Within-Host Ecology and Evolution of Nasal Colonizing Bacteria:Streptococcus pneumoniae, Haemophilus influenzae and Staphylococcusaureus Open Access
Margolis, Elisa Beth Hypata (2009)
Abstract
Many bacteria, including Streptococcus pneumoniae, Haemophilus influenzae and Staphylococcus aureus, colonize and reside harmlessly in the nasal passages of a substantial faction of the human population and are therefore considered commensal. However these same bacteria are also responsible for a great deal of morbidity and mortality. The goals of this dissertation are to understand the conditions under which these three species of bacteria can colonize the nasal passages, how they interact with each other and why they become invasive. Towards these ends, experiments in neonatal rats were performed on nasal colonization, intra- and inter-specific interactions and on the invasiveness of these bacteria. All three species readily colonized the nasal passages of neonatal rats and regardless of the inoculum size apparently reached a steady-state density. To ascertain how these three different species interacted, I introduced either the same or different species in rats already colonized with other strains or species. Established populations of S. aureus inhibit invasion of new S. aureus populations. And for both S. pneumoniae and H. influenzae the invading and established populations of the same species were able to co-exist. Previous reports had suggested that hydrogen peroxide (H2O2) production by S. pneumoniae reduces S. aureus' bacterial density, however in the neonatal rat model the density of S. aureus when colonized did not differ in the presence of a S. pneumoniae strain that was H2O2-secreting or non-H2O2 secreting. Surprisingly, the only multi-species interaction observed was H. influenzae reaching higher densities when S. aureus or S. pneumoniae were present. One hypothesis for the rare invasiveness of these commensal bacteria is the evolution of invasive mutants in the population of bacteria colonizing the nasal passage or within-host evolution. When H. influenzae isolated from the blood and the nasal passages were re-inoculated, one out of the six blood isolates had an increased propensity to invade the bloodstream. These results provide support for within-host evolution as one but not the sole explanation for the occasional invasive disease of normally commensal bacteria.
Table of Contents
Table of Contents Chapter 1: Introduction...1
1.1 Ecology of Nasal Colonizing Bacteria...2
1.1.1 Epidemiology of Nasal Colonizing Bacteria...2 1.1.2 Ecology of Bacterial Colonization...5
1.2 Evolution of Virulence...9
1.2.1 Within-host Evolution of Invasive Bacterial Disease...11 1.2.2 Contribution of Host's Immune System to Virulence...12
1.3 Neonatal Rat model...13 1.4 Outline of the Thesis...15
Chapter 2: Ecology of Nasal Colonization and Competition...17
2.1 Introduction...18 2.2 Methods and Materials...20
2.2.1 Bacterial strains, media and inoculum preparation...20 2.2.2 Infant Rat Model...21 2.2.3 Experimental Design...22 2.2.4 Immune Depletion...23 2.2.5 Statistical Analysis...23
2.3 Results...23
2.3.1 Population Dynamics...23 2.3.2 Invasion of Same Species in a Colonized Host...24 2.3.3 Invasion of Different Species in a Colonized Host...28
2.4 Discussion...31
2.4.1 Population Dynamics...31 2.4.2 Invasion of Same Species in a Colonized Host...32 2.4.3 Invasion of Different Species in a Colonized Host...33 2.4.4 Immune-mediated Competition...34 2.4.5 Limitations and Significance...35
2.5 Appendix...36
Chapter 3: Hydrogen Peroxide Mediated Competition...39
3.1 Introduction...40 3.2 Methods and Materials...41
3.2.1 Bacterial strains, media and inoculum preparation...41 3.2.2 Infant Rat Model...42 3.2.3 Experimental Design...42 3.2.4 Statistical Analysis...43
3.3 Results...44
3.3.1 Discussion...47
3.4 Appendix...50
Chapter 4: Evolution of Bacterial-Host Interactions: Virulence and the Immune Overresponse...52 Chapter 5: Mouse Wound Model...63
5.1 Introduction...63 5.2 Material and Methods...64
5.2.1 Bacterial strains, media and inoculum preparation...64 5.2.2 MouseWound Model...65 5.2.3 Attempts to Manipulate Abscess Formation...65
5.3 Results...67
5.3.1 MouseWound Model...67 5.3.2 Attempts to Manipulate Abscess Formation...68
5.4 Summary...71
Chapter: Within-Host Evolution for the Invasiveness of Commensal Bacteria...73 Chapter 7: Exploring the role of the immune response in preventing antibiotic resistance...81
7.1 Abstract...81 7.2 Introduction...82 7.3 The Mathematical Models...83
7.3.1 Bacteria and drug dynamics...83 7.3.2 Immune Responses...84
7.4 Results...88
7.4.1 Resistance emergence in the absence of an immune response...88 7.4.2 Resistance emergence in the presence of immune responses...88 7.4.3 Immune responses can reduce the mutant selection window...91 7.4.4 Immune responses can dampen the effect of imperfect adherence to treatment...94 7.4.5 Immune responses can change optimal dosing strategies...95
7.5 Discussion...98
7.5.1 Caveats and limitations...98 7.5.2 Predictions and hypotheses...99
Chapter 8: Conclusions...103
8.1 Part I: Ecology of Nasal Colonization...103 8.2 Part II: Evolution of Virulence...107
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