Variations of the signature-tagged mutagenesis (STM) technique are now possible and the method can be applied to most pathogens that have an STM-selectable phenotype in a host system. STM screening of 15,040 mutants from 11 bacterial species identified 323 in vivo attenuated mutants. As a genome-scanning tool, STM will yield information about genes with unknown functions as well as information crucial for understanding microbial pathogenesis.