Abstract: Live vaccines and methods for preparing the vaccines useful in protecting a host from infection by a pathogenic microorganism are provided. Vaccines are prepared from live Lactobacillus cells which have been transformed using DNA technology to express heterologous antigens of pathogenic microorganisms or other suitable biological material. Genes encoding antigenic determinants pathogenic in the mammalian gastrointestinal tract are inserted into expression cassettes and fused with genes encoding an aggregation factor and/or a mucin binding factor. The inserted genes are shown to transform L. reuteri cells. The aggregation enhancing and mucin binding genes have been isolated and sequenced. The vaccine can be ingested orally in a pharmaceutical carrier or in milk products.