Salmonella: now you see it, now you don't

Diseases caused by Salmonella species are characterized by bacterial invasion of host cells. Salmonella invasion requires a genetic locus (inv) with homology to bacterial systems involved in specific protein export and organelle assembly. Until recently, the actual Salmonella invasion factors exported or assembled by the inv system remained unidentified. It now appears that Salmonella produces novel appendages upon contact with host cells. These appendages are transient, appearing and disappearing rapidly from the bacterial surface. Appendages are altered in strains unable to invade due to mutations within the inv/spa locus. Therefore, a role for the invasion locus has been identified, providing another example of bacterial pathogens responding to signals provided by the host cell surface.