The
first immunization attempts using an L. casei strain that
contained the pspA gene of S. pneumoniae cloned on a plasmid
and under the control of an inducible lactose promoter
showed no stimulation of the immune response, probably due
to low levels of the antigen. Therefore, for the comparison of
the immunogenicity of the LAB strains carrying the pspA
gene, a strong constitutive promoter was employed.