Abstract: The present invention relates, e.g., to a Lactobacillus bacterium, which (1) expresses a recombinant polypeptide containing a lipoprotein signal sequence from the OspA protein of Borrelia burgdorferi, or an active variant of the leader sequence, operably linked to one or more heterologous polypeptide(s) of interest and/or (2) which comprises an expressible polynucleotide encoding a recombinant polypeptide, wherein the polynucleotide encodes a lipoprotein signal from the OspA protein of Borrelia burgdorferi, or an active variant thereof, which is operably linked to one or more heterologous polypeptide(s) of interest. In one embodiment, the heterologous polypeptide is from Yersinia pestis, the etiologic agent of plague. In another embodiment, the heterologous polypeptide is from Borrelia burgdorferi, the etiologic agent of Lyme disease.