On extrinsic and intrinsic organizational themes in gram negative bacteria and their role in evolution and virulence of the bacterial genus Salmonella spp

Abstract

Gram-negative bacteria are composed of intrinsic organizational themes such as outer and inner membrane, cell wall, and replicons. Themes that not only determine the body-plan and physical properties but also set the boundaries for choice of life style and evolution of the organisms. Bacteria also harbor extrinsic organizational themes that are represented by horizontally transferred genetic information such as plasmids, bacteriophages and pathogenicity island that augments the unique properties that define the separate organizational units of the individual bacteria.

Our studies have focused on two organizational themes and studied how they influence pathogenicity, virulence and evolution of species specificity of the Gram-negative rod Salmonella enterica serovar Typhimurium. We have determined the genetic structure and organization of the Salmonella enterica centisome 7 genomic island (SCI) located at the aspV loci in S.enterica subspecies I strains. The island encodes 37 putative proteins, including the saf fimbrial opreron and the sinR transcriptional regulator. Other open reading frames (designated sciA to Z) in the island encode putative proteins with homologies to virulence-associated proteins in a number of Gram-negative bacteria that have the ability to interact and manipulate eukaryotic cells. Deletion of the entire SCI region affects the ability of the organisms to enter eukaryotic cells.

We have investigated the immunogenicity in mice of Saf fimbrial components, and studied the protection induced by oral challenge with live Salmonella enterica. These studies show that a systemic and mucosal immune response is induced in immunized mice. Subcutaneously immunized mice showed significantly fewer bacteria in spleens after challenge than non-immunized. During bacterial growth the cell wall is constantly remodelled and the integrity of the murein is dependent on a balance between synthesizing and degrading activities and the turnover is coupled to a highly efficient recycling pathway. The recycling pathway is coupled to induction of antibiotic resistance in many Gram-negative bacteria.

Interference with the pathway does not affect cell growth in vitro and mutants in the membrane transport protein AmpG and the cytoplasmic muropeptide specific amidase AmpD are as viable as wild type cells. We constructed mutations in Salmonella enterica serovar Typhimurium that interferes with the recycling pathway in this bacterium. A Salmonella ampD knock out mutant has lower in vivo fitness in a competition experiment in Balb/c mice than the wild type. The ampD mutant is also less able to enter mouse macrophages. Furthermore, these studies show that the ampD mutant induces nitric oxide to a higher degree than the wild-type.

In conclusion, we have investigated, by using similar methodologies, how different Gram-negative organizational themes affect the pathogenicity and evolution in the important human pathogen Salmonella.