Potential of a lytic bacteriophage to disrupt Acinetobacter baumannii biofilms in vitro

Bacteriophages and their derived enzymes as promising alternatives for the treatment of Acinetobacter baumannii infections

Nosocomial infections with the opportunistic bacterium Acinetobacter baumannii pose a severe challenge to clinical treatment, which is aggravated by the increasing occurrence of multi-drug resistance, especially resistance to carbapenems. The use of phage therapy as an alternative and supplement to the current antibiotics has become an important research topic in the post-antibiotic era. This review summarizes in vivo and in vitro studies on phage therapy against multi-drug-resistant A. baumannii infection that have used different approaches, including treatment with a single phage, combination with other phages or non-phage agents, and administration of phage-derived enzymes. We also briefly discuss the current challenges of phage-based therapy as well as promising approaches for the treatment of A. baumannii infection in the future.

A systematic review of the use of bacteriophages for in vitro biofilm control

Bacteriophages (phages) are very promising biological agents for the prevention and control of bacterial biofilms. However, little is known about the parameters that can influence the efficacy of phages on biofilms. This systematic review provides a summary and analysis of the published data about the use of phages to control pre-formed biofilms in vitro, suggesting recommendations for future experiments in this area. A total of 68 articles, containing data on 605 experiments addressing the efficacy of phages to control biofilms in vitro were included, after a search conducted in Web of Science, Embase, and Medline (PubMed). The data collected from each experiment included information about biofilm growth conditions, phage characteristics, treatment conditions and biofilm reduction. In most cases, biofilms were formed in the surface of microtiter plates (82.5%); the median time for biofilm formation was 24 h, as is the median treatment duration. Quantification of biofilm biomass (52.6%), viable cells (25.5%) and metabolic activity (17.9%) were the most common biofilm assessment methods. Correlation analysis revealed that some phage parameters can influence the treatment outcome: higher phage concentrations were strongly associated with improved biofilm control, leading to higher levels of biofilm reduction, and phages with higher burst sizes and shorter latent periods seem to be the best candidates to control biofilms in vitro. However, the great variability of the methodologies used prompts the need for the development of standardized in vitro methodologies to characterize phage/biofilm interactions and to assess the efficacy of phages to control biofilms.

Determination of phage load and administration time in simulated occurrences of antibacterial treatments

The use of phages as antibacterials is becoming more and more common in Western countries. However, a successful phage-derived antibacterial treatment needs to account for additional features such as the loss of infective virions and the multiplication of the hosts. The parameters critical inoculation size (V F ) and failure threshold time (T F ) have been introduced to assure that the viral dose (V ϕ) and administration time (T ϕ) would lead to the extinction of the targeted bacteria. The problem with the definition of V F and T F is that they are non-linear equations with two unknowns; thus, obtaining their explicit values is cumbersome and not unique. The current study used machine learning to determine V F and T F for an effective antibacterial treatment. Within these ranges, a Pareto optimal solution of a multi-criterial optimization problem (MCOP) provided a pair of V ϕ and T ϕ to facilitate the user's work. The algorithm was tested on a series of in silico microbial consortia that described the outgrowth of a species at high cell density by another species initially present at low concentration. The results demonstrated that the MCOP-derived pairs of V ϕ and T ϕ could effectively wipe out the bacterial target within the context of the simulation. The present study also introduced the concept of mediated phage therapy, where targeting booster bacteria might decrease the virulence of a pathogen immune to phagial infection and highlighted the importance of microbial competition in attaining a successful antibacterial treatment. In summary, the present work developed a novel method for investigating phage/bacteria interactions that can help increase the effectiveness of the application of phages as antibacterials and ease the work of microbiologists.

Isolation and Characterization of a Novel Siphoviridae Phage, vB_AbaS_TCUP2199, Infecting Multidrug-Resistant Acinetobacter baumannii

Multidrug-resistant Acinetobacter baumannii (MDRAB) is a pathogen recognized as antimicrobial-resistant bacteria involved in healthcare-associated infections. Resistance to antibiotics has made alternative therapies necessary. Bacteriophage therapy is considered a potential solution to treat MDRAB. In this study, we isolated and characterized the phage vB_AbaS_TCUP2199 (TCUP2199), which can infect MDRAB. Morphological analysis revealed that TCUP2199 belongs to the Siphoviridae family. TCUP2199 has a wide host range, can adsorb rapidly (68.28% in 2 min), and has a burst size of 196 PFU/cell. At least 16 distinct structural proteins were visualized by SDS polyacrylamide gel electrophoresis. A stability test showed that TCUP2199 was stable at 37 °C and pH 7. Genome analysis of TCUP2199 showed that it consists of a double-stranded DNA genome of 79,572 bp with a G+C content of 40.39%, which contains 98 putative open reading frames, none of which is closely related to the bacteriophage genome sequence that was found in the public database. TCUP2199 shows similarity in genomic organization and putative packaging mechanism with Achromobacter phage JWF and Pseudoalteromonas phage KB12-38 based on protein BLAST and phylogenetic analysis. Because of those unique characteristics, we consider TCUP2199 to be a novel phage that is suitable for inclusion in a phage cocktail to treat A. baumannii infection.

Bacteriophage therapeutics to confront multidrug-resistant Acinetobacter baumannii - a global health menace

We have already entered the post-antibiotic era as the outbreaks of numerous multidrug-resistant strains in the community as well as hospital-acquired infections are ringing alarm bells in the health sector. Acinetobacter baumannii is one such pathogen that has been considered a worldwide threat as it acquires multidrug resistance. It is one of the most challenging hospital-acquired pathogens as World Health Organization has listed carbapenem-resistant A. baumannii as a critical priority pathogen with limited therapeutic options. There is an urgent need to develop novel strategies against such pathogens to tackle the global crisis. Bacteriophages (phages), especially the lytic ones have re-emerged as a potential therapeutic approach. This review encompasses vast majority of phages against A. baumannii strains with special references related to single phage or monophage therapy, use of phage cocktails, combination therapy with antibiotics, use of phage-derived enzymes like endolysins and depolymerases to combat the pathogen and explore their therapeutic aspects. The concurrent ecological as well as evolutionary interplay between the phages and host bacteria demands in depth-research and knowledge, so as to utilize the maximum potential of the bacteriophage therapy.

Preoptimized phage cocktail for use in aerosols against nosocomial transmission of carbapenem-resistant Acinetobacter baumannii: A 3-year prospective intervention study

Using bacteriophages (phages) as environmental sanitizers has been recognized as a potential alternative method to remove bacterial contamination in vitro; however, very few studies are available on the application of phages for infection control in hospitals. Here, we performed a 3-year prospective intervention study using aerosolized phage cocktails as biocontrol agents against carbapenem-resistant Acinetobacter baumannii (CRAB) infection in the hospital. When a CRAB-infected patient was identified in an intensive care unit (ICU), their surrounding environment was chosen for phage aerosol decontamination. Before decontamination, 501 clinical specimens from the patients were subjected to antibiotic resistance analysis and phage typing. The optimal phage cocktails were a combination of different phage families or were constructed by next-evolutionary phage typing with the highest score for the host lysis zone to prevent the development of environmental CRAB phage resistance. The phage infection percentage of the antibiotic-resistant A. baumannii strains was 97.1%, whereas the infection percentage in the antibiotic-susceptible strains was 79.3%. During the phage decontamination periods from 2017 to 2019, the percentage of carbapenem-resistant A. baumannii in test ICUs decreased significantly from 65.3% to 55%. The rate of new acquisitions of CRAB infection over the three years was 4.4 per 1000 patient-days, which was significantly lower than that in the control wards (8.9 per 1000 patient-days) where phage decontamination had never been performed. In conclusion, our results support the potential of phage cocktails to decrease CRAB infection rates, and the aerosol generation process may make this approach more comprehensive and time-saving.

Phage-Derived Depolymerase as an Antibiotic Adjuvant Against Multidrug-Resistant Acinetobacter baumannii

Bacteriophage-encoded depolymerases are responsible for degrading capsular polysaccharides (CPS), lipopolysaccharides (LPS), and exopolysaccharides (EPS) of the host bacteria during phage invasion. They have been considered as promising antivirulence agents in controlling bacterial infections, including those caused by multidrug-resistant (MDR) bacteria. This feature inspires hope of utilizing these enzymes to disarm the polysaccharide capsules of the bacterial cells, which then strengthens the action of antibiotics. Here we have identified, cloned, and expressed a depolymerase Dpo71 from a bacteriophage specific for the gram-negative bacterium Acinetobacter baumannii in a heterologous host Escherichia coli. Dpo71 sensitizes the MDR A. baumannii to the host immune attack, and also acts as an adjuvant to assist or boost the action of antibiotics, for example colistin. Specifically, Dpo71 at 10 μg/ml enables a complete bacterial eradication by human serum at 50% volume ratio. A mechanistic study shows that the enhanced bactericidal effect of colistin is attributed to the improved outer membrane destabilization capacity and binding rate to bacteria after stripping off the bacterial capsule by Dpo71. Dpo71 inhibits biofilm formation and disrupts the pre-formed biofilm. Combination of Dpo71 could significantly enhance the antibiofilm activity of colistin and improve the survival rate of A. baumannii infected Galleria mellonella. Dpo71 retains the strain-specificity of the parent phage from which Dpo71 is derived: the phage-sensitive A. baumannii strains respond to Dpo71 treatment, whereas the phage-insensitive strains do not. In summary, our work demonstrates the feasibility of using recombinant depolymerases as an antibiotic adjuvant to supplement the development of new antibacterials and to battle against MDR pathogens.

Convergence of Biofilm Formation and Antibiotic Resistance in Acinetobacter baumannii Infection

Acinetobacter baumannii (A. baumannii) is a leading cause of nosocomial infections as this pathogen has certain attributes that facilitate the subversion of natural defenses of the human body. A. baumannii acquires antibiotic resistance determinants easily and can thrive on both biotic and abiotic surfaces. Different resistance mechanisms or determinants, both transmissible and non-transmissible, have aided in this victory over antibiotics. In addition, the propensity to form biofilms (communities of organism attached to a surface) allows the organism to persist in hospitals on various medical surfaces (cardiac valves, artificial joints, catheters, endotracheal tubes, and ventilators) and also evade antibiotics simply by shielding the bacteria and increasing its ability to acquire foreign genetic material through lateral gene transfer. The biofilm formation rate in A. baumannii is higher than in other species. Recent research has shown how A. baumannii biofilm-forming capacity exerts its effect on resistance phenotypes, development of resistome, and dissemination of resistance genes within biofilms by conjugation or transformation, thereby making biofilm a hotspot for genetic exchange. Various genes control the formation of A. baumannii biofilms and a beneficial relationship between biofilm formation and "antimicrobial resistance" (AMR) exists in the organism. This review discusses these various attributes of the organism that act independently or synergistically to cause hospital infections. Evolution of AMR in A. baumannii, resistance mechanisms including both transmissible (hydrolyzing enzymes) and non-transmissible (efflux pumps and chromosomal mutations) are presented. Intrinsic factors [biofilm-associated protein, outer membrane protein A, chaperon-usher pilus, iron uptake mechanism, poly-β-(1, 6)-N-acetyl glucosamine, BfmS/BfmR two-component system, PER-1, quorum sensing] involved in biofilm production, extrinsic factors (surface property, growth temperature, growth medium) associated with the process, the impact of biofilms on high antimicrobial tolerance and regulation of the process, gene transfer within the biofilm, are elaborated. The infections associated with colonization of A. baumannii on medical devices are discussed. Each important device-related infection is dealt with and both adult and pediatric studies are separately mentioned. Furthermore, the strategies of preventing A. baumannii biofilms with antibiotic combinations, quorum sensing quenchers, natural products, efflux pump inhibitors, antimicrobial peptides, nanoparticles, and phage therapy are enumerated.

«Development of an anti-Acinetobacter baumannii biofilm phage cocktail: Genomic Adaptation to the Host»

The need for alternatives to antibiotic therapy due to the emergence of multidrug resistant bacteria (MDR), such as the nosocomial pathogen Acinetobacter baumannii, has led to the recovery of phage therapy. In addition, phages can be combined in cocktails to increase the host range. In this study, the evolutionary mechanism of adaptation was utilized in order to develop a phage adapted to A. baumannii, named phage Ab105-2phiΔCI404ad, from a mutant lytic phage, Ab105-2phiΔCI, previously developed by our group. The whole genome sequence of phage Ab105-2phiΔCI404ad was determined, showing that four genomic rearrangements events occurred in the tail morphogenesis module affecting the ORFs encoding the host receptor binding sites. As a consequence of the genomic rearrangements, 10 ORFs were lost and four new ORFs were obtained, all encoding tail proteins; two inverted regions were also derived from these events. The adaptation process increased the host range of the adapted phage by almost three folds. In addition, a depolymerase-expressing phenotype, indicated by formation of a halo, which was not observed in the ancestral phage, was obtained in 81% of the infected strains. A phage cocktail was formed by combining this phage with the A. baumannii phage vB_AbaP_B3, known to express a depolymerase. Both the individual phages and the phage cocktail showed strong antimicrobial activity against 5 clinical strains and 1 reference strain of A. baumannii tested. However, in all cases resistance to the bacterial strains was also observed. The antibiofilm activity of the individual phages and the cocktail was assayed. The phage cocktail displayed strong antibiofilm activity.

Biofilms in Diabetic Foot Ulcers: Impact, Risk Factors and Control Strategies

Diabetic foot ulcers (DFUs) are a serious complication from diabetes mellitus, with a huge economic, social and psychological impact on the patients' life. One of the main reasons why DFUs are so difficult to heal is related to the presence of biofilms. Biofilms promote wound inflammation and a remarkable lack of response to host defences/treatment options, which can lead to disease progression and chronicity. In fact, appropriate treatment for the elimination of these microbial communities can prevent the disease evolution and, in some cases, even avoid more serious outcomes, such as amputation or death. However, the detection of biofilm-associated DFUs is difficult due to the lack of methods for diagnostics in clinical settings. In this review, the current knowledge on the involvement of biofilms in DFUs is discussed, as well as how the surrounding environment influences biofilm formation and regulation, along with its clinical implications. A special focus is also given to biofilm-associated DFU diagnosis and therapeutic strategies. An overview on promising alternative therapeutics is provided and an algorithm considering biofilm detection and treatment is proposed.

Modulation of Quorum Sensing and Biofilms in Less Investigated Gram-Negative ESKAPE Pathogens

Pathogenic bacteria have the ability to sense their versatile environment and adapt by behavioral changes both to the external reservoirs and the infected host, which, in response to microbial colonization, mobilizes equally sophisticated anti-infectious strategies. One of the most important adaptive processes is the ability of pathogenic bacteria to turn from the free, floating, or planktonic state to the adherent one and to develop biofilms on alive and inert substrata; this social lifestyle, based on very complex communication networks, namely, the quorum sensing (QS) and response system, confers them an increased phenotypic or behavioral resistance to different stress factors, including host defense mechanisms and antibiotics. As a consequence, biofilm infections can be difficult to diagnose and treat, requiring complex multidrug therapeutic regimens, which often fail to resolve the infection. One of the most promising avenues for discovering novel and efficient antibiofilm strategies is targeting individual cells and their QS mechanisms. A huge amount of data related to the inhibition of QS and biofilm formation in pathogenic bacteria have been obtained using the well-established gram-positive Staphylococcus aureus and gram-negative Pseudomonas aeruginosa models. The purpose of this paper was to revise the progress on the development of antibiofilm and anti-QS strategies in the less investigated gram-negative ESKAPE pathogens Klebsiella pneumoniae, Acinetobacter baumannii, and Enterobacter sp. and identify promising leads for the therapeutic management of these clinically significant and highly resistant opportunistic pathogens.

Bacteriophages as tools for biofilm biocontrol in different fields

Microbial biofilms are difficult to control due to the limited accessibility that antimicrobial drugs and chemicals have to the entrapped inner cells. The extracellular matrix, binds water, contributes to altered cell physiology within biofilms and act as a barrier for most antiproliferative molecules. Thus, new strategies need to be developed to overcome biofilm vitality. In this review, based on 223 documents, the advantages, recommendations, and limitations of using bacteriophages as 'biofilm predators' are presented. The plausibility of using phages (bacteriophages and mycoviruses) to control biofilms grown in different environments is also discussed. The topics covered here include recent historical experiences in biofilm control/eradication using phages in medicine, dentistry, veterinary, and food industries, the pros and cons of their use, and the development of microbial resistance/immunity to such viruses.

Bacteriophages for ESKAPE: role in pathogenicity and measures of control

INTRODUCTION

The quest to combat bacterial infections has dreaded humankind for centuries. Infections involving ESKAPE (Enterococcus spp., Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp.) impose therapeutic challenges due to the emergence of antimicrobial drug resistance. Recently, investigations with bacteriophages have led to the development of novel strategies against ESKAPE infections. Also, bacteriophages have been demonstrated to be instrumental in the dissemination of virulence markers in ESKAPE pathogens.

AREAS COVERED

The review highlights the potential of bacteriophage in and against the pathogenicity of antibiotic-resistant ESKAPE pathogens. The review also emphasizes the challenges of employing bacteriophage in treating ESKAPE pathogens and the knowledge gap in the bacteriophage mediated antibiotic resistance and pathogenicity in ESKAPE infections.

EXPERT OPINION

Bacteriophage infection can kill the host bacteria but in survivors can transfer genes that contribute toward the survival of the pathogens in the host and resistance toward multiple antimicrobials. The knowledge on the dual role of bacteriophages in the treatment and pathogenicity will assist in the prediction and development of novel therapeutics targeting antimicrobial-resistant ESKAPE. Therefore, extensive investigations on the efficacy of synthetic bacteriophage, bacteriophage cocktails, and bacteriophage in combination with antibiotics are needed to develop effective therapeutics against ESKAPE infections.

Characterization, Antibiofilm, and Depolymerizing Activity of Two Phages Active on Carbapenem-Resistant Acinetobacter baumannii

Acinetobacter baumannii is a leading cause of healthcare-associated infections worldwide. Its various intrinsic and acquired mechanisms of antibiotic resistance make the therapeutic challenge even more serious. One of the promising alternative treatments that is increasingly highlighted is phage therapy, the therapeutic use of bacteriophages to treat bacterial infections. Two phages active against nosocomial carbapenem-resistant A. baumannii strain 6077/12, vB_AbaM_ISTD, and vB_AbaM_NOVI, were isolated from Belgrade wastewaters, purified, and concentrated using CsCl gradient ultracentrifugation. The phages were screened against 103 clinical isolates of A. baumannii from a laboratory collection and characterized based on plaque and virion morphology, host range, adsorption rate, and one-step growth curve. Given that phage ISTD showed a broader host range, better adsorption rate, shorter latent period, and larger burst size, its ability to lyse planktonic and biofilm-embedded cells was tested in detail. Phage ISTD yielded a 3.5- and 2-log reduction in planktonic and biofilm-associated viable bacterial cell count, respectively, but the effect was time-dependent. Both phages produced growing turbid halos around plaques indicating the synthesis of depolymerases, enzymes capable of degrading bacterial exopolysaccharides. Halos tested positive for presence of phages in the proximity of the plaque, but not further from the plaque, which indicates that the observed halo enlargement is a consequence of enzyme diffusion through the agar, independently of the phages. This notion was also supported by the growing halos induced by phage preparations applied on pregrown bacterial lawns, indicating that depolymerizing effect was achieved also on non-dividing sensitive cells. Overall, good rates of growth, fast adsorption rate, broad host range, and high depolymerizing activity, as well as antibacterial effectiveness against planktonic and biofilm-associated bacteria, make these phages good candidates for potential application in combating A. baumannii infections.

Identification of a Novel Acinetobacter baumannii Phage-Derived Depolymerase and Its Therapeutic Application in Mice

The rapid expansion of Acinetobacter baumannii clinical isolates exhibiting resistance to most or all available antibiotics is a global concern. Current treatments for infections caused by this bacterium have become less effective, and the need to explore new alternative therapies is urgent. Depolymerases derived from phages are emerging as attractive anti-virulence agents. In this study, a previously isolated A. baumannii phage (designated as vB_AbaM_IME285) was characterized, and genomic study was carried out using various bioinformatics tools. A gene predicted as encoding for the depolymerase was cloned and expressed, and the depolymerase activity of the recombinant enzyme (Dp49) was identified both in vitro and in experimental mice. The results showed that phage IME285 formed translucent halos around the plaques when inoculated onto a lawn of the host bacteria, exibiting depolymerase activity against this strain. On the basis of complete genome sequencing and bioinformatics analysis, ORF49 was speculated to be a gene encoding for the putative capsule depolymerase. The expressed recombinant Dp49 displayed an effective depolymerase activity and had a spectrum of activity similar to its parental phage IME285, which was active against 25 out of 49 A. baumannii strains. It was found that Dp49 greatly improved the inhibitory effect of serum on bacterial growth in vitro, and the administration of this enzyme significantly increased the survival rates of A. baumannii-infected mice in the animal experiment. In conclusion, the phage-encoded depolymerase Dp49 might be a promising alternative means of controlling infections mediated by multidrug-resistant A. baumannii.

Development of Antimicrobial Therapy Methods to Overcome the Antibiotic Resistance of Acinetobacter baumannii

The spread of antibiotic resistance among pathogens represents a threat to human health around the world. In 2017, the World Health Organization published a list of 12 top-priority antibiotic-resistant pathogenic bacteria for which new effective antibiotics or new ways of treating the infections caused by them are needed. This review focuses on Acinetobacter baumannii, one of these top-priority pathogens. The pathogenic bacterium A. baumannii is one of the most frequently encountered infectious agents in the world; its clinically significant features include resistance to UV light, drying, disinfectants, and antibiotics. This review looks at the various attempts that have been made to tackle the problem of drug resistance relating to A. baumannii variants without the use of antibiotics. The potential of bacteriophages and antimicrobial peptides in the treatment of infections caused by A. baumannii in both planktonic and biofilm form is assessed. Such topics as research into the development of vaccines based on the outer membrane proteins of A. baumannii and the use of silver nanoparticles, as well as photodynamic and chelate therapy, are also covered.

Antibacterial Efficacy of Two Commercially Available Bacteriophage Formulations, Staphylococcal Bacteriophage and PYO Bacteriophage, Against Methicillin-Resistant Staphylococcus aureus: Prevention and Eradication of Biofilm Formation and Control of a Systemic Infection of Galleria mellonella Larvae

Sessile bacteria growing on surfaces are more resistant to standard antibiotics than their planktonic counterpart. Due to their antimicrobial properties, bacteriophages have re-emerged as a promising approach to treat bacterial biofilm-associated infections. Here, we evaluated the ability of two commercially available phage formulations, Staphylococcal bacteriophage (containing the monophage Sb-1) and PYO bacteriophage (a polyphage), in preventing and eradicating an in vitro biofilm of methicillin-resistant Staphylococcus aureus (MRSA) by isothermal microcalorimetry and high-resolution confocal laser scanning microscopy (CLSM). Moreover, to assess the potential in vivo efficacy of both phage preparations, a Galleria mellonella model of MRSA systemic infection was used. Microcalorimetry measurement showed that 10⁷ PFU/ml (the highest tested titer) of both phage formulations were able to inhibit planktonic growth in a concentration-dependent manner. However, MRSA biofilm was eradicated only by co-incubation of 5–7 days with the highest phage titers, respectively. In the experiments of biofilm prevention, isothermal microcalorimetry revealed that the heat production was completely abolished in the presence of sub-inhibitory titers (10⁴ PFU/ml) of phages. These data were also confirmed by confocal laser scanning microscopy. Both phage formulations increased the survival of G. mellonella larvae preventing or treating MRSA infection compared to untreated control. In conclusion, tested phage formulations are promising for preventing device colonization and killing biofilm bacteria attached on a surface. Novel strategies for direct coating and release of phages from material should be investigated.

Protective and therapeutic application of the depolymerase derived from a novel KN1 genotype of Klebsiella pneumoniae bacteriophage in mice

Klebsiella pneumoniae is one of the major Gram-negative bacterial pathogens causing hospital-acquired multidrug-resistant infections, and the antimicrobial treatment options are scarce. The lack of available antimicrobials has prompted the development of alternative strategies for the treatment of these infections. In this study, a K. pneumoniae bacteriophage (vB_KpnP_IME321) targeting a KN1 capsular type strain, Kp409, was isolated, characterized and sequenced. This bacteriophage has a latent period of 20 min and a burst size of approximately 410 pfu/cell. It contained 49 predicted open reading frames, of which ORF42 was identified as encoding the putative capsule depolymerase. The enzyme expressed and purified in the Escherichia coli BL21 system, namely Dp42, could depolymerize the capsular polysaccharide of Kp409 and form translucent halos on the plates. The phage-encoded depolymerase could increase the inhibitory effect of serum on the growth of bacteria in vitro. Pre-treated with Dp42 rescued 100% of mice following lethal Kp409 challenge, and administration of this enzyme after infection significantly increased survival rates of infected mice in the animal experiment. In conclusion, the phage-encoded depolymerase Dp42 represents a potential alternative strategy for controlling infections mediated by K. pneumoniae expressing the KN1 capsular polysaccharide.

Isolation and characterization of a bacteriophage and its potential to disrupt multi-drug resistant Pseudomonas aeruginosa biofilms

A lytic Pseudomonas aeruginosa bacteriophage, vB_PaeM_LS1, was isolated and characterized herein. To examine the eligibility of bacteriophage vB_PaeM_LS1 as a therapeutic bacteriophage, we analysed its genome and compared it to similar bacteriophages. Genome of bacteriophage vB_PaeM_LS1 consisted of a linear, double-stranded DNA molecule 66,095 bp in length and with 55.7% G + C content. Neighbor-joining analysis of the large subunit terminase showed that bacteriophage vB_PaeM_LS1 had similarity to the Pbunavirus genus. The potential of the lytic bacteriophage to disrupt Pseudomonas aeruginosa biofilms was assessed by scanning electron microscopy and bacterial counts. This study revealed that the bacteriophage vB_PaeM_LS1 with its lytic effect showed a high potential impact on the inhibition of the growth of Pseudomonas aeruginosa biofilm formation.

Identification and characterization of capsule depolymerase Dpo48 from Acinetobacter baumannii phage IME200

Background

The emergence of multidrug- or extensively drug-resistant Acinetobacter baumannii has made it difficult to treat and control infections caused by this bacterium. It is urgently necessary to search for alternatives to conventional antibiotics for control of severe A. baumannii infections. In recent years, bacteriophages and their derivatives, such as depolymerases, showed great potential as antibacterial or antivirulence agents against bacterial infections. Nonetheless, unlike broad-spectrum bactericidal antibiotics, phage-encoded depolymerase targets only a limited number of bacterial strains. Therefore, identification of novel depolymerases and evaluation of their ability to control A. baumannii infections is important.

Methods

A bacteriophage was isolated from hospital sewage using an extensively drug-resistant A. baumannii strain as the host bacterium, and the phage’s plaque morphology and genomic composition were studied. A polysaccharide depolymerase (Dpo48) was expressed and identified, and the effects of pH and temperature on its activity were determined. Besides, a serum killing assay was conducted, and amino acid sequences homologous to those of putative polysaccharide depolymerases were compared.

Results

Phage IME200 yielded clear plaques surrounded by enlarged halos, with polysaccharide depolymerase activity against the host bacterium. A tail fiber protein with a Pectate_lyase_3 domain was identified as Dpo48 and characterized . Dpo48 was found to degrade the capsule polysaccharide of the bacterial surface, as revealed by Alcian blue staining. Dpo48 manifested stable activity over a broad range of pH (5.0–9.0) and temperatures (20–70 °C). Results from in vitro serum killing assays indicated that 50% serum was sufficient to cause a five log reduction of overnight enzyme-treated bacteria, with serum complement playing an important role in these killing assays. Moreover, Dpo48 had a spectrum of activity exactly the same as its parental phage IME200, which was active against 10 out of 41 A. baumannii strains. Amino acid sequence alignment showed that the putative tail fiber proteins had a relatively short, highly conserved domain in their N-terminal sequences, but their amino acid sequences containing pectate lyase domains, found in the C-terminal regions, were highly diverse.

Conclusions

Phage-encoded capsule depolymerases may become promising antivirulence agents for preventing and controlling A. baumannii infections.

Bacteriophage Therapy: Clinical Trials and Regulatory Hurdles

Increasing reports of antimicrobial resistance and limited new antibiotic discoveries and development have fuelled innovation in other research fields and led to a revitalization of bacteriophage (phage) studies in the Western world. Phage therapy mainly utilizes obligately lytic phages to kill their respective bacterial hosts, while leaving human cells intact and reducing the broader impact on commensal bacteria that often results from antibiotic use. Phage therapy is rapidly evolving and has resulted in cases of life-saving therapeutic use and multiple clinical trials. However, one of the biggest challenges this antibiotic alternative faces relates to regulations and policy surrounding clinical use and implementation beyond compassionate cases. This review discusses the multi-drug resistant Gram-negative pathogens of highest critical priority and summarizes the current state-of-the-art in phage therapy targeting these organisms. It also examines phage therapy in humans in general and the approaches different countries have taken to introduce it into clinical practice and policy. We aim to highlight the rapidly advancing field of phage therapy and the challenges that lie ahead as the world shifts away from complete reliance on antibiotics.