The genetic modification of the poultry virulent form of S. typhimurium strain 82/6915, was performed using enterobacteria phage P22 transduction of a DNA vector containing the S. typhimurium LT2 aroA554::Tn10 transposon originating from strain 1545.

The insertion site of the transposon Tn10 is in the aroA-serC operon. aroA specifies the enzyme 3-enolpyruvylshikimate-5-phosphate synthetase. Strains contining this transposon cannot grow on chemically defined media unless provided with the essential metabolites derived from chorismic acid, i.e. the aromatic amino acids tyrosine, phenylalanine, and tryptophan and two minor aromatic metabolites, p-aminobenzoate, needed as a precursor of folic acid, and 2,3-dihydroxybenzoic acid, as a precursor of the iron-chelating compound enterobactin (enterochelin). serC encodes phosphoserine aminotransferase, an enzyme in the biosynthesis of serine. Insertion of the transposon also imparts tetracycline resistance.

Transduction of the receipient strain S. typhimurium 82/6915 was carried out in the presence of tetracycline supplemented medium.  Transductant clones that were found to be resistant to tetracycline were selected.

Colonies were isolated and further screened for loss of tetracycline resistance. The isolation of tetracycline-sensitive variants is facilitated by the fact that tetracycline, at appropriate concentrations, prevents multiplication of tetracycline-sensitive bacteria, but does not kill them, whereas penicillin kills multiplying bacteria but spares non-multiplying bacteria. The technique of penicillin-selection was used for isolation of tetracycline-sensitive variants.

A tetracycline-sensitive mutant that remained auxotrophic for aromatic metabolites and serine was identified. The resulting S. typhimurium strain 82/6915 aroA-serC deletion mutant, was denominated as strain STM-1.

The requirement for aromatic metabolites and serine,is not met when S. typhimurium is present in vertebrate tissues, resulting in retardation of growth in vivo or outside the host. Deficient lineages of this modified-live vaccine remain in host tissue for several days, but without causing symptoms and eventually be eliminated by immunological defence mechanisms while still imparting protection against salmonella.