The nadB gene of Salmonella typhimurium complements the nicotinic acid auxotrophy of Shigella flexneri.

Shigella species are characteristically nicotinic acid (NA) auxotrophs. The invasive S. flexneri strain M90T, transformed with the multicopy plasmid pZT349 encoding the nadB gene of Salmonella typhimurium, can grow in minimal glucose medium without exogenous NA, whereas, M90T containing the control vector, pUC18 does not, suggesting that this species lacks L-aspartic acid oxidase, the first enzyme in the de novo NAD biosynthetic pathway. The estimated growth rate of strain M90T (pZT349) in HeLa cells was identical to that of M90T (pUC18), indicating the available intracellular concentration of NA is not limiting for bacterial growth.