The population and evolutionary dynamics of competition between Pseudomonas aeruginosa and Staphylococcus aureus Público

Abstract

Pseudomonas aeruginosa and Staphylococcus aureus regularly co-infect the lungs of cystic fibrosis (CF) patients. However, P. aeruginosa either kills or inhibits the growth of S. aureus when co-cultured in vitro. The current consensus is that the P. aeruginosa PQS quorum sensing system, specifically 4- hydroxy-2-heptoquinoline-N-oxide (HQNO), is responsible for this antagonism. Although much is known the role of HQNO in this antagonism at the molecular and cellular level, there remain fundamental unanswered ecological and evolutionary questions about the antagonistic interaction between these Gram-negative and Gram-positive bacteria. The goal of this jointly theoretical and in vitro experimental study is to address and answer the following questions: 1) Are there population interactions, other than those mediated by HQNO, that contribute to this antagonism? 2) To what extent does antagonism by P. aeruginosa contribute to the ecology of these populations? 3) Is the mechanism of this antagonism purely extracellular, or does it require cell-to-cell contact? 4) What are the selective forces responsible for the evolution of this antagonistic phenomenon? Addressing these questions will provide a framework to better understand the ecology the CF lung infections. Using a combination of mathematical modelling and in vitro experiments with P. aeruginosa and S. aureus, we address and provide answers to these questions. We present evidence for frequency and density dependence of P. aeruginosa antagonism towards S. aureus. Simple resource competition, extracellular allelopathy, and contact-dependent inhibition alone cannot explain the observed dynamics. S. aureus has a growth disadvantage when cultured in the presence of sonicated P. aeruginosa supernatant, implying the existence of an extracellular layer to the mechanism. Nevertheless, the presence of P. aeruginosa cell-free extract does alter the change in growth fold of S. aureus. A significant decline in S. aureus population is, however, observed when it is cultured in the presence of non-replicating P. aeruginosa culture, implying that the killing itself is either dependent of cell-cell contact or induction by an S. aureus exoproduct. Variations on models wherein P. aeruginosa lyses S. aureus and uses resources liberated from the ruptured cells, or in which P. aeruginosa utilizes passively released S. aureus exoproducts as a resource, best reflect the observed population dynamics and antagonistic phenomena. 

Table of Contents

Introduction...........................................................................................................................................1

Materials and Methods........................................................................................................................5 Results....................................................................................................................................................6

Figure 1: Schematic diagrams of the basic theoretical models..............................8

Table 1: Parameters and variables used for numerical solutions....................................11

Table 2: Growth Parameters for Strains Used in the Study .............................................12

Figure 2: Simulations of resource competition..................................................................13

Figure 3: S. aureus JE2 decline and eradication during serial passage with P. aeruginosa PAO1 occurs in a frequency-dependent manner..................................................15

Figure 4: In extracellular allelopathy and contact-dependent models, P. aeruginosa

cannot predominate if initially rare, and the two models can exhibit indistinguishable dynamics......................................................................................17

Figure 5: P. aeruginosa antagonism towards S. aureus occurs late in growth..............18

Table 3: Enriched media experiments.................................................................................19

Figure 6: P. aeruginosa antagonism involves direct killing of S. aureus and requires the presence of both living P. aeruginosa cells and HQNO........................................22

Figure 7: S. aureus JE2 has a growth disadvantage in the presence of sonicated PAO1 WT supernatant.........................................................................................................23

Figure 8: Schematic diagrams of “predation” models.......................................................25

Table 4: Parameters and Variables for Predation and Passively Released S. aureus Resource Models.......................................................................................................27

Figure 9: A contact-dependent model with predation does not explain the observed dynamics when P. aeruginosa is initially rare.......................................................28

Figure 10: Schematic diagrams of theoretical models, with a passively released S. aureus-derived resource..........................................................................................29

Figure 11: A model where a product passively produced S. aureus induces P. aeruginosa antagonism and is used by P. aeruginosa as a resource simulates the results observed experimentally.....................................................................32

Figure 12: P. aeruginosa antagonism towards S. aureus occurs in a density dependent manner and requires a short P. aeruginosa-S.aureus intercellular distance. P. aeruginosa and S. aureus colonies can grow in the same vicinity, with S. aureus visible even with initial populations (No) ~1E7 cells.............................................35

Discussion.............................................................................................................................................36

Figure 13: Possible model that explains SSS observations................................................40

References............................................................................................................................................43 

About this Honors Thesis

Rights statement

• Permission granted by the author to include this thesis or dissertation in this repository. All rights reserved by the author. Please contact the author for information regarding the reproduction and use of this thesis or dissertation.

School	Emory College
Department	Biology
Degree	B.S.
Submission	Honors Thesis
Language	English
Research Field	Biology, Microbiology Biology, Ecology Biology, Evolution and Development
Palavra-chave	S. aureus Bacteria Competition Population Cystic Fibrosis P. aeruginosa
Committee Chair / Thesis Advisor	Bruce R. Levin, Emory University
Committee Members	Marvin Whiteley, Emory University Joanna Goldberg, Emory University Nic Vega, Emory University

Última modificação

Primary PDF

Thumbnail	Title	Date Uploaded	Actions
	The population and evolutionary dynamics of competition between Pseudomonas aeruginosa and Staphylococcus aureus ()	2020-04-09 19:48:26 -0400	Press to Select an action Download

Supplemental Files

Thumbnail	Title	Date Uploaded	Actions