Important progress in the elucidation of the mechanisms influencing bacterial pathogenicity has recently been made through the introduction of modem genetic techniques. Molecular cloning allows the isolation of genes for pheno- types that epidemiological surveys have suggested play an important role in pathogenesis. The structural analysis of determinants for pathogenic traits can lead to the identifica- tion not only of the primary sequence but also of the possi- ble secondary and tertiary structures for important viru- lence factors such as toxins and adhesins. From these data, the...