Bacterial Mobile Genetic Elements
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5 results
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The Mosaic Type IV Secretion Systems
- Author: Peter J. Christie
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Citation: Christie P. 2016. The Mosaic Type IV Secretion Systems, EcoSal Plus 2016; doi:10.1128/ecosalplus.ESP-0020-2015
- DOI 10.1128/ecosalplus.ESP-0020-2015
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Abstract:
Escherichia coli and other Gram-negative and -positive bacteria employ type IV secretion systems (T4SSs) to translocate DNA and protein substrates, generally by contact-dependent mechanisms, to other cells. The T4SSs functionally encompass two major subfamilies, the conjugation systems and the effector translocators. The conjugation systems are responsible for interbacterial transfer of antibiotic resistance genes, virulence determinants, and genes encoding other traits of potential benefit to the bacterial host. The effector translocators are used by many Gram-negative pathogens for delivery of potentially hundreds of virulence proteins termed effectors to eukaryotic cells during infection. In E. coli and other species of Enterobacteriaceae, T4SSs identified to date function exclusively in conjugative DNA transfer. In these species, the plasmid-encoded systems can be classified as the P, F, and I types. The P-type systems are the simplest in terms of subunit composition and architecture, and members of this subfamily share features in common with the paradigmatic Agrobacterium tumefaciens VirB/VirD4 T4SS. This review will summarize our current knowledge of the E. coli systems and the A. tumefaciens P-type system, with emphasis on the structural diversity of the T4SSs. Ancestral P-, F-, and I-type systems were adapted throughout evolution to yield the extant effector translocators, and information about well-characterized effector translocators also is included to further illustrate the adaptive and mosaic nature of these highly versatile machines.
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The Role of Conjugation in the Evolution of Bacteria
- Author: Fernando de la Cruz
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Source: Microbes and Evolution , pp 133-138
Publication Date :
January 2012
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Abstract:
There are apparently different mechanisms to achieve conjugation if one judges by the lack of sequence similarity among conjugative plasmids. The process of conjugation can be conveniently divided into three basic steps: initiation (or DNA processing in the donor), DNA transport, and termination (or DNA processing in the recipient). The plasmid R388 is an ideal model to study conjugation because of its simplicity. In an envisaged scenario, plasmids are the cavalry in the army of bacterial evolution, the first to arrive to the battlefield. The rationale behind this idea is that by inhibiting conjugation, a two-pronged weapon is used to combat bacterial disease: on one side the spread of antibiotic resistance is avoided, and on the other, some mechanisms of virulence that directly use type IV secretion systems can be directly attacked. In summary, conjugation is an essential feature in the physiology of plasmids. Conjugation plays an important role in the evolution of bacterial genomes. Human pathogenic bacteria have acquired many of their virulent traits, as well as antibiotic resistance, by conjugation. If one learns how to control conjugation, an additional potent weapon to combat human bacterial disease can be obtained.
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Pulsed-Field Gel Electrophoresis: Laboratory and Epidemiologic Considerations for Interpretation of Data
- Authors: Richard V. Goering, Efrain M. Ribot, Peter Gerner-Smidt
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Source: Molecular Microbiology , pp 167-177
Publication Date :
January 2011
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Abstract:
This chapter reviews pulsed-field gel electrophoresis (PFGE) as an epidemiological tool, considering (i) factors that influence the electrophoretic process, (ii) methodological streamlining, (iii) the troubleshooting of common problems, (iv) quality assurance, (v) use of PFGE for continuous surveillance, and (vi) issues of data interpretation. To be suitable for reliable PFGE analysis, intact chromosomal DNA must be isolated in a protected environment free from mechanical, chemical, and enzymatic degradation to yield a clear and reproducible macrorestriction fragment pattern. As PFGE analysis is applied to larger study populations, the need for computer-assisted analysis (CAA) of banding patterns becomes increasingly evident. At the laboratory level the quality assurance/ quality control (QA/QC) system consists of strict adherence to each of the PFGE standard operating procedures (SOPs) as described in the laboratory QA/QC manual. It is important to emphasize that the successful establishment of dynamic databases is dependent on strict adherence to well-defined QA and QC criteria. An important component of the protocol standardization and QA/QC program for PulseNet is the annual update meeting. Molecular typing, along with a variety of other microbiological assays is clearly moving toward sequence-based analysis. However, this approach is still being validated for a variety of applications including strain typing. Thus far, none of the new sequence-based typing methods are as broadly applicable as PFGE. Therefore, while this problem will undoubtedly be solved in the future, at present PFGE will clearly continue to provide meaningful epidemiological data on molecular typing in a variety of important settings for years to come.
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Genetic Exchange in the Respiratory Tract
- Author: Christopher G. Dowson
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Source: Colonization of Mucosal Surfaces , pp 131-140
Publication Date :
January 2005
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Abstract:
This chapter focuses on three important pathogens and the role that genetic exchange has played in their continuing evolution. There are numerous examples where genetic exchange has been responsible for the evolution of antibiotic resistance and virulence determinants and wholesale acquisition of pathogenicity islands, and there is increasing evidence that recombination is important in populations of both naturally transformable and nontransformable organisms. The prevalence of genetic exchange varies among different bacteria, as does the major mechanism of exchange. If these mechanisms enable the transmission of extrachromosomal elements, plasmids transposons, or lysogenic bacteriophage, bacteria can acquire completely novel resistance or virulence determinants. For chromosomal DNA, genetic exchange is mediated primarily by homologous recombination. Genetic exchange involving the acquisition of plasmids or conjugative transposons carrying novel resistance determinants is undoubtedly the most widespread route by which bacteria have acquired resistance to antibiotics. In many countries, lack of susceptibility to tetracyclines is the most frequently observed resistance phenotype in pneumococci. It would appear that different bacterial species are genetically variable for most of these factors. For example, restriction systems which differ widely between organisms function primarily against incoming double-stranded DNA but may also play a role in recombination. Although the future for the pathogenic commensal organisms of the upper respiratory tract is uncertain, it is clear that genetic exchange will continue to play an important role in helping to shape that future.
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Mobile Genetic Elements and Bacterial Pathogenesis
- Authors: Brigid M. Davis, Matthew K. Waldor
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Source: Mobile DNA II , pp 1040-1059
Publication Date :
January 2002
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Abstract:
Horizontally transferred genes were frequently disseminated among bacterial populations as components of mobile genetic elements, such as plasmids, phages, and transposons. This chapter focuses on bacterial pathogens, and in particular, on the role of sequence-specific mechanisms for intercellular gene transfer in the evolution of such organisms. It concentrates on virulence factors encoded by genes on mobilizable or formerly mobilizable genetic elements, especially plasmids, bacteriophages, and pathogenicity islands (PAIs). Finally, analyses based on the different classes of mobile elements can highlight the processes by which virulence genes have been transported and illuminate the evolutionary relationships between types of mobile elements. In the chapter, the last-mentioned approach is used, and bacteriophages, plasmids, and PAIs are sequentially discussed. Finally, some plasmids, such as the large Yersinia virulence plasmid and the enterohemorrhagic Escherichia coli (EHEC) virulence plasmid, contain remnants of DNA transfer systems, suggesting that they were previously capable of self-mobilization. Although the virulence genes acquired as components of mobile elements may not have been subject to host regulatory processes immediately after acquisition, they clearly have not remained autonomous agents, independent of host functions. Mobile genetic elements have clearly distributed a diverse collection of virulence genes and thereby played essential roles in the evolution of bacterial pathogens. However, a subset of virulence factors does not seem to confer any advantages on the bacterial hosts, so the forces underlying their continued production are a mystery.