Evolutionary Epidemiology Consequences of Trait-Dependent Control of Heterogeneous Parasites.

AbstractDisease control can induce both demographic and evolutionary responses in host-parasite systems. Foreseeing the outcome of control therefore requires knowledge of the eco-evolutionary feedback between control and system. Previous work has assumed that control strategies have a homogeneous effect on the parasite population. However, this is not true when control targets those traits that confer to the parasite heterogeneous levels of resistance, which can additionally be related to other key parasite traits through evolutionary trade-offs. In this work, we develop a minimal model coupling epidemiological and evolutionary dynamics to explore possible trait-dependent effects of control strategies. In particular, we consider a parasite expressing continuous levels of a trait-determining resource exploitation and a control treatment that can be either positively or negatively correlated with that trait. We demonstrate the potential of trait-dependent control by considering that the decision maker may want to minimize both the damage caused by the disease and the use of treatment, due to possible environmental or economic costs. We identify efficient strategies showing that the optimal type of treatment depends on the amount applied. Our results pave the way for the study of control strategies based on evolutionary constraints, such as collateral sensitivity and resistance costs, which are receiving increasing attention for both public health and agricultural purposes.

Mitigation of evolved bacterial resistance to phage therapy.

The ease with which bacteria can evolve resistance to phages is a key consideration for development of phage therapy. Here, we review recent work on the different evolutionary and ecological approaches to mitigate the problem. The approaches are broadly categorised into two areas: Minimising evolved phage resistance; and Directing phage-resistance evolution towards therapeutically beneficial outcomes.

High genomic diversity of novel phages infecting the plant pathogen Ralstonia solanacearum, isolated in Mauritius and Reunion islands.

Bacterial wilt caused by the Ralstonia solanacearum species complex (RSSC) is among the most important plant diseases worldwide, severely affecting a high number of crops and ornamental plants in tropical regions. Only a limited number of phages infecting R. solanacearum have been isolated over the years, despite the importance of this bacterium and the associated plant disease. The antibacterial effect or morphological traits of these R. solanacearum viruses have been well studied, but not their genomic features, which need deeper consideration. This study reports the full genome of 23 new phages infecting RSSC isolated from agricultural samples collected in Mauritius and Reunion islands, particularly affected by this plant bacterial pathogen and considered biodiversity hotspots in the Southwest Indian Ocean. The complete genomic information and phylogenetic classification is provided, revealing high genetic diversity between them and weak similarities with previous related phages. The results support our proposal of 13 new species and seven new genera of R. solanacearum phages. Our findings highlight the wide prevalence of phages of RSSC in infected agricultural settings and the underlying genetic diversity. Discoveries of this kind lead more insight into the diversity of phages in general and to optimizing their use as biocontrol agents of bacterial diseases of plants in agriculture.

[Antibacterial applications of bacteriophages]. / Les applications antibactériennes des bactériophages.

In the 1917 article in which Félix d'Hérelle describes his first observations and proposes the name of bacteriophage, he also reports the first use of these viruses to treat bacterial infections, thus giving birth to phage therapy. Soon after antibiotics supplanted bacteriophages. Today, bacteria resistant to multiple antibiotics become a growing public health issue worldwide. This situation has revived research aiming at developing the antibacterial activity of bacteriophages to treat patients as well as diseases in animals and plants. In fact, the areas of applications of bacteriophages as antibacterial are widening as current solutions of chemical nature are questioned. This review summarizes the basic principles of therapeutic applications of bacteriophages and presents recent data in areas where commercial exploitation is occurring or about to emerge.

[A century of research on bacteriophages]. / Un siècle de recherche sur les bactériophages.

Bacteriophages have a prominent place in the living world. They participate to our understanding of the living world through three main aspects : (i) the dissection of the most intimist aspects of viral infection molecular mechanisms (molecular biology), (ii) the description and functioning mechanisms of ecosystems (ecology), and (iii) the adaptive dynamics of integrated viral and host-cell populations (evolution). This review looks back at the genesis of these fundamental findings and draws a picture of the most active fields of current research.

The disparate effects of bacteriophages on antibiotic-resistant bacteria.

Faced with the crisis of multidrug-resistant bacteria, bacteriophages, viruses that infect and replicate within bacteria, have been reported to have both beneficial and detrimental effects with respect to disease management. Bacteriophages (phages) have important ecological and evolutionary impacts on their bacterial hosts and have been associated with therapeutic use to kill bacterial pathogens, but can lead to the transmission of antibiotic resistance. Although the process known as transduction has been reported for many bacterial species by classic and modern genetic approaches, its contribution to the spread of antibiotic resistance in nature remains unclear. In addition, detailed molecular studies have identified phages residing in bacterial genomes, revealing unexpected interactions between phages and their bacterial hosts. Importantly, antibiotics can induce the production of phages and phage-encoded products, disseminating these viruses and virulence-related genes, which have dangerous consequences for disease severity. These unwanted side-effects of antibiotics cast doubt on the suitability of some antimicrobial treatments and may require new strategies to prevent and limit the selection for virulence. Foremost among these treatments is phage therapy, which could be used to treat many bacterial infectious diseases and confront the pressing problem of antibiotic resistance in pathogenic bacteria. This review discusses the interactions between bacteriophages, antibiotics, and bacteria and provides an integrated perspective that aims to inspire the development of successful antibacterial therapies.

Transient negative effects of antibiotics on phages do not jeopardise the advantages of combination therapies.

Phages, the viruses of bacteria, have been proposed as antibacterial agents to complement or replace antibiotics due to the growing problem of resistance. In nature and in the clinic, antibiotics are ubiquitous and may affect phages indirectly via impacts on bacterial hosts. Even if the synergistic association of phages and antibiotics has been shown in several studies, the focus is often on bacteria with little known about the impact on phages. Evolutionary studies have demonstrated that time scale is an important factor in understanding the consequences of antimicrobial strategies, but this perspective is generally overlooked in phage-antibiotic combination studies. Here, we explore the effects of antibiotics on phages targeting the opportunistic pathogen Pseudomonas aeruginosa. We go beyond previous studies by testing the interaction between several types of antibiotics and phages, and evaluate the effects on several important phage parameters during 8 days of experimental co-evolution with bacteria. Our study reveals that antibiotics had a negative effect on phage density and efficacy early on, but not in the later stages of the experiment. The results indicate that antibiotics can affect phage adaptation, but that phages can nevertheless contribute to managing antibiotic resistance levels.

Phage Therapy Faces Evolutionary Challenges.

Antibiotic resistance evolution in bacteria indicates that one of the challenges faced by phage therapy is that, sooner or later, bacteria will evolve resistance to phages. Evidently, this is the case of every known antimicrobial therapy, but here this is also part of a ubiquitous natural process of co-evolution between phages and bacteria. Fundamental evolutionary studies hold some clues that are crucial to limit the problematic process of bacterial resistance during phage applications. First, I discuss here the importance of defining evolutionary and ecological factors influencing bacterial resistance and phage counter-defense mechanisms. Then, I comment on the interest of determining the co-evolutionary dynamics between phages and bacteria that may allow for selecting the conditions that will increase the probability of therapeutic success. I go on to suggest the varied strategies that may ensure the long-term success of phage therapy, including analysis of internal phage parameters and personalized treatments. In practical terms, these types of approaches will define evolutionary criteria regarding how to develop, and when to apply, therapeutic phage cocktails. Integrating this perspective in antimicrobial treatments, such as phage therapy, is among the necessary steps to expand its use in the near future, and to ensure its durability and success.

"French Phage Network"-Second Meeting Report.

The study of bacteriophages (viruses of bacteria) includes a variety of approaches, such as structural biology, genetics, ecology, and evolution, with increasingly important implications for therapeutic and industrial uses. Researchers working with phages in France have recently established a network to facilitate the exchange on complementary approaches, but also to engage new collaborations. Here, we provide a summary of the topics presented during the second meeting of the French Phage Network that took place in Marseille in November 2016.

Antibiotic stress selects against cooperation in the pathogenic bacterium Pseudomonas aeruginosa.

Cheats are a pervasive threat to public goods production in natural and human communities, as they benefit from the commons without contributing to it. Although ecological antagonisms such as predation, parasitism, competition, and abiotic environmental stress play key roles in shaping population biology, it is unknown how such stresses generally affect the ability of cheats to undermine cooperation. We used theory and experiments to address this question in the pathogenic bacterium, Pseudomonas aeruginosa Although public goods producers were selected against in all populations, our competition experiments showed that antibiotics significantly increased the advantage of nonproducers. Moreover, the dominance of nonproducers in mixed cultures was associated with higher resistance to antibiotics than in either monoculture. Mathematical modeling indicates that accentuated costs to producer phenotypes underlie the observed patterns. Mathematical analysis further shows how these patterns should generalize to other taxa with public goods behaviors. Our findings suggest that explaining the maintenance of cooperative public goods behaviors in certain natural systems will be more challenging than previously thought. Our results also have specific implications for the control of pathogenic bacteria using antibiotics and for understanding natural bacterial ecosystems, where subinhibitory concentrations of antimicrobials frequently occur.

Size evolution in microorganisms masks trade-offs predicted by the growth rate hypothesis.

Adaptation to local resource availability depends on responses in growth rate and nutrient acquisition. The growth rate hypothesis (GRH) suggests that growing fast should impair competitive abilities for phosphorus and nitrogen due to high demand for biosynthesis. However, in microorganisms, size influences both growth and uptake rates, which may mask trade-offs and instead generate a positive relationship between these traits (size hypothesis, SH). Here, we evolved a gradient of maximum growth rate (µmax) from a single bacterium ancestor to test the relationship among µmax, competitive ability for nutrients and cell size, while controlling for evolutionary history. We found a strong positive correlation between µmax and competitive ability for phosphorus, associated with a trade-off between µmax and cell size: strains selected for high µmax were smaller and better competitors for phosphorus. Our results strongly support the SH, while the trade-offs expected under GRH were not apparent. Beyond plasticity, unicellular populations can respond rapidly to selection pressure through joint evolution of their size and maximum growth rate. Our study stresses that physiological links between these traits tightly shape the evolution of competitive strategies.

Long-term effects of single and combined introductions of antibiotics and bacteriophages on populations of Pseudomonas aeruginosa.

With escalating resistance to antibiotics, there is an urgent need to develop alternative therapies against bacterial pathogens and pests. One of the most promising is the employment of bacteriophages (phages), which may be highly specific and evolve to counter antiphage resistance. Despite an increased understanding of how phages interact with bacteria, we know very little about how their interactions may be modified in antibiotic environments and, reciprocally, how phage may affect the evolution of antibiotic resistance. We experimentally evaluated the impacts of single and combined applications of antibiotics (different doses and different types) and phages on in vitro evolving populations of the opportunistic pathogen Pseudomonas aeruginosa PAO1. We also assessed the effects of past treatments on bacterial virulence in vivo, employing larvae of Galleria mellonella to survey the treatment consequences for the pathogen. We find a strong synergistic effect of combining antibiotics and phages on bacterial population density and in limiting their recovery rate. Our long-term study establishes that antibiotic dose is important, but that effects are relatively insensitive to antibiotic type. From an applied perspective, our results indicate that phages can contribute to managing antibiotic resistance levels, with limited consequences for the evolution of bacterial virulence.

Evolutionary Rationale for Phages as Complements of Antibiotics.

Antibiotic-resistant bacterial infections are a major concern to public health. Phage therapy has been proposed as a promising alternative to antibiotics, but an increasing number of studies suggest that both of these antimicrobial agents in combination are more effective in controlling pathogenic bacteria than either alone. We advocate the use of phages in combination with antibiotics and present the evolutionary basis for our claim. In addition, we identify compelling challenges for the realistic application of phage-antibiotic combined therapy.

Phage selection for bacterial cheats leads to population decline.

While predators and parasites are known for their effects on bacterial population biology, their impact on the dynamics of bacterial social evolution remains largely unclear. Siderophores are iron-chelating molecules that are key to the survival of certain bacterial species in iron-limited environments, but their production can be subject to cheating by non-producing genotypes. In a selection experiment conducted over approximately 20 bacterial generations and involving 140 populations of the pathogenic bacterium Pseudomonas aeruginosa PAO1, we assessed the impact of a lytic phage on competition between siderophore producers and non-producers. We show that the presence of lytic phages favours the non-producing genotype in competition, regardless of whether iron use relies on siderophores. Interestingly, phage pressure resulted in higher siderophore production, which constitutes a cost to the producers and may explain why they were outcompeted by non-producers. By the end of the experiment, however, cheating load reduced the fitness of mixed populations relative to producer monocultures, and only monocultures of producers managed to grow in the presence of phage in situations where siderophores were necessary to access iron. These results suggest that public goods production may be modulated in the presence of natural enemies with consequences for the evolution of social strategies.

The SOS response increases bacterial fitness, but not evolvability, under a sublethal dose of antibiotic.

Exposure to antibiotics induces the expression of mutagenic bacterial stress-response pathways, but the evolutionary benefits of these responses remain unclear. One possibility is that stress-response pathways provide a short-term advantage by protecting bacteria against the toxic effects of antibiotics. Second, it is possible that stress-induced mutagenesis provides a long-term advantage by accelerating the evolution of resistance. Here, we directly measure the contribution of the Pseudomonas aeruginosa SOS pathway to bacterial fitness and evolvability in the presence of sublethal doses of ciprofloxacin. Using short-term competition experiments, we demonstrate that the SOS pathway increases competitive fitness in the presence of ciprofloxacin. Continued exposure to ciprofloxacin results in the rapid evolution of increased fitness and antibiotic resistance, but we find no evidence that SOS-induced mutagenesis accelerates the rate of adaptation to ciprofloxacin during a 200 generation selection experiment. Intriguingly, we find that the expression of the SOS pathway decreases during adaptation to ciprofloxacin, and this helps to explain why this pathway does not increase long-term evolvability. Furthermore, we argue that the SOS pathway fails to accelerate adaptation to ciprofloxacin because the modest increase in the mutation rate associated with SOS mutagenesis is offset by a decrease in the effective strength of selection for increased resistance at a population level. Our findings suggest that the primary evolutionary benefit of the SOS response is to increase bacterial competitive ability, and that stress-induced mutagenesis is an unwanted side effect, and not a selected attribute, of this pathway.

A window of opportunity to control the bacterial pathogen Pseudomonas aeruginosa combining antibiotics and phages.

The evolution of antibiotic resistance in bacteria is a global concern and the use of bacteriophages alone or in combined therapies is attracting increasing attention as an alternative. Evolutionary theory predicts that the probability of bacterial resistance to both phages and antibiotics will be lower than to either separately, due for example to fitness costs or to trade-offs between phage resistance mechanisms and bacterial growth. In this study, we assess the population impacts of either individual or combined treatments of a bacteriophage and streptomycin on the nosocomial pathogen Pseudomonas aeruginosa. We show that combining phage and antibiotics substantially increases bacterial control compared to either separately, and that there is a specific time delay in antibiotic introduction independent of antibiotic dose, that minimizes both bacterial density and resistance to either antibiotics or phage. These results have implications for optimal combined therapeutic approaches.

A trade-off between oxidative stress resistance and DNA repair plays a role in the evolution of elevated mutation rates in bacteria.

The dominant paradigm for the evolution of mutator alleles in bacterial populations is that they spread by indirect selection for linked beneficial mutations when bacteria are poorly adapted. In this paper, we challenge the ubiquity of this paradigm by demonstrating that a clinically important stressor, hydrogen peroxide, generates direct selection for an elevated mutation rate in the pathogenic bacterium Pseudomonas aeruginosa as a consequence of a trade-off between the fidelity of DNA repair and hydrogen peroxide resistance. We demonstrate that the biochemical mechanism underlying this trade-off in the case of mutS is the elevated secretion of catalase by the mutator strain. Our results provide, to our knowledge, the first experimental evidence that direct selection can favour mutator alleles in bacterial populations, and pave the way for future studies to understand how mutation and DNA repair are linked to stress responses and how this affects the evolution of bacterial mutation rates.

Evaluating evolutionary models of stress-induced mutagenesis in bacteria.

Increased mutation rates under stress allow bacterial populations to adapt rapidly to stressors, including antibiotics. Here we evaluate existing models for the evolution of stress-induced mutagenesis and present a new model arguing that it evolves as a result of a complex interplay between direct selection for increased stress tolerance, second-order selection for increased evolvability and genetic drift. Further progress in our understanding of the evolutionary biology of stress and mutagenesis will require a more detailed understanding both of the patterns of stress encountered by bacteria in nature and of the mutations that are produced under stress.

HC-Pro hypo- and hypersuppressor mutants: differences in viral siRNA accumulation in vivo and siRNA binding activity in vitro.

Viruses have evolved mechanisms to suppress the RNA silencing defense of their hosts, allowing replication and systemic colonization. In a recent study, we found that the effect of mutations in the RNA silencing suppressor of tobacco etch virus (TEV) was variable, ranging from complete abolition of suppressor activity to significantly stronger suppression. Whereas hyposuppressor mutants were less virulent and accumulated fewer viral particles than the wild type, hypersuppressors induced symptoms similar to those of the wild type and accumulated particles to similar levels. Here, we further characterize a set of these mutants in terms of their ability to bind in vitro and induce accumulation in vivo of virus-derived siRNAs. Hyposuppressor alleles are less efficient at binding siRNAs than hypersuppressors, whereas the latter are not different from the wild type. As a consequence of lower viral accumulation, plants infected with virus bearing a hyposuppressor allele also accumulate less virus-derived siRNA.