Coordinate intracellular expression of Salmonella genes induced during infection

Salmonella typhimurium in vivo-induced (ivi) genes were grouped by their co ordinate behavior in response to a wide variety of environmental and geneti c signals, including pH, Mg2+, Fe2+, and PhoPQ, All of the seven ivi fusion s that are induced by both low pH and low Mg2+ (e.g., iviVI-A) are activate d by the PhoPQ regulatory system. Iron-responsive ivi fusions include those induced under iron limitation (e.g., entF) as well as one induced by iron excess but only in the absence of PhoP (pdu). Intracellular expression stud ies showed that each of the pH- and Mg2+-responsive fusions is induced upon entry into and growth within three distinct mammalian cell lines: RAW 264. 7 murine macrophages and two cultured human epithelial cell lines: HEp-2 an d Henle-407, Each ivi fusion has a characteristic level of induction consis tent within all three cell types, suggesting that this class of coordinatel y expressed ivi genes responds to general intracellular signals that are pr esent both in initial and in progressive stages of infection and may reflec t their responses to similar vacuolar microenvironments in these cell types . Investigation of ivi expression patterns reveals not only the inherent ve rsatility of pathogens to express a given gene(s) at various host sites but also the ability to modify their expression within the context of differen t animal hosts, tissues, cell types, or subcellular compartments.