Salmonella enterica serovar Typhimurium strain 82/6915 was attenuated by transposon mutagenesis (Tn10) generating the vaccine strain STM-1, which resulted in functional inactivation of aroA (Alderton et al., 1991). Inactivation of aroA renders the strain auxotrophic for aromatic amino acids (tryptophan, tyrosine and phenylalanine) and is associated with the loss of detectable H2S production on XLD media. A Salmonella spp. with functional AroA results in cream coloured colonies with black centres (H2S), which is characteristic of Salmonella spp. on XLD media. The parent strain can produce H2S on XLD media. Observations in the field and in in vivo chicken trials when STM-1 was recovered from mixed samples containing common chicken intestinal microflora, e.g., E. coli, identified colonies of STM-1 that appeared to produce H2S on XLD agar. The role of co-culturing an Escherichia spp. with STM-1 in compensating for the aroA deletion and restoring the H2S producing phenotype was investigated by cross-streaking and co-culturing in broth then co-plating on XLD agar.
H2S production is observed in Salmonella Typhimurium DaroA strains when co-cultured with E. coli and to a lesser extent, E. fergusonii. H2S production is restored only when STM-1 is in physical contact with an Escherichia colony. Growing STM-1 in the presence of filter sterilised supernatant from an Escherichia culture does not lead to H2S production in STM-1. It is likely that the compound responsible for phenotypic reversion to H2S production is not in sufficiently high concentrations in the supernatant or is not stable in the environment, which is why colony to colony contact is required. Continual cell turnover is the likely source of compounds produced downstream of aroA in the aromatic amino acid biosynthesis pathway, e.g., the branch chain intermediate chorismate.