Novel Fri1-like Viruses Infecting Acinetobacter baumannii-vB_AbaP_AS11 and vB_AbaP_AS12-Characterization, Comparative Genomic Analysis, and Host-Recognition Strategy

Characterization and therapeutic potential of MRABP9, a novel lytic bacteriophage infecting multidrug-resistant Acinetobacter baumannii clinical strains

Acinetobacter baumannii is one of the most important pathogens of healthcare-associated infections. The rising prevalence of multidrug-resistant A. baumannii (MRAB) strains and biofilm formation impact the outcome of conventional treatment. Phage-related therapy is a promising strategy to tame troublesome multidrug-resistant bacteria. Here, we isolated and evaluated a highly efficient lytic phage called MRABP9 from hospital sewage. The phage was a novel species within the genus Friunavirus and exhibited lytic activity against 2 other identified MRAB strains. Genomic analysis revealed it was a safe virulent phage and a pectate lyase domain was identified within its tail spike protein. MRABP9 showed potent bactericidal and anti-biofilm activity against MRAB, significantly delaying the time point of bacterial regrowth in vitro. Phage administration could rescue the mice from acute lethal MRAB infection. Considering its features, MRABP9 has the potential as an efficient candidate for prophylactic and therapeutic use against acute infections caused by MRAB strains.

Lytic Capsule-Specific Acinetobacter Bacteriophages Encoding Polysaccharide-Degrading Enzymes

The genus Acinetobacter comprises both environmental and clinically relevant species associated with hospital-acquired infections. Among them, Acinetobacter baumannii is a critical priority bacterial pathogen, for which the research and development of new strategies for antimicrobial treatment are urgently needed. Acinetobacter spp. produce a variety of structurally diverse capsular polysaccharides (CPSs), which surround the bacterial cells with a thick protective layer. These surface structures are primary receptors for capsule-specific bacteriophages, that is, phages carrying tailspikes with CPS-depolymerizing/modifying activities. Phage tailspike proteins (TSPs) exhibit hydrolase, lyase, or esterase activities toward the corresponding CPSs of a certain structure. In this study, the data on all lytic capsule-specific phages infecting Acinetobacter spp. with genomes deposited in the NCBI GenBank database by January 2024 were summarized. Among the 149 identified TSPs encoded in the genomes of 143 phages, the capsular specificity (K specificity) of 46 proteins has been experimentally determined or predicted previously. The specificity of 63 TSPs toward CPSs, produced by various Acinetobacter K types, was predicted in this study using a bioinformatic analysis. A comprehensive phylogenetic analysis confirmed the prediction and revealed the possibility of the genetic exchange of gene regions corresponding to the CPS-recognizing/degrading parts of different TSPs between morphologically and taxonomically distant groups of capsule-specific Acinetobacter phages.

Functional domains of Acinetobacter bacteriophage tail fibers

A rapid increase in antimicrobial resistant bacterial infections around the world is causing a global health crisis. The Gram-negative bacterium Acinetobacter baumannii is categorized as a Priority 1 pathogen for research and development of new antimicrobials by the World Health Organization due to its numerous intrinsic antibiotic resistance mechanisms and ability to quickly acquire new resistance determinants. Specialized phage enzymes, called depolymerases, degrade the bacterial capsule polysaccharide layer and show therapeutic potential by sensitizing the bacterium to phages, select antibiotics, and serum killing. The functional domains responsible for the capsule degradation activity are often found in the tail fibers of select A. baumannii phages. To further explore the functional domains associated with depolymerase activity, tail-associated proteins of 71 sequenced and fully characterized phages were identified from published literature and analyzed for functional domains using InterProScan. Multisequence alignments and phylogenetic analyses were conducted on the domain groups and assessed in the context of noted halo formation or depolymerase characterization. Proteins derived from phages noted to have halo formation or a functional depolymerase, but no functional domain hits, were modeled with AlphaFold2 Multimer, and compared to other protein models using the DALI server. The domains associated with depolymerase function were pectin lyase-like (SSF51126), tailspike binding (cd20481), (Trans)glycosidases (SSF51445), and potentially SGNH hydrolases. These findings expand our knowledge on phage depolymerases, enabling researchers to better exploit these enzymes for therapeutic use in combating the antimicrobial resistance crisis.

Fitness Trade-Offs between Phage and Antibiotic Sensitivity in Phage-Resistant Variants: Molecular Action and Insights into Clinical Applications for Phage Therapy

In recent decades, phage therapy has been overshadowed by the widespread use of antibiotics in Western countries. However, it has been revitalized as a powerful approach due to the increasing prevalence of antimicrobial-resistant bacteria. Although bacterial resistance to phages has been reported in clinical cases, recent studies on the fitness trade-offs between phage and antibiotic resistance have revealed new avenues in the field of phage therapy. This strategy aims to restore the antibiotic susceptibility of antimicrobial-resistant bacteria, even if phage-resistant variants develop. Here, we summarize the basic virological properties of phages and their applications within the context of antimicrobial resistance. In addition, we review the occurrence of phage resistance in clinical cases, and examine fitness trade-offs between phage and antibiotic sensitivity, exploring the potential of an evolutionary fitness cost as a countermeasure against phage resistance in therapy. Finally, we discuss future strategies and directions for phage-based therapy from the aspect of fitness trade-offs. This approach is expected to provide robust options when combined with antibiotics in this era of phage 're'-discovery.

Response Surface Methodology Application for Bacteriophage-Antibiotic Antibiofilm Activity Optimization

Phage-antibiotic combination-based protocols are presently under heightened investigation. This paradigm extends to engagements with bacterial biofilms, necessitating novel computational approaches to comprehensively characterize and optimize the outcomes achievable via these combinations. This study aimed to explore the Response Surface Methodology (RSM) in optimizing the antibiofilm activity of bacteriophage-antibiotic combinations. We employ a combination of antibiotics (gentamicin, meropenem, amikacin, ceftazidime, fosfomycin, imipenem, and colistin) alongside the bacteriophage vB_AbaP_AGC01 to combat Acinetobacter baumannii biofilm. Based on the conducted biofilm challenge assays analyzed using the RSM, the optimal points of antibiofilm activity efficacy were effectively selected by applying this methodology, enabling the quantifiable mathematical representations. Subsequent optimization showed the synergistic potential of the anti-biofilm that arises when antibiotics are judiciously combined with the AGC01 bacteriophage, reducing biofilm biomass by up to 80% depending on the antibiotic used. The data suggest that the phage-imipenem combination demonstrates the highest efficacy, with an 88.74% reduction. Notably, the lower concentrations characterized by a high maximum reduction in biofilm biomass were observed in the phage-amikacin combination at cA = 0.00195 and cP = 0.38 as the option that required minimum resources. It is worth noting that only gentamicin antagonism between the phage and the antibiotic was detected.

Isolation and characterization of three novel Acinetobacter baumannii phages from Beninese hospital wastewater

Acinetobacter baumannii is an opportunistic pathogen that is mostly associated with hospital-acquired infections. The rapid emergence of multi- and pan-drug-resistant Acinetobacter strains poses an increasing challenge in hospitals. Phage therapy offers one treatment option for infections caused by A. baumannii. We isolated three phages from Beninese hospital wastewater - fBenAci001, fBenAci002, and fBenAci003 - that infected clinical A. baumannii strains from Finnish patients. Phylogenetic analysis showed that these phages resemble phages of the genus Friunavirus, family Autographiviridae. The isolated phages meet the requirements set for phages used for phage therapy. However, they were found to have a narrow host range, which may limit their therapeutic use.

Assessing Acinetobacter baumannii virulence and treatment with a bacteriophage using zebrafish embryos

Acinetobacter baumannii is the leading bacteria causative of nosocomial infections, with high fatality rates, mostly due to their multi-resistance to antibiotics. The capsular polysaccharide (k-type) is a major virulence factor. Bacteriophages are viruses that specifically infect bacteria and have been used to control drug-resistant bacterial pathogens. In particular, A. baumannii phages can recognize specific capsules, from a diversity of >125 that exist. This high specificity demands the in vivo identification of the most virulent A. baumannii k-types that need to be targeted by phage therapy. Currently, the zebrafish embryo has particularly attained interest for in vivo infection modeling. In this study, an A. baumannii infection was successfully established, through the bath immersion of tail-injured zebrafish embryos, to study the virulence of eight capsule types (K1, K2, K9, K32, K38, K44, K45, and K67). The model revealed itself as capable of discerning the most virulent (K2, K9, K32, and K45), middle (K1, K38, and K67), and the less virulent (K44) strains. Additionally, the infection of the most virulent strains was controlled in vivo resorting to the same technique, with previously identified phages (K2, K9, K32, and K45 phages). Phage treatments were able to increase the average survival from 35.2% to up to 74.1% (K32 strain). All the phages performed equally well. Collectively, the results show the potential of the model to not only evaluate virulence of bacteria such as A. baumannii but also assess novel treatments' effectiveness.

Friunavirus Phage-Encoded Depolymerases Specific to Different Capsular Types of Acinetobacter baumannii

Acinetobacter baumannii is a critical priority nosocomial pathogen that produces a variety of capsular polysaccharides (CPSs), the primary receptors for specific depolymerase-carrying phages. In this study, the tailspike depolymerases (TSDs) encoded in genomes of six novel Friunaviruses, APK09, APK14, APK16, APK86, APK127v, APK128, and one previously described Friunavirus phage, APK37.1, were characterized. For all TSDs, the mechanism of specific cleavage of corresponding A. baumannii capsular polysaccharides (CPSs) was established. The structures of oligosaccharide fragments derived from K9, K14, K16, K37/K3-v1, K86, K127, and K128 CPSs degradation by the recombinant depolymerases have been determined. The crystal structures of three of the studied TSDs were obtained. A significant reduction in mortality of Galleria mellonella larvae infected with A. baumannii of K9 capsular type was shown in the example of recombinant TSD APK09_gp48. The data obtained will provide a better understanding of the interaction of phage-bacterial host systems and will contribute to the formation of principles of rational usage of lytic phages and phage-derived enzymes as antibacterial agents.

Characterization of Virulent T4-Like Acinetobacter baumannii Bacteriophages DLP1 and DLP2

The world is currently facing a global health crisis due to the rapid increase in antimicrobial-resistant bacterial infections. One of the most concerning pathogens is Acinetobacter baumannii, which is listed as a Priority 1 pathogen by the World Health Organization. This Gram-negative bacterium has many intrinsic antibiotic resistance mechanisms and the ability to quickly acquire new resistance determinants from its environment. A limited number of effective antibiotics against this pathogen complicates the treatment of A. baumannii infections. A potential treatment option that is rapidly gaining interest is “phage therapy”, or the clinical application of bacteriophages to selectively kill bacteria. The myoviruses DLP1 and DLP2 (vB_AbaM-DLP_1 and vB_AbaM-DLP_2, respectively) were isolated from sewage samples using a capsule minus variant of A. baumannii strain AB5075. Host range analysis of these phages against 107 A. baumannii strains shows a limited host range, infecting 15 and 21 for phages DLP1 and DLP2, respectively. Phage DLP1 has a large burst size of 239 PFU/cell, a latency period of 20 min, and virulence index of 0.93. In contrast, DLP2 has a smaller burst size of 24 PFU/cell, a latency period of 20 min, and virulence index of 0.86. Both phages show potential for use as therapeutics to combat A. baumannii infections.

Acinetobacter Baumannii Phages: Past, Present and Future

Acinetobacter baumannii (A. baumannii) is one of the most common clinical pathogens and a typical multi-drug resistant (MDR) bacterium. With the increase of drug-resistant A. baumannii infections, it is urgent to find some new treatment strategies, such as phage therapy. In this paper, we described the different drug resistances of A. baumannii and some basic properties of A. baumannii phages, analyzed the interaction between phages and their hosts, and focused on A. baumannii phage therapies. Finally, we discussed the chance and challenge of phage therapy. This paper aims to provide a more comprehensive understanding of A. baumannii phages and theoretical support for the clinical application of A. baumannii phages.

Loss of a Branch Sugar in the Acinetobacter baumannii K3-Type Capsular Polysaccharide Due To Frameshifts in the gtr6 Glycosyltransferase Gene Leads To Susceptibility To Phage APK37.1

The type of capsular polysaccharide (CPS) on the cell surface of Acinetobacter baumannii can determine the specificity of lytic bacteriophage under consideration for therapeutic use. Here, we report the isolation of a phage on an extensively antibiotic resistant ST2 A. baumannii isolate AB5001 that carries the KL3 CPS biosynthesis gene cluster predicting a K3-type CPS. As the phage did not infect isolates carrying KL3 or KL22 and known to produce K3 CPS, the structure of the CPS isolated from A. baumannii AB5001 was determined. AB5001 produced a variant CPS form, K3-v1, that lacks the β-d-GlсpNAc side chain attached to the d-Galp residue in the K3 structure. Inspection of the KL3 sequence in the genomes of AB5001 and other phage-susceptible isolates with a KL3 locus revealed single-base deletions in gtr6, causing loss of the Gtr6 glycosyltransferase that adds the missing d-GlсpNAc side chain to the K3 CPS. Hence, the presence of this sugar profoundly restricts the ability of the phage to digest the CPS. The 41-kb linear double-stranded DNA (dsDNA) phage genome was identical to the genome of a phage isolated on a K37-producing isolate and thus was named APK37.1. APK37.1 also infected isolates carrying KL116. Consistent with this, K3-v1 resembles the K37 and K116 structures. APK37.1 is a Friunavirus belonging to the Autographiviridae family. The phage-encoded tail spike depolymerase DpoAPK37.1 was not closely related to Dpo encoded by other sequenced Friunaviruses, including APK37 and APK116. IMPORTANCE Lytic bacteriophage have potential for the treatment of otherwise untreatable extensively antibiotic-resistant bacteria. For Acinetobacter baumannii, most phage exhibit specificity for the type of capsular polysaccharide (CPS) produced on the cell surface. However, resistance can arise via mutations in CPS genes that abolish this phage receptor. Here, we show that single-base deletions in a CPS gene result in alteration of the final structure rather than deletion of the capsule layer and hence affect the ability of a newly reported podophage to infect strains producing the K3 CPS.

Comparative genomics of Acinetobacter baumannii and therapeutic bacteriophages from a patient undergoing phage therapy

In 2016, a 68-year-old patient with a disseminated multidrug-resistant Acinetobacter baumannii infection was successfully treated using lytic bacteriophages. Here we report the genomes of the nine phages used for treatment and three strains of A. baumannii isolated prior to and during treatment. The phages used in the initial treatment are related, T4-like myophages. Analysis of 19 A. baumannii isolates collected before and during phage treatment shows that resistance to the T4-like phages appeared two days following the start of treatment. We generate complete genomic sequences for three A. baumannii strains (TP1, TP2 and TP3) collected before and during treatment, supporting a clonal relationship. Furthermore, we use strain TP1 to select for increased resistance to five of the phages in vitro, and identify mutations that are also found in phage-insensitive isolates TP2 and TP3 (which evolved in vivo during phage treatment). These results support that in vitro investigations can produce results that are relevant to the in vivo environment.

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.

Characterization of Three Novel Virulent Aeromonas Phages Provides Insights into the Diversity of the Autographiviridae Family

In this study, we isolated and characterized three novel virulent Autographiviridae bacteriophages, vB_AspA_Bolek, vB_AspA_Lolek, and vB_AspA_Tola, which infect different Aeromonas strains. These three host-pathogen pairs were derived from the same sampling location-the arsenic-containing microbial mats of the Zloty Stok gold mine. Functional analysis showed they are psychrotolerant (4-25 °C), albeit with a much wider temperature range of propagation for the hosts (≤37 °C). Comparative genomic analyses revealed a high nucleotide and amino acid sequence similarity of vB_AspA_Bolek and vB_AspA_Lolek, with significant differences exclusively in the C-terminal region of their tail fibers, which might explain their host range discrimination. The protein-based phage network, together with a phylogenetic analysis of the marker proteins, allowed us to assign vB_AspA_Bolek and vB_AspA_Lolek to the Beijerinckvirinae and vB_AspA_Tola to the Colwellvirinae subfamilies, but as three novel species, due to their low nucleotide sequence coverage and identity with other known phage genomes. Global comparative analysis showed that the studied phages are also markedly different from most of the 24 Aeromonas autographiviruses known so far. Finally, this study provides in-depth insight into the diversity of the Autographiviridae phages and reveals genomic similarities between selected groups of this family as well as between autographiviruses and their relatives of other Caudoviricetes families.

Capsule-Targeting Depolymerases Derived from Acinetobacter baumannii Prophage Regions

In this study, several different depolymerases encoded in the prophage regions of Acinetobacter baumannii genomes have been bioinformatically predicted and recombinantly produced. The identified depolymerases possessed multi-domain structures and were identical or closely homologous to various proteins encoded in other A. baumannii genomes. This means that prophage-derived depolymerases are widespread, and different bacterial genomes can be the source of proteins with polysaccharide-degrading activities. For two depolymerases, the specificity to capsular polysaccharides (CPSs) of A. baumannii belonging to K1 and K92 capsular types (K types) was determined. The data obtained showed that the prophage-derived depolymerases were glycosidases that cleaved the A. baumannii CPSs by the hydrolytic mechanism to yield monomers and oligomers of the K units. The recombinant proteins with established enzymatic activity significantly reduced the mortality of Galleria mellonella larvae infected with A. baumannii of K1 and K92 capsular types. Therefore, these enzymes can be considered as suitable candidates for the development of new antibacterials against corresponding A. baumannii K types.

Bacteriophage Tail Proteins as a Tool for Bacterial Pathogen Recognition—A Literature Review

In recent years, a number of bacterial detection methods have been developed to replace time-consuming culture methods. One interesting approach is to mobilize the ability of phage tail proteins to recognize and bind to bacterial hosts. In this paper, the authors provide an overview of the current methodologies in which phage proteins play major roles in detecting pathogenic bacteria. Authors focus on proteins capable of recognizing highly pathogenic strains, such as Acinetobacter baumannii, Campylobacter spp., Yersinia pestis, Pseudomonas aeruginosa, Listeria monocytogenes, Staphylococcus aureus, Enterococcus spp., Salmonella spp., and Shigella. These pathogens may be diagnosed by capture-based detection methods involving the use of phage protein-coated nanoparticles, ELISA (enzyme-linked immunosorbent assay)-based methods, or biosensors. The reviewed studies show that phage proteins are becoming an important diagnostic tool due to the discovery of new phages and the increasing knowledge of understanding the specificity and functions of phage tail proteins.

Genomic Diversity of Bacteriophages Infecting the Genus Acinetobacter

The number of sequenced Acinetobacter phage genomes in the International Nucleotide Sequence Database Collaboration has increased significantly in recent years, from 37 in 2017 to a total of 139 as of January 2021 with genome sizes ranging from 31 to 378 kb. Here, we explored the genetic diversity of the Acinetobacter phages using comparative genomics approaches that included assessment of nucleotide similarity, shared gene content, single gene phylogeny, and the network-based classification tool vConTACT2. Phages infecting Acinetobacter sp. are genetically diverse and can be grouped into 8 clusters (subfamilies) and 46 sub-clusters (genera), of which 8 represent genomic singletons (additional genera). We propose the creation of five new subfamilies and suggest a reorganisation of the genus Obolenskvirus. These results provide an updated view of the viruses infecting Acinetobacter species, providing insights into their diversity.

Unpuzzling Friunavirus-Host Interactions One Piece at a Time: Phage Recognizes Acinetobacter pittii via a New K38 Capsule Depolymerase

Acinetobacter pittii is a species that belong to the Acinetobacter calcoaceticus-baumannii complex, increasingly recognized as major nosocomial bacterial pathogens, often associated with multiple drug-resistances. The capsule surrounding the bacteria represents a main virulence factor, helping cells avoid phage predation and host immunity. Accordingly, a better understanding of the phage infection mechanisms is required to efficiently develop phage therapy against Acinetobacter of different capsular types. Here, we report the isolation of the novel A. pittii-infecting Fri1-like phage vB_Api_3043-K38 (3043-K38) of the Podoviridae morphotype, from sewage samples. Its 41,580 bp linear double-stranded DNA genome harbours 53 open reading frames and 302 bp of terminal repeats. We show that all studied Acinetobacter Fri1-like viruses have highly similar genomes, which differentiate only at the genes coding for tailspike, likely to adapt to different host receptors. The isolated phage 3043-K38 specifically recognizes an untapped Acinetobacter K38 capsule type via a novel tailspike with K38 depolymerase activity. The recombinant K38 depolymerase region of the tailspike (center-end region) forms a thermostable trimer, and quickly degrades capsules. When the K38 depolymerase is applied to the cells, it makes them resistant to phage predation. Interestingly, while K38 depolymerase treatments do not synergize with antibiotics, it makes bacterial cells highly susceptible to the host serum complement. In summary, we characterized a novel phage-encoded K38 depolymerase, which not only advances our understanding of phage-host interactions, but could also be further explored as a new antibacterial agent against drug-resistant Acinetobacter.

Phage Selective Pressure Reduces Virulence of Hypervirulent Klebsiella pneumoniae Through Mutation of the wzc Gene

Hypervirulent Klebsiella pneumoniae (hvKp), one of the major community-acquired pathogens, can cause invasive infections such as liver abscess. In recent years, bacteriophages have been used in the treatment of K. pneumoniae, but the characteristics of the phage-resistant bacteria produced in the process of phage therapy need to be evaluated. In this study, two Podoviridae phages, hvKpP1 and hvKpP2, were isolated and characterized. In vitro and in vivo experiments demonstrated that the virulence of the resistant bacteria was significantly reduced compared with that of the wild type. Comparative genomic analysis of monoclonal sequencing showed that nucleotide deletion mutations of wzc and wcaJ genes led to phage resistance, and the electron microscopy and mucoviscosity results showed that mutations led to the loss of the capsule. Meanwhile, animal assay indicated that loss of capsule reduced the virulence of hvKp. These findings contribute to a better understanding of bacteriophage therapy, which not only can kill bacteria directly but also can reduce the virulence of bacteria by phage screening.

Novel Acinetobacter baumannii Bacteriophage Aristophanes Encoding Structural Polysaccharide Deacetylase

Acinetobacter baumannii appears to be one of the most crucial nosocomial pathogens. A possible component of antimicrobial therapy for infections caused by extremely drug-resistant A. baumannii strains may be specific lytic bacteriophages or phage-derived enzymes. In the present study, we observe the biological features, genomic organization, and phage–host interaction strategy of novel virulent bacteriophage Aristophanes isolated on A. baumannii strain having K26 capsular polysaccharide structure. According to phylogenetic analysis phage Aristophanes can be classified as a representative of a new distinct genus of the subfamily Beijerinckvirinae of the family Autographiviridae. This is the first reported A. baumannii phage carrying tailspike deacetylase, which caused O-acetylation of one of the K26 sugar residues.

Novel Acinetobacter baumannii Myovirus TaPaz Encoding Two Tailspike Depolymerases: Characterization and Host-Recognition Strategy

Acinetobacter baumannii, one of the most significant nosocomial pathogens, is capable of producing structurally diverse capsular polysaccharides (CPSs) which are the primary receptors for A. baumannii bacteriophages encoding polysaccharide-degrading enzymes. To date, bacterial viruses specifically infecting A. baumannii strains belonging to more than ten various capsular types (K types) were isolated and characterized. In the present study, we investigate the biological properties, genomic organization, and virus-bacterial host interaction strategy of novel myovirus TaPaz isolated on the bacterial lawn of A. baumannii strain with a K47 capsular polysaccharide structure. The phage linear double-stranded DNA genome of 93,703 bp contains 178 open reading frames. Genes encoding two different tailspike depolymerases (TSDs) were identified in the phage genome. Recombinant TSDs were purified and tested against the collection of A. baumannii strains belonging to 56 different K types. One of the TSDs was demonstrated to be a specific glycosidase that cleaves the K47 CPS by the hydrolytic mechanism.

Phage φAB6-Borne Depolymerase Combats Acinetobacter baumannii Biofilm Formation and Infection

Biofilm formation is one of the main causes of increased antibiotic resistance in Acinetobacter baumannii infections. Bacteriophages and their derivatives, such as tail proteins with depolymerase activity, have shown considerable potential as antibacterial or antivirulence agents against bacterial infections. Here, we gained insights into the activity of a capsular polysaccharide (CPS) depolymerase, derived from the tailspike protein (TSP) of φAB6 phage, to degrade A. baumannii biofilm in vitro. Recombinant TSP showed enzymatic activity and was able to significantly inhibit biofilm formation and degrade formed biofilms; as low as 0.78 ng, the inhibition zone can still be formed on the bacterial lawn. Additionally, TSP inhibited the colonization of A. baumannii on the surface of Foley catheter sections, indicating that it can be used to prevent the adhesion of A. baumannii to medical device surfaces. Transmission and scanning electron microscopy demonstrated membrane leakage of bacterial cells treated with TSP, resulting in cell death. The therapeutic effect of TSP in zebrafish was also evaluated and the results showed that the survival rate was significantly improved (80%) compared with that of the untreated control group (10%). Altogether, we show that TSP derived from φAB6 is expected to become a new antibiotic against multi-drug resistant A. baumannii and a biocontrol agent that prevents the formation of biofilms on medical devices.

Specific Interaction of Novel Friunavirus Phages Encoding Tailspike Depolymerases with Corresponding Acinetobacter baumannii Capsular Types

Acinetobacter baumannii is one of the most clinically important nosocomial pathogens. The World Health Organisation refers it to its «critical priority» category to develop new strategies for effective therapy. This microorganism is capable of producing structurally diverse capsular polysaccharides (CPSs), which serve as primary receptors for A. baumannii bacteriophages carrying polysaccharide-depolymerasing enzymes. In this study, eight novel bacterial viruses that specifically infect A. baumannii strains belonging to K2/K93, K32, K37, K44, K48, K87, K89 and K116 capsular types were isolated and characterized. The overall genomic architecture demonstrated that these viruses are representatives of the Friunavirus genus of the family Autographiviridae The linear double-stranded DNA phage genomes of 41,105-42,402 bp share high nucleotide sequence identity, except for genes encoding structural depolymerases or tailspikes which determine the host specificity. Deletion mutants lacking N-terminal domains of tailspike proteins were cloned, expressed and purified. The structurally defined CPSs of the phage bacterial hosts were cleaved with the specific recombinant depolymerases, and the resultant oligosaccharides that corresponded to monomers or/and dimers of the CPS repeats (K-units) were isolated. Structures of the derived oligosaccharides were established by nuclear magnetic resonance spectroscopy and high-resolution electrospray ionization mass spectrometry. The data obtained showed that all depolymerases studied were glycosidases that cleave specifically the A. baumannii CPSs by the hydrolytic mechanism, in most cases, by the linkage between the K-units.IMPORTANCE Acinetobacter baumannii, a nonfermentative, Gram-negative, aerobic bacterium, is one of the most significant nosocomial pathogens. The pathogenicity of A. baumannii is based on the cooperative action of many factors, one of them being the production of capsular polysaccharides (CPSs) that surround bacterial cells with a thick protective layer. Polymorphism of the chromosomal capsule loci is responsible for the observed high structural diversity of the CPSs. In this study, we describe eight novel lytic phages which have different tailspike depolymerases (TSDs) determining the interaction of the viruses with corresponding A. baumannii capsular types (K-types). Moreover, we elucidate the structures of oligosaccharide products obtained by cleavage of the CPSs by the recombinant depolymerases. We believe that as the TSDs determine phage specificity, the diversity of their structures should be taken into consideration as selection criteria for inclusion of certain phage candidate to the cocktail designed to control A. baumannii with different K-types.

Mechanisms of Acinetobacter baumannii Capsular Polysaccharide Cleavage by Phage Depolymerases

Aerobic gram-negative bacterium Acinetobacter baumannii has recently become one of the most relevant pathogens associated with hospital-acquired infections worldwide. A. baumannii produces a capsule around the cell, which represents a thick viscous layer of structurally variable capsular polysaccharide (CPS). The capsule protects the bacteria against unfavorable environmental factors and biological systems, including bacteriophages and host immune system. Many A. baumannii phages have structural depolymerases (tailspikes) that specifically recognize and digest bacterial CPS. In this work, we studied the interaction of tailspike proteins of four lytic depolymerase-carrying phages with A. baumannii CPS. Depolymerases of three bacteriophages (Fri1, AS12, and BS46) were identified as specific glycosidases that cleave the CPS of A. baumannii strains 28, 1432, and B05, respectively, by the hydrolytic mechanism. The gp54 depolymerase from bacteriophage AP22 was characterized as a polysaccharide lyase that cleaves the CPS of A. baumannii strain 1053 by β-elimination at hexuronic acid (ManNAcA) residues.

Lysin LysMK34 of Acinetobacter baumannii Bacteriophage PMK34 Has a Turgor Pressure-Dependent Intrinsic Antibacterial Activity and Reverts Colistin Resistance

The prevalence of extensively and pandrug-resistant strains of Acinetobacter baumannii leaves little or no therapeutic options for treatment for this bacterial pathogen. Bacteriophages and their lysins represent attractive alternative antibacterial strategies in this regard. We used the extensively drug-resistant A. baumannii strain MK34 to isolate the bacteriophage PMK34 (vB_AbaP_PMK34). This phage shows fast adsorption and lacks virulence genes; nonetheless, its narrow host spectrum based on capsule recognition limits broad application. PMK34 is a Fri1virus member of the Autographiviridae and has a 41.8-kb genome (50 open reading frames), encoding an endolysin (LysMK34) with potent muralytic activity (1,499.9 ± 131 U/μM), a typical mesophilic thermal stability up to 55°C, and a broad pH activity range (4 to 10). LysMK34 has an intrinsic antibacterial activity up to 4.8 and 2.4 log units for A. baumannii and Pseudomonas aeruginosa strains, respectively, but only when a high turgor pressure is present. The addition of 0.5 mM EDTA or application of an osmotic shock after treatment can compensate for the lack of a high turgor pressure. The combination of LysMK34 and colistin results in up to 32-fold reduction of the MIC of colistin, and colistin-resistant strains are resensitized in both Mueller-Hinton broth and 50% human serum. As such, LysMK34 may be used to safeguard the applicability of colistin as a last-resort antibiotic.IMPORTANCE A. baumannii is one of the most challenging pathogens for which development of new and effective antimicrobials is urgently needed. Colistin is a last-resort antibiotic, and even colistin-resistant A. baumannii strains exist. Here, we present a lysin that sensitizes A. baumannii for colistin and can revert colistin resistance to colistin susceptibility. The lysin also shows a strong, turgor pressure-dependent intrinsic antibacterial activity, providing new insights in the mode of action of lysins with intrinsic activity against Gram-negative bacteria.

Isolation and Characterization of AbTJ, an Acinetobacter baumannii Phage, and Functional Identification of Its Receptor-Binding Modules

A. baumannii is an opportunistic pathogen and a major cause of various community-acquired infections. Strains of this species can be resistant to multiple antimicrobial agents, leaving limited therapeutic options, also lacking in methods for accurate and prompt diagnosis. In this context, AbTJ, a novel phage that infects A. baumannii MDR-TJ, was isolated and characterized, together with its two tail fiber proteins. Morphological analysis revealed that it belongs to Podoviridae family. Its host range, growth characteristics, stability under various conditions, and genomic sequence, were systematically investigated. Bioinformatic analysis showed that AbTJ consists of a circular, double-stranded 42670-bp DNA molecule which contains 62 putative open reading frames (ORFs). Genome comparison revealed that the phage AbTJ is related to the Acinetobacter phage Ab105-1phi (No. KT588074). Tail fiber protein (TFPs) gp52 and gp53 were then identified and confirmed as species-specific proteins. By using a combination of bioluminescent methods and magnetic beads, these TFPs exhibit excellent specificity to detect A. baumannii. The findings of this study can be used to help control opportunistic infections and to provide pathogen-binding modules for further construction of engineered bacteria of diagnosis and treatment.

Diversity and Function of Phage Encoded Depolymerases

Bacteriophages of the Podoviridae family often exhibit so-called depolymerases as structural components of the virion. These enzymes appear as tail spike proteins (TSPs). After specific binding to capsular polysaccharides (CPS), exopolysaccharides (EPS) or lipopolysaccharide (LPS) of the host bacteria, polysaccharide-repeating units are specifically cleaved. Finally, the phage reaches the last barrier, the cell wall, injects its DNA, and infects the cell. Recently, similar enzymes from bacteriophages of the Ackermannviridae, Myoviridae, and Siphoviridae families were also described. In this mini-review the diversity and function of phage encoded CPS-, EPS-, and LPS-degrading depolymerases is summarized. The function of the enzymes is described in terms of substrate specificity and applications in biotechnology.

Characterization of myophage AM24 infecting Acinetobacter baumannii of the K9 capsular type

In the present study, we investigate the biological properties and genomic organization of virulent bacteriophage AM24, which specifically infects multidrug-resistant clinical Acinetobacter baumannii strains with a K9 capsular polysaccharide structure. The phage was identified as a member of the family Myoviridae by transmission electron microscopy. The AM24 linear double-stranded DNA genome of 97,177 bp contains 167 open reading frames. Putative functions were assigned for products of 40 predicted genes, including proteins involved in nucleotide metabolism and DNA replication, packaging of DNA into the capsid, phage assembly and structural proteins, and bacterial cell lysis. The gene encoding the tailspike, which possesses depolymerase activity towards the corresponding capsular polysaccharides, is situated in the phage genome outside of the structural module, upstream of the genes responsible for packaging of DNA into the capsid. The data on characterization of depolymerase-carrying phage AM24 contributes to our knowledge of the diversity of viruses infecting different capsular types of A. baumannii.

Functional Analysis and Antivirulence Properties of a New Depolymerase from a Myovirus That Infects Acinetobacter baumannii Capsule K45

Acinetobacter baumannii is an important pathogen causative of health care-associated infections and is able to rapidly develop resistance to all known antibiotics, including colistin. As an alternative therapeutic agent, we have isolated a novel myovirus (vB_AbaM_B9) which specifically infects and makes lysis from without in strains of the K45 and K30 capsule types, respectively. Phage B9 has a genome of 93,641 bp and encodes 167 predicted proteins, of which 29 were identified by mass spectrometry. This phage holds a capsule depolymerase (B9gp69) able to digest extracted exopolysaccharides of both K30 and K45 strains and remains active in a wide range of pH values (5 to 9), ionic strengths (0 to 500 mM), and temperatures (20 to 80°C). B9gp69 was demonstrated to be nontoxic in a cell line model of the human lung and to make the K45 strain fully susceptible to serum killing in vitro Contrary to the case with phage, no resistance development was observed by bacteria targeted with the B9gp69. Therefore, capsular depolymerases may represent attractive antimicrobial agents against A. baumannii infections.IMPORTANCE Currently, phage therapy has revived interest for controlling hard-to-treat bacterial infections. Acinetobacter baumannii is an emerging Gram-negative pathogen able to cause a variety of nosocomial infections. Additionally, this species is becoming more resistant to several classes of antibiotics. Here we describe the isolation of a novel lytic myophage B9 and its recombinant depolymerase. While the phage can be a promising alternative antibacterial agent, its success in the market will ultimately depend on new regulatory frameworks and general public acceptance. We therefore characterized the phage-encoded depolymerase, which is a natural enzyme that can be more easily managed and used. To our knowledge, the therapeutic potential of phage depolymerase against A. baumannii is still unknown. We show for the first time that the K45 capsule type is an important virulence factor of A. baumannii and that capsule removal via the recombinant depolymerase activity helps the host immune system to combat the bacterial infection.

Complete Genome Sequences of Five Acinetobacter baumannii Phages from Abidjan, Côte d'Ivoire

Five bacteriophages of Acinetobacter baumannii were isolated from sewage water in Abidjan, Côte d’Ivoire. Phages Aci01-1, Aci02-2, and Aci05 belong to an unclassified genus of the Myoviridae family, with double-stranded DNA (dsDNA) genomes, whereas Aci07 and Aci08 belong to the Fri1virus genus of the Podoviridae family of phages.

Five bacteriophages of Acinetobacter baumannii were isolated from sewage water in Abidjan, Côte d’Ivoire. Phages Aci01-1, Aci02-2, and Aci05 belong to an unclassified genus of the Myoviridae family, with double-stranded DNA (dsDNA) genomes, whereas Aci07 and Aci08 belong to the Fri1virus genus of the Podoviridae family of phages.

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.

Enhanced antibacterial effect of the novel T4-like bacteriophage KARL-1 in combination with antibiotics against multi-drug resistant Acinetobacter baumannii

The continuing rise of infections caused by multi-drug resistant bacteria has led to a renewed interest in bacteriophage therapy. Here we characterize phage vB_AbaM-KARL-1 with lytic activity against multi-drug resistant clinical isolates of Acinetobacter baumannii (AB). Besides genomic and phenotypic phage analysis, the objective of our study was to investigate the antibacterial outcome when the phage acts in concert with distinct antibiotics. KARL-1 belongs to the family of Myoviridae and is able to lyse 8 of 20 (40%) tested clinical isolates. Its double-stranded DNA genome consists of 166,560 bp encoding for 253 open reading frames. Genome wide comparison suggests that KARL-1 is a novel species within the subfamily Tevenvirinae, sharing 77% nucleotide identity (coverage 58%) with phage ZZ1. The antibacterial efficacy at various multiplicities of infection (MOI) was monitored either alone or in combination with meropenem, ciprofloxacin, and colistin. A complete clearance of liquid cultures was achieved with KARL-1 at an MOI of 10^-1 and meropenem (>128 mg/l). KARL-1 was still effective at an MOI of 10^-7, but antibacterial activity was significantly augmented with meropenem. While ciprofloxacin did generally not support phage activity, the application of KARL-1 at an MOI of 10^-7 and therapeutic doses of colistin significantly elevated bacterial suppression. Hence, KARL-1 represents a novel candidate for use against multi-drug resistant AB and the therapeutic outcome may be positively influenced by the addition of traditional antibiotics.