Mercedes Berlanga

Berlanga, Mercedes

Hemeroteca Cientifica Catalana

Int. MIcrobIol. Vol. 16, 2013BOOK REVIEWS 67Horizontal gene transfer (HGT) is a major force in micro-bial evolution and a great source of genetic innovation in prokaryotes. The first evidence of HGT was reported in 1928 by Frederick Griffith, who noted that bacteria (pneu-mococci) are capable of transferring genetic information (virulence determinants) through a process known as trans-formation. Later, the identification of gene transfer medi-ated by plasmids (conjugation) and viruses (bacteriophag-es, transduction) and the recognition of transposable ele-ments further improved our knowledge of gene flux and the importance of mobile genetic elements. The book Horizontal gene transfer in microorganisms provides state of the art information on this fascinating topic. The nine chapters are written by several experts in the field, includ-ing the book’s editor M. Pilar Francino.The flow of genes between different species is a form of genetic variation whose implications have yet to be fully appreciated. Strains of the same bacterial species frequent-ly differ in up to 30–35% of their total genes in the ge-nome, resulting in organisms with contrasting phenotypes and/or ecological strategies (e.g., highly invasive patho-gens vs. commensal strains of Escherichia coli). Mobile genetic elements, such as transposons, phages and plas-mids, account for most of the differences in gene content among closely related strains. This genomic flexibility contributes to the ability of bacteria to adapt to varying environmental challenges and thus ensure survival. The persistence of new genes depends on whether they are rep-licated as parts of integrated genomes. Genomes and genes evolve in response to natural selection at the organismal level, as they regulate gene interactions and permanence (Chapter 1).In prokaryotes, usually around 80% of the genome en-codes proteins. Bacterial chromosomes also seem to be or-ganized into regions with different gene expression levels. Functionally related genes are usually transcribed in oper-ons or superoperons. Genomics and genome sequencing studies have confirmed that the position of genes in the chromosome is not random but the result of selection. However, despite these common organizing principles, prokaryotic genomes are extremely diverse. Bacterial ge-nome sequencing has become so easy and accessible that the genomes of multiple strains of an increasing number of individual species have been and will be rapidly deter-mined. These data sets provide for in-depth analysis of in-tra-species diversity. The pan-genome is the sum of the core genome, i.e., those genes common to all strains, and the dispensable ge-nome. The latter group of genes may confer adaptive ad-vantages under certain environmental conditions, such as in the presence of antibiotics, xenobiotics, and other com-pounds. These genes also confer the important characteris-tics that allow the colonization of new ecological niches governed by biotic factors such as symbiotic and patho-genic relationships. Pan-genome analyses are aimed at in-terpreting the sequence data among widespread lateral gene flow. A comparison of 20 strains of Escherichia coli showed that the core genome consists of only 2000 genes, less than half the average genome size in a typical strain. Modifications of gene repertoires can strongly affect cellular Horizontal gene transfer in microorganismsM. pIlar francIno (ed)2012, Caister Acad Press, Norfolk, UK 202 pp, 16 × 24 cm Price:  £159ISBN: 978-1-908230-10-2Int. MIcrobIol. Vol. 16, 2013 BOOK REVIEWS64metabolism and morphology, and such changes can provide prokaryotes with the opportunities for adaptation to new ecological niches (Chapter 2). The mammalian-adapted bacterium Bartonella and plant-adapted rhizobia are dis-cussed as examples in Chapter 3.Horizontal gene transfer operates by a variety of mech-anisms, relies on different ecological circumstances, and results in a wide range of fitness effects on the recipient organisms. Chapter 4 explores the barriers that limit HGT among microorganisms, such as DNA restriction. Exoge-nous DNA that survives restriction must be integrated into the bacterial host chromosome and then expressed in order to persist in the recipient cell. Transferred genes may con-tinue evolving in different genomic and ecological envi-ronments. Chapter 5 analyzes the process of sequence di-vergence of transferred genes at different evolutionary time-scales in the Xanthomonadales, a group of plant path-ogens belonging to Gammaproteobacteria. The genomes of the Xanthomonadales are the result of multiple HGT events from other Proteobacteria to the ancestral genome of the group. Chapter 6 discusses the environmental regu-lation of HGT, as opposed to genetic regulation at the cel-lular level as explained in Chapter 4. HGT has been studied in natural ecosystems, including soil, water, and the gut microbiota. Conditions in the gut are favorable for HGT because of the constant body temperature and inflow of nutrients, which support high population densities and vast diversity. Bacterial responses to host stress hormones can facilitate conjugative gene transfer between enteric bacte-ria. In in-vitro experiments, the physiological concentra-tion of noradrenaline was shown to stimulate the transfer of a conjugative plasmid from a clinical strain of Salmo-nella sp. to an E. coli recipient. These mediators of host stress may affect HGT within the in vivo environment as well.Plasmids, bacteriophage elements, transposons, and in-sertion sequences are mobile genetic elements because they can move between microbial cells. The total of all the mobile genetic elements in a cell is referred to as the mob-ilome. Despite the ability of plasmids to transfer across dif-ferent taxonomic groups of bacteria, it is unclear which factors are responsible for plasmid continuance among bacterial populations. Chapter 7 describes the different hy-potheses explaining the maintenance of plasmids.Several approaches can be used to detect mobile genetic elements. In Chapter 8, four different experimental method-ologies are described: sequencing studies, sequence-based screening, capture studies (hybridization with magnetic beads), and functional studies. The appearance and dis-semination of antibiotic-resistance in bacterial pathogens have stimulated investigations of the genetic aspects of the different phenomena associated with resistance develop-ment, such as gene pickup, heterologous expression, HGT, and mutation. Chapter 9 focuses on HGT and recombina-tion in the evolution of antibiotic resistance genes. Essen-tially any mobile genetic element found in bacteria can acquire resistance genes and promote their transmission; the type of element involved varies with the genus of the pathogen, but plasmid-mediated transmission is far and away the most common mechanism of HGT. Moreover, the existing processes of gene acquisition, transfer, modifi-cation, and expression are expanding and their presence in the modern biosphere is accelerating in response to anthro-pogenic influences.Clearly, prokaryotes, are under different forms of selec-tive pressures with respect to DNA exchange than most multicellular organisms. The theory of prokaryotic evolu-tion needs further work as it must take into account the population genetics and molecular evolution of organisms that have no traditional species boundaries but share genes across considerable evolutionary distance. Horizontal gene transfer in microorganisms is an up-to-date and nec-essary review of the most topical aspects of HGT. Mercedes BerlanGaUniversity of Barcelonamberlanga@ub.edu

Horizontal gene transfer in microorganisms

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Horizontal gene transfer in microorganisms

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Hemeroteca Cientifica Catalana