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Fitness of Salmonella mutants resistant to antimicrobial peptides
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Medical Biochemistry and Microbiology.
Uppsala University, Disciplinary Domain of Medicine and Pharmacy, Faculty of Medicine, Department of Medical Biochemistry and Microbiology.
2015 (English)In: Journal of Antimicrobial Chemotherapy, ISSN 0305-7453, E-ISSN 1460-2091, Vol. 70, no 2, 432-440 p.Article in journal (Refereed) Published
Abstract [en]

Objectives: To examine the effects of mutations in the waaY, phoP and pmrB genes, which confer resistance to antimicrobial peptides (AMPs), on fitness of Salmonella Typhimurium. Methods: Survival during low pH, oxidative stress, stationary-phase incubation, exposure to serum and bile and growth in mice and laboratory media were determined by time-kills, disc inhibition assays, competition experiments and optical density measurements. Results: Individual mutations in the waaY gene (involved in LPS core biosynthesis) and in the phoP and pmrB genes (part of two different two-component regulatory systems, phoPQ and pmrAB) conferred no or minor effects on bacterial survival during stressful in vitro conditions or in mice. In contrast, a waaY-phoP-pmrB triple mutant was compromised under most assay conditions. Conclusions: Results from this study show that AMP resistance can be cost-free, as assessed by several assays that attempt to mimic the conditions a bacterium might encounter within a host. Our findings imply that future therapeutic use of AMPs could select for fit mutants with cross-resistance to human defence peptides and that potential resistance development in response to therapeutic use of AMPs needs to be carefully monitored.

Place, publisher, year, edition, pages
2015. Vol. 70, no 2, 432-440 p.
Keyword [en]
Salmonella Typhimurium, resistance, bile, serum, mice, pH response, oxidative stress, growth rate
National Category
Pharmacology and Toxicology
URN: urn:nbn:se:uu:diva-249022DOI: 10.1093/jac/dku423ISI: 000350211600011PubMedID: 25362575OAI: oai:DiVA.org:uu-249022DiVA: diva2:807788
Available from: 2015-04-24 Created: 2015-04-10 Last updated: 2016-06-15
In thesis
1. Mechanisms and Biological Costs of Bacterial Resistance to Antimicrobial Peptides
Open this publication in new window or tab >>Mechanisms and Biological Costs of Bacterial Resistance to Antimicrobial Peptides
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The global increasing problem of antibiotic resistance necessarily drives the pursuit and discovery of new antimicrobial agents. Antimicrobial peptides (AMPs) initially seemed like promising new drug candidates. Already members of the innate immune system, it was assumed that they would be bioactive and non-toxic. Their common trait for fundamental, non-specific mode of action also seemed likely to reduce resistance development.

In this thesis, we demonstrate the ease with which two species of pathogenic bacteria, the gram-negative Salmonella typhimurium (S. typhimurium), and the gram-positive Staphylococcus aureus (S. aureus), can gain increased tolerance and stable resistance to various AMPs. By serially passaging each bacterial species separately under increasing AMP selection pressure we observed increasing AMP tolerance. Resulting in independent bacterial lineages exposed to four different AMPs (including a two-AMP combination) that exhibited 2 to 16-fold increases in MIC. Substantial cross-resistance between the AMPs was observed. Additionally, the S. aureus mutants were found to be cross-resistant to human beta-defensins 1, 2, 3, and 4.

The LPS molecule, with mutations in the waaY, pmrB and phoP genes, was the principal target for S. typhimurium resistance development. The main target for S. aureus remained elusive. Reduced membrane potential was a common change for two of the mutants, but not for the others. All sequenced mutants had one or more mutations in various stress response pathways.

Fitness of the resistant mutants was assayed by growth rate analysis and in vitro virulence factor testing (e.g. survival response to bile, superoxide, acidic pH). Furthermore an in vivo survival/virulence test involving a mouse competition experiment (S. typhimurium) and sepsis model (S. aureus) was performed. In the absence of AMPs there was often little or no fitness reduction in the mutants. Our results suggest that AMP resistance mechanisms do not irrevocably weaken either species with regard to virulence characteristics or survival within the host.

In light of these findings, we suggest that the progression of therapeutic use of AMPs should proceed with great caution since otherwise we might select for AMP resistant mutants that are more resistant to our innate host defenses and thereby potentially more virulent.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. 61 p.
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Medicine, ISSN 1651-6206 ; 1226
antimicrobial peptides, antibiotic resistance, fitness cost, Salmonella Typhimurium, Staphylococcus aureus, bile, serum, pH response, growth rate, mice, phoP, pmrB, waaY, LPS, LL-37, defensins, membrane potential
National Category
Microbiology in the medical area
Research subject
urn:nbn:se:uu:diva-284119 (URN)978-91-554-9579-4 (ISBN)
Public defence
2016-06-14, C8:305, BMC, Husargatan 3, Uppsala, 13:00 (English)
Available from: 2016-05-20 Created: 2016-04-15 Last updated: 2016-06-15

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Lofton, HavaAndersson, Dan I.
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