Signal Transduction and Virulence Gene Regulation in Shigella spp.: Temperature and (maybe) a Whole Lot More
Many bacterial pathogens of man pass through several different environments before coming into contact with and colonizing the host. These may include another mammal (zoonotic infection), insects (arthropod-borne infections), or specific niches in the ter
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CHAPTER 7
SIGNAL TRANSDUCTION AND VIRULENCE GENE REGULATION IN SHIGELLA SPP.: TEMPERATURE AND (MAYBE) AWHOLE Lor MoRE Catherine M. C. O'Connell, Robin C. Sandlin and Anthony T. Maurelli 1. INTRODUCTION
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any bacterial pathogens of man pass through several different environments before coming into contact with and colonizing the host. These may include another mammal (zoonotic infection), insects (arthropod-borne infections), or specific niches in the terrestrial or aquatic ecosystems, e.g. contaminated food or water. Even after gaining access to a host, a pathogen often passes through several different environments before reaching the site it will ultimately colonize. Thus, for most bacterial pathogens one can imagine a life cycle which consists of a segment outside the host and a segment within the host, the latter which can be subdivided into the various anatomical sites through which the organism passes on its way to the site of colonization. Bacteria economize energy by regulating gene expression such that genes are turned on only when they are needed. Virulence genes are no exception and the various systems in the pathogens described in The opinions or assertions contained herein are the private ones of the authors and are not to be construed as official or reflecting the views ofthe Department ofDefense or the Uniformed Services University of the Health Sciences.
Signal Transduction and Bacterial Virulence, edited by Rino Rappuoli, Vincenzo Scarlato and Beatrice Arico.© 1995 R.G. Landes Company.
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this book attest to this fact. For the most part, virulence genes are nonessential for survival outside the host. Expression of some virulence genes may even be detrimental to the organism when expressed outside the host. At the very least, such expression would be a waste of valuable energy. Therefore, bacterial pathogens have evolved to tightly control expression of their virulence genes. To express these genes only when in the host, bacteria need to be able to know when they have arrived in the host. The ability to sense signals which indicate to a bacterium that it is in a host must then be integrated into a regulatory loop such that the virulence genes can be turned on. Thus, signal sensing and signal transduction within the host is a critical element for success as a pathogen. The organism which we study, Shigella flexneri, has proven to be an excellent model for the study of virulence gene regulation through environmental sensing. Bacteria of the species Shigella (S. dysenteriae, S. flexneri, S. boydii, and S. sonnei) are Gram-negative rods which are the causative agents of bacillary dysentery or shigellosis. Clinical disease is a result of invasion of the bacteria into the epithelial cells of the large intestine, multiplication within the cytoplasm of these cells and subsequent death of the infected cells. In the course of the infection, the bacteria spread laterally into adjacent cells in the epithelium, thus propagating the infection. These steps in the life cycle of Shigella in the human host can
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