonella enterica serovar typhimurium strains expressing escherichia coli mutant heat-labile enterotoxin - Covone, M Giuseppina; Brocchi, Marcelo; Palla, Emanuela; Silveira, W Dias da; Rappuoli, Rino; Galeotti, Cesira L (Infection and Immunity, 1998)