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Contribution of gene amplification to evolution of increased antibiotic resistance in Salmonella typhimurium
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Medical Biochemistry and Microbiology.
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Evolution, Genomics and Systematics, Molecular Evolution.
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Medical Biochemistry and Microbiology.
2009 (English)In: Genetics, ISSN 0016-6731, E-ISSN 1943-2631, Vol. 182, no 4, 1183-1195 p.Article in journal (Refereed) Published
Abstract [en]

The use of beta-lactam antibiotics has led to the evolution and global spread of a variety of resistance mechanisms, including beta-lactamases, a group of enzymes that degrade the beta-lactam ring. The evolution of increased beta-lactam resistance was studied by exposing independent lineages of Salmonella typhimurium to progressive increases in cephalosporin concentration. Each lineage carried a beta-lactamase gene (bla(TEM-1)) that provided very low resistance. In most lineages, the initial response to selection was an amplification of the bla(TEM-1) gene copy number. Amplification was followed in some lineages by mutations (envZ, cpxA, or nmpC) that reduced expression of the uptake functions, the OmpC, OmpD, and OmpF porins. The initial resistance provided by bla(TEM-1) amplification allowed the population to expand sufficiently to realize rare secondary point mutations. Mathematical modeling showed that amplification often is likely to be the initial response because events that duplicate or further amplify a gene are much more frequent than point mutations. These models show the importance of the population size to appearance of later point mutations. Transient gene amplification is likely to be a common initial mechanism and an intermediate in stable adaptive improvement. If later point mutations (allowed by amplification) provide sufficient adaptive improvement, the amplification may be lost.

Place, publisher, year, edition, pages
2009. Vol. 182, no 4, 1183-1195 p.
National Category
Biological Sciences
URN: urn:nbn:se:uu:diva-119727DOI: 10.1534/genetics.109.103028ISI: 000270214000021PubMedID: 19474201OAI: oai:DiVA.org:uu-119727DiVA: diva2:300748
Available from: 2010-03-01 Created: 2010-03-01 Last updated: 2012-08-01Bibliographically approved
In thesis
1. Dynamics and Mechanisms of Adaptive Evolution in Bacteria
Open this publication in new window or tab >>Dynamics and Mechanisms of Adaptive Evolution in Bacteria
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Determining the properties of mutations is fundamental to understanding the mechanisms of adaptive evolution. The major goal of this thesis is to investigate the mechanisms of bacterial adaptation to new environments using experimental evolution. Different types of mutations were under investigations with a particular focus on genome rearrangements. Adaptive evolution experiments were focused on the development of bacterial resistance to antibiotics.

In paper I, we performed stochastic simulations to examine the role of gene amplification in promoting the establishment of new gene functions. The results show that gene amplification can contribute to creation of new gene functions in nature. In paper II, the evolution of β-lactam resistance was studied by evolving S. typhimurium carrying a β-lactamase gene towards increased resistance against cephalosporins. Our results suggest that gene amplification is likely to provide an immediate solution at the early stage of adaptive evolution and subsequently facilitate further stable adaptation. In paper III, we isolated spontaneous deletion mutants with increased competitive fitness, which indicated that genome reduction could be driven by selection. To test this hypothesis, independent lineages of wild type S. typhimurium were serially passaged for 1000 generations and we observed fixation of deletions that significantly increased bacterial fitness when reconstructed in wild type genetic background. In paper IV, we developed a new strategy combining 454 pyrosequencing technology and a ‘split mapping’ computational method to identify unique junction sequences formed by spontaneous genome rearrangements. A high steady-state frequency of rearrangements in unselected bacterial populations was suggested from our results. In paper V, the rates, mechanisms and fitness effects of colistin resistance in S. typhimurium were determined. The high mutation rate and low fitness costs suggest that colistin resistance could develop in clinical settings. In paper VI, a novel Metallo-β-lactamase (MBL) with low resistance against β-lactam antibiotics was employed as the ancestral protein in a directed evolution experiment to examine how an enzyme evolves towards increased resistance. For most isolated mutants, in spite of their significantly increased resistance, both mRNA and protein levels were decreased as compared with the parental protein, suggesting that the catalytic activity had increased.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2012. 64 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine, ISSN 1651-6206 ; 770
adaptive evolution, mutation, genome rearrangements, antibiotic resistance, gene amplification, genome reduction, directed evolution
National Category
Research subject
Biology with specialization in Microbiology
urn:nbn:se:uu:diva-172786 (URN)978-91-554-8354-8 (ISBN)
Public defence
2012-06-05, C10:305, BMC, Husargatan 3, Uppsala, 09:00 (English)
Available from: 2012-05-14 Created: 2012-04-15 Last updated: 2012-08-01Bibliographically approved

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