Type III secretion by Salmonella typhimurium does not require contact with a eukaryotic host.

The type III secretion system encoded by pathogenicity island I in Salmonella typhimurium delivers proteins to the external milieu and into the eukaryotic host cell. The principal factor in induction of the secretion system was found to be a change in the pH of the culture medium from acidic to mildly alkaline. The synthesis of components of the secretion machinery and the production and secretion of substrates occur simultaneously and do not require contact with a eukaryotic host cell. This argues against the concept that type III secretion in S. typhimurium is a process in which the delivery of a presynthesized pool of substrates is triggered by contact with a eukaryotic host cell.