Nature Reviews Microbiology - Reviews: "Many of the -proteobacteria establish long-term, often chronic, interactions with higher eukaryotes. These interactions range from pericellular colonization through facultative intracellular multiplication to obligate intracellular lifestyles. A common feature in this wide range of interactions is modulation of host-cell proliferation, which sometimes leads to the formation of tumour-like structures in which the bacteria can grow. Comparative genome analyses reveal genome reduction by gene loss in the intracellular -proteobacterial lineages, and genome expansion by gene duplication and horizontal gene transfer in the free-living species. In this review, we discuss -proteobacterial genome evolution and highlight strategies and mechanisms used by these bacteria to infect and multiply in eukaryotic cells.

The -subdivision of the Proteobacteria is a very large and diverse group of Gram-negative microorganisms. They show great variability in metabolic capacities and inhabit diverse ecological niches. The authors discuss the different mechanisms used by these bacteria to associate with eukaryotic hosts, either as symbionts or as pathogens.

The genomes of the -proteobacteria vary in both size (from 1 to 9 Mb) and organization (from single circular replicons to multiple replicons, both circular and linear). The genome of the common ancestor of the -proteobacteria was estimated to contain 3,000−5,000 genes. Two major trends associated with the conversion of this ancestral gene pool into the genomes of the modern species can be recognized: intracellular bacteria associated with invertebrates, animals and humans have evolved by gene loss, whereas the soil-growing, plant-associated bacteria have evolved through genome expansion.

Despite the close phylogenetic relationships between these bacteria, there is no clear 'common mechanism' that allows these bacteria to interact with their eukaryotic hosts. Is this surprising considering the diversity of the hosts? The strategies and mechanisms used by these bacteria to infect the host, avoid host defences and then adapt to the host are described, compared and contrasted.

A recurrent feature of the interaction of -proteobacteria with their hosts is an effect on host-cell proliferation, creating a new niche in which, or on which, the bacteria can survive. This involves inhibiting cell death and then inducing cell proliferation. Here again, each bacterium uses a different mechanism, ranging from the production of diverse molecules that affect host cell biology, often affecting growth hormones, to 'genetically manipulating' the host cell to induce tumorigenesis."

Jacques Batut, Siv G. E. Andersson & David O'Callaghan THE EVOLUTION OF CHRONIC INFECTION STRATEGIES IN THE α-PROTEOBACTERIA Nature Reviews Microbiology 3, 933-945 (2005); doi:10.1038/nrmicro1044

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