Expert Opinion on Three Phage Therapy Related Topics: Bacterial Phage Resistance, Phage Training and Prophages in Bacterial Production Strains

Pharmacologically Aware Phage Therapy: Pharmacodynamic and Pharmacokinetic Obstacles to Phage Antibacterial Action in Animal and Human Bodies

The use of viruses infecting bacteria (bacteriophages or phages) to treat bacterial infections has been ongoing clinically for approximately 100 years. Despite that long history, the growing international crisis of resistance to standard antibiotics, abundant anecdotal evidence of efficacy, and one successful modern clinical trial of efficacy, this phage therapy is not yet a mainstream approach in medicine. One explanation for why phage therapy has not been subject to more widespread implementation is that phage therapy research, both preclinical and clinical, can be insufficiently pharmacologically aware. Consequently, here we consider the pharmacological obstacles to phage therapy effectiveness, with phages in phage therapy explicitly being considered to serve as drug equivalents. The study of pharmacology has traditionally been differentiated into pharmacokinetic and pharmacodynamic aspects. We therefore separately consider the difficulties that phages as virions can have in traveling through body compartments toward reaching their target bacteria (pharmacokinetics) and the difficulties that phages can have in exerting antibacterial activity once they have reached those bacteria (pharmacodynamics). The latter difficulties, at least in part, are functions of phage host range and bacterial resistance to phages. Given the apparently low toxicity of phages and the minimal side effects of phage therapy as practiced, phage therapy should be successful so long as phages can reach the targeted bacteria in sufficiently high numbers, adsorb, and then kill those bacteria. Greater awareness of what obstacles to this success generally or specifically can exist, as documented in this review, should aid in the further development of phage therapy toward wider use.

Combined Use of the Ab105-2φΔCI Lytic Mutant Phage and Different Antibiotics in Clinical Isolates of Multi-Resistant Acinetobacter baumannii

Phage therapy is an abandoned antimicrobial therapy that has been resumed in recent years. In this study, we mutated a lysogenic phage from Acinetobacter baumannii into a lytic phage (Ab105-2phiΔCI) that displayed antimicrobial activity against A. baumannii clinical strain Ab177_GEIH-2000 (isolated in the GEIH-REIPI Spanish Multicenter A. baumannii Study II 2000/2010, Umbrella Genbank Bioproject PRJNA422585, and for which meropenem and imipenem MICs of respectively, 32 µg/mL, and 16 µg/mL were obtained). We observed an in vitro synergistic antimicrobial effect (reduction of 4 log–7 log CFU/mL) between meropenem and the lytic phage in all combinations analyzed (Ab105-2phiΔCI mutant at 0.1, 1 and 10 MOI and meropenem at 1/4 and 1/8 MIC). Moreover, bacterial growth was reduced by 8 log CFU/mL for the combination of imipenem at 1/4 MIC plus lytic phage (Ab105-2phiΔCI mutant) and by 4 log CFU/mL for the combination of imipenem at 1/8 MIC plus lytic phage (Ab105-2phiΔCI mutant) at both MOI 1 and 10. These results were confirmed in an in vivo model (G. mellonella), and the combination of imipenem and mutant Ab105-2phiΔCI was most effective (p < 0.05). This approach could help to reduce the emergence of phage resistant bacteria and restore sensitivity to antibiotics used to combat multi-resistant strains of Acinetobacter baumannii.

Host Range of Bacteriophages Against a World-Wide Collection of Erwinia amylovora Determined Using a Quantitative PCR Assay

Erwinia amylovora is a globally devastating pathogen of apple, pear, and other Rosaceous plants. The use of lytic bacteriophages for disease management continues to garner attention as a possible supplement or alternative to antibiotics. A quantitative productive host range was established for 10 Erwinia phages using 106 wild type global isolates of E. amylovora, and the closely related Erwinia pyrifoliae, to investigate the potential regional efficacy of these phages within a biopesticide. Each host was individually infected with each of the 10 Erwinia phages and phage production after 8 h incubation was measured using quantitative real time PCR (qPCR) in conjunction with a standardized plasmid. PCR amplicons for all phages used in the study were incorporated into a single plasmid, allowing standardized quantification of the phage genome copy number after the infection process. Nine of the tested phages exhibited a broad host range, replicating their genomes by at least one log in over 88% of tested hosts. Also, every Amygdaloideae infecting E. amylovora host was able to increase at least one phage by three logs. Bacterial hosts isolated in western North America were less susceptible to most phages, as the mean genomic titre produced dropped by nearly two logs, and this phenomenon was strongly correlated to the amount of exopolysaccharide produced by the host. This method of host range analysis is faster and requires less effort than traditional plaque assay techniques, and the resulting quantitative data highlight subtle differences in phage host preference not observable with typical plaque-based host range assays. These quantitative host range data will be useful to determine which phages should be incorporated into a phage-mediated biocontrol formulation to be tested for regional and universal control of E. amylovora.

Resistance Evolution against Phage Combinations Depends on the Timing and Order of Exposure

Phage therapy is a promising alternative to chemotherapeutic antibiotics for the treatment of bacterial infections. However, despite recent clinical uses of combinations of phages to treat multidrug-resistant infections, a mechanistic understanding of how bacteria evolve resistance against multiple phages is lacking, limiting our ability to deploy phage combinations optimally. Here, we show, using Pseudomonas aeruginosa and pairs of phages targeting shared or distinct surface receptors, that the timing and order of phage exposure determine the strength, cost, and mutational basis of resistance. Whereas sequential exposure allowed bacteria to acquire multiple resistance mutations effective against both phages, this evolutionary trajectory was prevented by simultaneous exposure, resulting in quantitatively weaker resistance. The order of phage exposure determined the fitness costs of sequential resistance, such that certain sequential orders imposed much higher fitness costs than the same phage pair in the reverse order. Together, these data suggest that phage combinations can be optimized to limit the strength of evolved resistances while maximizing their associated fitness costs to promote the long-term efficacy of phage therapy.IMPORTANCE Globally rising rates of antibiotic resistance have renewed interest in phage therapy where combinations of phages have been successfully used to treat multidrug-resistant infections. To optimize phage therapy, we first need to understand how bacteria evolve resistance against combinations of multiple phages. Here, we use simple laboratory experiments and genome sequencing to show that the timing and order of phage exposure determine the strength, cost, and mutational basis of resistance evolution in the opportunistic pathogen Pseudomonas aeruginosa These findings suggest that phage combinations can be optimized to limit the emergence and persistence of resistance, thereby promoting the long-term usefulness of phage therapy.

Diversity and Host Specificity Revealed by Biological Characterization and Whole Genome Sequencing of Bacteriophages Infecting Salmonella enterica

Phages infecting members of the opportunistic human pathogen, Salmonella enterica, are widespread in natural environments and offer a potential source of agents that could be used for controlling populations of this bacterium; yet, relatively little is known about these phages. Here we describe the isolation and characterization of 45 phages of Salmonella enterica from disparate geographic locations within British Columbia, Canada. Host-range profiling revealed host-specific patterns of susceptibility and resistance, with several phages identified that have a broad-host range (i.e., able to lyse >40% of bacterial hosts tested). One phage in particular, SE13, is able to lyse 51 out of the 61 Salmonella strains tested. Comparative genomic analyses also revealed an abundance of sequence diversity in the sequenced phages. Alignment of the genomes grouped the phages into 12 clusters with three singletons. Phages within certain clusters exhibited extraordinarily high genome homology (>98% nucleotide identity), yet between clusters, genomes exhibited a span of diversity (<50% nucleotide identity). Alignment of the major capsid protein also supported the clustering pattern observed with alignment of the whole genomes. We further observed associations between genomic relatedness and the site of isolation, as well as genetic elements related to DNA metabolism and host virulence. Our data support the knowledge framework for phage diversity and phage-host interactions that are required for developing phage-based applications for various sectors, including biocontrol, detection and typing.

Factors determining phage stability/activity: challenges in practical phage application

Introduction: Phages consist of nucleic acids and proteins that may lose their activity under different physico-chemical conditions. The production process of phage formulations may decrease phage infectivity. Ingredients present in the preparation may influence phage particles, although preparation and storage conditions may also cause variations in phage titer. Significant factors are the manner of phage application, the patient's immune system status, the type of medication being taken, and diet. Areas covered: We discuss factors determining phage activity and stability, which is relevant for the preparation and application of phage formulations with the highest therapeutic efficacy. Our article should be helpful for more insightful implementation of clinical trials, which could pave the way for successful phage therapy. Expert opinion: The number of naturally occurring phages is practically unlimited and phages vary in their susceptibility to external factors. Modern methods offer engineering techniques which should lead to enhanced precision in phage delivery and anti-bacterial activity. Recent data suggesting that phages may also be used in treating nonbacterial infections as well as anti-inflammatory and immunomodulatory agents add further weight to such studies. It may be anticipated that different phage activities could have varying susceptibility to factors determining their actions.

Advantages and Limitations of Bacteriophages for the Treatment of Bacterial Infections

Bacteriophages (BPs) are viruses that can infect and kill bacteria without any negative effect on human or animal cells. For this reason, it is supposed that they can be used, alone or in combination with antibiotics, to treat bacterial infections. In this narrative review, the advantages and limitations of BPs for use in humans will be discussed. PubMed was used to search for all of the studies published from January 2008 to December 2018 using the key words: “BPs” or “phages” and “bacterial infection” or “antibiotic” or “infectious diseases.” More than 100 articles were found, but only those published in English or providing evidence-based data were included in the evaluation. Literature review showed that the rapid rise of multi-drug-resistant bacteria worldwide coupled with a decline in the development and production of novel antibacterial agents have led scientists to consider BPs for treatment of bacterial infection. Use of BPs to overcome the problem of increasing bacterial resistance to antibiotics is attractive, and some research data seem to indicate that it might be a rational measure. However, present knowledge seems insufficient to allow the use of BPs for this purpose. To date, the problem of how to prepare the formulations for clinical use and how to avoid or limit the risk of emergence of bacterial resistance through the transmission of genetic material are not completely solved problems. Further studies specifically devoted to solve these problems are needed before BPs can be used in humans.

Lytic and genomic properties of spontaneous host-range Kayvirus mutants prove their suitability for upgrading phage therapeutics against staphylococci

Lytic bacteriophages are valuable therapeutic agents against bacterial infections. There is continual effort to obtain new phages to increase the effectivity of phage preparations against emerging phage-resistant strains. Here we described the genomic diversity of spontaneous host-range mutants of kayvirus 812. Five mutant phages were isolated as rare plaques on phage-resistant Staphylococcus aureus strains. The host range of phage 812-derived mutants was 42% higher than the wild type, determined on a set of 186 methicillin-resistant S. aureus strains representing the globally circulating human and livestock-associated clones. Comparative genomics revealed that single-nucleotide polymorphisms from the parental phage 812 population were fixed in next-step mutants, mostly in genes for tail and baseplate components, and the acquired point mutations led to diverse receptor binding proteins in the phage mutants. Numerous genome changes associated with rearrangements between direct repeat motifs or intron loss were found. Alterations occurred in host-takeover and terminal genomic regions or the endolysin gene of mutants that exhibited the highest lytic activity, which implied various mechanisms of overcoming bacterial resistance. The genomic data revealed that Kayvirus spontaneous mutants are free from undesirable genes and their lytic properties proved their suitability for rapidly updating phage therapeutics.

Characterization of Two Pseudomonas aeruginosa Viruses vB_PaeM_SCUT-S1 and vB_PaeM_SCUT-S2

The sophisticated antibiotic resistance mechanism of Pseudomonas aeruginosa has urged the development of alternative antibacterial strategies. Phage therapy has been proven successful for the treatment of multidrug-resistant infections. In this study, we reported two virulent P. aeruginosa phages, vB_PaeM_SCUT-S1 (S1) and vB_PaeM_SCUT-S2 (S2), which were characterized at morphological, genomic, and proteomic levels. Phages S1 and S2 were assigned to the Myoviridae family. The genome sequencing showed that the genome size of Phage S1 was 66,046 bp and that of Phage S2 was 94,434 bp. The phylogenetic tree indicated that the two phages were distantly related to each other and were classified in the genera Pbunavirus and Pakpunavirus respectively. Thirty-one proteins were identified for each phage by mass spectrometry and were used to substantiate the function of the predicted coding genes. The two phages inhibited the growth of P. aeruginosa strain PAO1 at low multiplicity of infection levels and had good performance both on preventing biofilm formation and eradicating preformed biofilms. They were also stable over a wide range of temperature and pH values, supporting their potential use in the treatment of P. aeruginosa infections.

Emerging Strategies to Combat ESKAPE Pathogens in the Era of Antimicrobial Resistance: A Review

The acronym ESKAPE includes six nosocomial pathogens that exhibit multidrug resistance and virulence: Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp. Persistent use of antibiotics has provoked the emergence of multidrug resistant (MDR) and extensively drug resistant (XDR) bacteria, which render even the most effective drugs ineffective. Extended spectrum β-lactamase (ESBL) and carbapenemase producing Gram negative bacteria have emerged as an important therapeutic challenge. Development of novel therapeutics to treat drug resistant infections, especially those caused by ESKAPE pathogens is the need of the hour. Alternative therapies such as use of antibiotics in combination or with adjuvants, bacteriophages, antimicrobial peptides, nanoparticles, and photodynamic light therapy are widely reported. Many reviews published till date describe these therapies with respect to the various agents used, their dosage details and mechanism of action against MDR pathogens but very few have focused specifically on ESKAPE. The objective of this review is to describe the alternative therapies reported to treat ESKAPE infections, their advantages and limitations, potential application in vivo, and status in clinical trials. The review further highlights the importance of a combinatorial approach, wherein two or more therapies are used in combination in order to overcome their individual limitations, additional studies on which are warranted, before translating them into clinical practice. These advances could possibly give an alternate solution or extend the lifetime of current antimicrobials.

Towards Inhaled Phage Therapy in Western Europe

The emergence of multidrug-resistant bacteria constitutes a great challenge for modern medicine, recognized by leading medical experts and politicians worldwide. Rediscovery and implementation of bacteriophage therapy by Western medicine might be one solution to the problem of increasing antibiotic failure. In some Eastern European countries phage therapy is used for treating infectious diseases. However, while the European Medicines Agency (EMA) advised that the development of bacteriophage-based therapies should be expedited due to its significant potential, EMA emphasized that phages cannot be recommended for approval before efficacy and safety have been proven by appropriately designed preclinical and clinical trials. More evidence-based data is required, particularly in the areas of pharmacokinetics, repeat applications, immunological reactions to the application of phages as well as the interactions and effects on bacterial biofilms and organ-specific environments. In this brief review we summarize advantages and disadvantages of phage therapy and discuss challenges to the establishment of phage therapy as approved treatment for multidrug-resistant bacteria.

Clinical application of bacteriophages in Europe

Bacteriophages could help address the antibiotic resistance crisis that impacts health systems all over the world. In 2011, the European Commission formally confirmed that phage products used as therapeutics are medicinal products and thus manufacturers need to navigate the extremely arduous and enormously expensive medicine development and marketing pathway. However, up until now, not one therapeutic phage product has made it to the European market, and yet clinicians are under increasing pressure to use phages in the treatment of multidrug-resistant bacterial infections. While a handful of small European enterprises are struggling to squeeze therapeutic phage products through the conventional and centralised European medicinal products funnel, some clinicians and academics are exploring (European) national solutions to accelerate the availability of phages for the treatment of an increasing number of desperate patients. This mini-review summarises the actual status and perspectives of clinical phage application in Europe.

Bacteriophages: A Therapy Concept against Multi-Drug-Resistant Bacteria

Bacteriophages (phages) are viruses that kill bacteria specifically but cannot infect other kinds of organisms. They have attracted new attention since the increasing antibiotic resistance developed into a global crisis. Phage therapy, a 100-year-old form of antibacterial treatment in medicine, is gaining momentum because phages represent a therapy concept without such negative side effects as toxicity; phages are the only therapeutic agent that regulates itself at the sites of infection and decays when the infectious bacteria have been killed. Nature is an almost infinite phage resource: New ones can be isolated for most kinds of problem bacteria as needed; bacteria and their phages constantly co-evolve. This is important as new pathogenic bacterial variants evolve and new challenging situations arise. In human therapy, "cocktails" of multiple phages may reduce the probability of selecting bacteria that developed resistance to a certain phage. Antibiotic agents can be applied together with phages in many circumstances; the two often function synergistically. Phages cannot be expected to replace antibiotic agents in our medical arsenal, but can be used where antibiotic agents fail. The selected phages, however, must be obligately virulent, well-characterized, and highly purified before application. Countless patients and their physicians are waiting for re-establishing phage therapy as a flexible, tailored medicine; infrastructures should be built in all countries urgently: The 2015 World Health Organization assembly resolution 68.7.3. called for national action plans by May 2017 to combat the antimicrobial drug resistance crisis. This article discusses the therapeutic potential of phages and describes challenges and recent developments.

Microbiota and Phage Therapy: Future Challenges in Medicine

An imbalance of bacterial quantity and quality of gut microbiota has been linked to several pathologies. New strategies of microbiota manipulation have been developed such as fecal microbiota transplantation (FMT); the use of pre/probiotics; an appropriate diet; and phage therapy. The presence of bacteriophages has been largely underestimated and their presence is a relevant component for the microbiome equilibrium. As a promising treatment, phage therapy has been extensively used in Eastern Europe to reduce pathogenic bacteria and has arisen as a new method to modulate microbiota diversity. Phages have been selected and “trained” to infect a wide spectrum of bacteria or tailored to infect specific antibiotic resistant bacteria present in patients. The new development of genetically modified phages may be an efficient tool to treat the gut microbiota dysbiosis associated with different pathologies and increased production of bacterial metabolites and subsequently decrease systemic low-grade chronic inflammation associated with chronic diseases. Microbiota quality and mitochondria dynamics can be remodulated and manipulated by phages to restore the equilibrium and homeostasis of the system. Our aim is to highlight the great interest for phages not only to eliminate and control pathogenic bacterial infection but also in the near future to modulate the microbiota by adding new functions to selected bacteria species and rebalance the dynamic among phages and bacteria. The challenge for the medicine of tomorrow is to re-think and redesign strategies differently and far from our traditional thinking.

Applications of Bacteriophages in the Treatment of Localized Infections in Humans

In the recent years, multidrug-resistant bacteria have become a global threat, and phage therapy may to be used as an alternative to antibiotics or, at least, as a supplementary approach to treatment of some bacterial infections. Here, we describe the results of bacteriophage application in clinical practice for the treatment of localized infections in wounds, burns, and trophic ulcers, including diabetic foot ulcers. This mini-review includes data from various studies available in English, as well as serial case reports published in Russian scientific literature (with, at least, abstracts accessible in English). Since, it would be impossible to describe all historical Russian publications; we focused on publications included clear data on dosage and rout of phage administration.

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.

Phage Therapy: What Have We Learned?

In this article we explain how current events in the field of phage therapy may positively influence its future development. We discuss the shift in position of the authorities, academia, media, non-governmental organizations, regulatory agencies, patients, and doctors which could enable further advances in the research and application of the therapy. In addition, we discuss methods to obtain optimal phage preparations and suggest the potential of novel applications of phage therapy extending beyond its anti-bacterial action.