A genetic system termed in vivo expression technology was devised that positively selects for microbial genes that are specifically induced when microbes infect their host. The method of this invention comprises complementing the growth of an auxotrophic or antibiotic sensitive microorganism by integrating an expression vector by way of homologous recombination into the auxotrophic or antibiotic sensitive microorganism's chromosome and inducing the expression of a synthetic operon which encodes transcripts, the expression of which are easily monitored both in vitro and in vivo.