Experimental bacteriophage treatment of honeybees (Apis mellifera) infected with Paenibacillus larvae, the causative agent of American Foulbrood Disease

Bacteriophage resistance evolution in a honey bee pathogen

Honey bee (Apis mellifera) larvae are susceptible to the bacterial pathogen Paenibacillus larvae, which causes severe damage to bee colonies. Antibiotic treatment requires veterinary supervision in the United States, is not used in many parts of the world, perpetuates problems associated with antibiotic resistance, and can necessitate residual testing in bee products. There is interest in using bacteriophages to treat infected colonies (bacteriophage therapy) and several trials are promising. Nevertheless, the safety of using biological agents in the environment must be scrutinized. In this study we analyzed the ability of P. larvae to evolve resistance to several different bacteriophages. We found that bacteriophage resistance is rapidly developed in culture but often results in growth defects. Mutations in the bacteriophage-resistant isolates are concentrated in genes encoding potential surface receptors but are also observed in genes controlling general cellular functions, and in two cases-lysogeny. Testing one of these isolates in bee larvae, we found it to have reduced virulence compared to the parental P. larvae strain. We also found that bacteriophages are likely able to counteract resistance evolution. This work suggests that while bacteriophage-resistance may arise, its impact will likely be mitigated by reduced pathogenicity and secondary bacteriophage mutations that overcome resistance.

Genomic Analysis of 96 Paenibacillus larvae Bacteriophages Including 26 from Aotearoa, New Zealand

The bacterium Paenibacillus larvae is responsible for the devastating honey bee (Apis mellifera) disease American Foulbrood. Research into bacteriophages that infect P. larvae is growing rapidly due to increasing antibiotic resistance and restrictions on antibiotic use in beehives in some countries. In this study, we present the sequenced and annotated genomes of 26 novel P. larvae phages recently isolated in New Zealand, which brings the total number of sequenced and annotated P. larvae phages to 96. The 26 novel phages belong to the pre-existing Vegas or Harrison clusters. We performed a comprehensive genomic analysis of all 96 phage genomes, grouping them into five divergent clusters and two singletons. The majority of these phages are temperate, with the possible exception of three phages that may be lytic. All 96 of these phages encode an N-acteylmuramoyl-L-alanine amidase that serves as their lysin. The amidases are from two divergent clusters, both of which show a high degree of intra-cluster similarity. Six phages and a prophage contain the Plx1 P. larvae toxin gene, which we suggest may be mobilizable. This study expands our knowledge of P. larvae phages from around the world.

Probing the interaction of Paenibacillus larvae bacteriophage as a biological agent to control the american foulbrood disease in honeybee

American foulbrood (AFB) is a harmful honeybee disease primarily caused by Paenibacillus larvae. The study aims to isolate and identify the AFB causative agent P. larvae and their specific phages to use as a new biological method for AFB disease control. Eight apiaries were inspected for AFB infections. Symptoms of diseased brood comb, were odd brood cells with soft brown decayed brood amongst healthy brood, were identified in the field and demonstrated the prevalence of AFB in every apiary. Three P. larvae isolates were identified using traditional techniques using a 452-bp PCR amplicon specific to the bacterial 16SrRNA gene and was compared between Paenibacillus isolates. Additionally, specific phages of P. larvae strains were applied to examine their efficiency in reducing the infection rate under the apiary condition. The infection rate was reduced to approximately 94.6 to 100 % through the application of a phage mixture, as opposed to 20 to 85.7 % when each phage was administered individually or 78.6 to 88.9 % when antibiotic treatment was implemented. Histological studies on phage-treated bee larvae revealed some cells regaining normal shape, with prominent nuclei and microvilli. The gastrointestinal tract showed normal longitudinal and circular muscles, unlike bee larvae treated with bacterial strains with abnormal and destroyed tissues, as shown by the basement membrane surrounding the mid-gut epithelium. Phage techniques exhibited promise in resolving the issue of AFB in honeybees due to their ease of application, comparatively lower cost, and practicality for beekeepers in terms of laboratory preparation.

Paenibacillus larvae and their phages; a community science approach to discovery and initial testing of prophylactic phage cocktails against American Foulbrood in New Zealand

Background: American foulbrood (AFB) is a devastating disease of the European honey bee (Apis mellifera) and is found throughout the world. AFB is caused by the bacterium Paenibacillus larvae (P. larvae). Treatment with antibiotics is strictly forbidden in many regions, including New Zealand. Safe and natural prophylactic solutions to protect honey bees from AFB are needed. Bacteriophages are a well-studied alternative to antibiotics and have been shown to be effective against P. larvae in other countries. Methods: We employed a community science approach to obtaining samples from around New Zealand to discover novel bacteriophages. Standard isolation approaches were employed for both bacteria and bacteriophages. Host range testing was performed by agar overlay spot tests, and cocktail formulation and in vitro testing were performed in 96-well plate assays, followed by sub-sampling and CFU visualization on agar plates. Results: Herein, we describe the discovery and isolation of eight P. larvae bacterial isolates and 26 P. larvae bacteriophages that are novel and native to New Zealand. The phage genomes were sequenced and annotated, and their genomes were compared to extant sequenced P. larvae phage genomes. We test the host ranges of the bacteriophages and formulate cocktails to undertake in vitro testing on a set of representative bacterial strains. These results form the basis of a promising solution for protecting honey bees in New Zealand from AFB.

In Vitro Evolution to Increase the Titers of Difficult Bacteriophages: RAMP-UP Protocol

Background

Bacteriophages are becoming increasingly important in the race to find alternatives to antibiotics. Unfortunately, bacteriophages that might otherwise be useful are sometimes discarded due to low titers making them unsuitable for downstream applications.

Methods

Here, we present two distinct approaches used to experimentally evolve novel New Zealand Paenibacillus larvae bacteriophages. The first approach uses the traditional agar-overlay method, whereas the other was a 96-well plate liquid infection protocol that improved phage titers in as little as four days. We also used a mathematical model to probe the parameters and limits of the RAMP-UP approach to rapidly select mutants that improve bacteriophage titers.

Results

Both experimental approaches resulted in an increase in plaque-forming units (PFU/mL). The liquid infection approach developed here, which we call RAMP-UP for Rapid Adaptive Mutation of Phage - UP, was significantly faster and simpler, and allowed us to evolve high titer bacteriophages in as little as four days. Titers were increased from 100-100,000-fold relative to their ancestors. The resultant titers were sufficient to extract and sequence DNA from these bacteriophages. An analysis of these phage genomes is provided.

Conclusion

The RAMP-UP protocol is an effective method for experimentally evolving previously intractable bacteriophages in a high-throughput and expeditious manner.

Analysis of intact prophages in genomes of Paenibacillus larvae: An important pathogen for bees

Paenibacillus larvae is the etiological agent of American Foulbrood (AFB), a highly contagious and worldwide spread bacterial disease that affects honeybee brood. In this study, all complete P. larvae genomes available on the NCBI database were analyzed in order to detect presence of prophages using the PHASTER software. A total of 55 intact prophages were identified in 11 P. larvae genomes (5.0 ± 2.3 per genome) and were further investigated for the presence of genes encoding relevant traits related to P. larvae. A closer look at the prophage genomes revealed the presence of several putative genes such as metabolic and antimicrobial resistance genes, toxins or bacteriocins, potentially influencing host performance. Some of the coding DNA sequences (CDS) were present in all ERIC-genotypes, while others were only found in a specific genotype. While CDS encoding toxins and antitoxins such as HicB and MazE were found in prophages of all bacterial genotypes, others, from the same category, were provided by prophages particularly to ERIC I (enhancin-like toxin), ERIC II (antitoxin SocA) and ERIC V strains (subunit of Panton-Valentine leukocidin system (PVL) LukF-PV). This is the first in-depth analysis of P. larvae prophages. It provides better knowledge on their impact in the evolution of virulence and fitness of P. larvae, by discovering new features assigned by the viruses.

Frontiers in effective control of problem parasites in beekeeping

Demand for better control of certain parasites in managed western honey bees (Apis mellifera L.) remains apparent amongst beekeepers in both Europe and North America, and is of widespread public, scientific, and agricultural concern. Academically, interest from numerous fields including veterinary sciences has led to many exemplary reviews of the parasites of honey bees and the treatment options available. However, summaries of current research frontiers in treating both novel and long-known parasites of managed honey bees are lacking. This review complements the currently comprehensive body of literature summarizing the effectiveness of parasite control in managed honey bees by outlining where significant gaps in development, implementation, and uptake lie, including integration into IPM frameworks and separation of cultural, biological, and chemical controls. In particular, I distinguish where challenges in identifying appropriate controls exist in the lab compared to where we encounter hurdles in technology transfer due to regulatory, economic, or cultural contexts. I overview how exciting frontiers in honey bee parasite control research are clearly demonstrated by the abundance of recent publications on novel control approaches, but also caution that temperance must be levied on the applied end of the research engine in believing that what can be achieved in a laboratory research environment can be quickly and effectively marketed for deployment in the field.

Anti-Virulence Strategy against the Honey Bee Pathogenic Bacterium Paenibacillus larvae via Small Molecule Inhibitors of the Bacterial Toxin Plx2A

American Foulbrood, caused by Paenibacillus larvae, is the most devastating bacterial honey bee brood disease. Finding a treatment against American Foulbrood would be a huge breakthrough in the battle against the disease. Recently, small molecule inhibitors against virulence factors have been suggested as candidates for the development of anti-virulence strategies against bacterial infections. We therefore screened an in-house library of synthetic small molecules and a library of flavonoid natural products, identifying the synthetic compound M3 and two natural, plant-derived small molecules, Acacetin and Baicalein, as putative inhibitors of the recently identified P. larvae toxin Plx2A. All three inhibitors were potent in in vitro enzyme activity assays and two compounds were shown to protect insect cells against Plx2A intoxication. However, when tested in exposure bioassays with honey bee larvae, no effect on mortality could be observed for the synthetic or the plant-derived inhibitors, thus suggesting that the pathogenesis strategies of P. larvae are likely to be too complex to be disarmed in an anti-virulence strategy aimed at a single virulence factor. Our study also underscores the importance of not only testing substances in in vitro or cell culture assays, but also testing the compounds in P. larvae-infected honey bee larvae.

Detection Bioactive Metabolites of Fructobacillus fructosus Strain HI-1 Isolated from Honey Bee's Digestive Tract Against Paenibacillus larvae

American foulbrood is a devastating disease of honey bee, causing economic loss in the beekeeping industry. The disease mainly causes reduction in honey bee populations which negatively affect the honey bee's major role as natural pollinators of significant crops and wildflowers. Thus, it is crucial to develop safe efficient strategies to control the disease and to improve bee colony health. Using lactic acid bacteria (LAB) as an alternative to chemical treatments is a promising novel technique for tackling honey bee diseases and improving their immunity. The endogenous LAB isolates were recovered from honey bee gut samples collected from different apiaries in two Egyptian governorates and screened for antagonistic activities against Paenibacillus larvae (pathogen of AFB disease). The results showed that 53.3% of tested LAB isolates (n = 120) exhibited antagonistic activities against P. larvae. The minimum inhibitory concentration and minimum bactericidal concentration of the most potent LAB isolate (with an inhibition zone of 44 mm) were 100 and 125 µL/mL, respectively. 16S rRNA sequencing identified the most potent isolate as Fructobacillus fructosus HI-1. The bioactive metabolites of F. fructosus were extracted with ethyl acetate and fractionated on thin-layer chromatography (TLC); also, bioactive fractions were detected. Heptyl 2-methylbutyrate, di-isobutyl phthalate, D-turanose, heptakis (trimethylsilyl), di-isooctyl phthalate, and hyodeoxycholic acid compounds were identified in the bioactive fractions. The result explores the promising administration of probiotic metabolites to control honey bee AFB disease, as a natural tool to substitute antibiotics and chemicals in disease-controlling strategies.

Isolation and Characterization of Phages Active against Paenibacillus larvae Causing American Foulbrood in Honeybees in Poland

The aim of this study was the isolation and characterization, including the phage effect on honeybees in laboratory conditions, of phages active against Paenibacillus larvae, the causative agent of American Foulbrood-a highly infective and easily spreading disease occurring in honeybee larva, and subsequently the development of a preparation to prevent and treat this dangerous disease. From the tested material (over 2500 samples) 35 Paenibacillus spp. strains were obtained and used to search for phages. Five phages specific to Paenibacillus were isolated and characterized (ultrastructure, morphology, biological properties, storage stability, and genome sequence). The characteristics were performed to obtain knowledge of their lytic potential and compose the final phage cocktail with high antibacterial potential and intended use of future field application. Preliminary safety studies have also been carried out on healthy bees, which suggest that the phage preparation administered is harmless.

Characterization of CRISPR Spacer and Protospacer Sequences in Paenibacillus larvae and Its Bacteriophages

The bacterium Paenibacillus larvae is the causative agent of American foulbrood, the most devastating bacterial disease of honeybees. Because P. larvae is antibiotic resistant, phages that infect it are currently used as alternative treatments. However, the acquisition by P. larvae of CRISPR spacer sequences from the phages could be an obstacle to treatment efforts. We searched nine complete genomes of P. larvae strains and identified 714 CRISPR spacer sequences, of which 384 are unique. Of the four epidemiologically important P. larvae strains, three of these have fewer than 20 spacers, while one strain has over 150 spacers. Of the 384 unique spacers, 18 are found as protospacers in the genomes of 49 currently sequenced P. larvae phages. One P. larvae strain does not have any protospacers found in phages, while another has eight. Protospacer distribution in the phages is uneven, with two phages having up to four protospacers, while a third of phages have none. Some phages lack protospacers found in closely related phages due to point mutations, indicating a possible escape mechanism. This study serve a point of reference for future studies on the CRISPR-Cas system in P. larvae as well as for comparative studies of other phage–host systems.

Paenibacillus larvae bacteriophages: obscure past, promising future

Paenibacillus larvae is a Gram-positive, spore-forming bacterium that is the causative agent of American foulbrood (AFB), the most devastating bacterial disease of the honeybee. P. larvae is antibiotic resistant, complicating treatment efforts. Bacteriophages that target P. larvae are rapidly emerging as a promising treatment. The first P. larvae phages were isolated in the 1950s, but as P. larvae was not antibiotic resistant at the time, interest in them remained scant. Interest in P. larvae phages has grown rapidly since the first P. larvae phage genome was sequenced in 2013. Since then, the number of sequenced P. larvae phage genomes has reached 48 and is set to grow further. All sequenced P. larvae phages encode a conserved N-acetylmuramoyl-l-alanine amidase that is responsible for cleaving the peptidoglycan cell wall of P. larvae. All P. larvae phages also encode either an integrase, excisionase or Cro/CI, indicating that they are temperate. In the last few years, several studies have been published on using P. larvae phages and the P. larvae phage amidase as treatments for AFB. Studies were conducted on infected larvae in vitro and also on hives in the field. The phages have a prophylactic effect, preventing infection, and also a curative effect, helping resolve infection. P. larvae phages have a narrow range, lysing only P. larvae, and are unable to lyse even related Paenibacillus species. P. larvae phages thus appear to be safe to use and effective as treatment for AFB, and interest in them in the coming years will continue to grow.

Phages in Therapy and Prophylaxis of American Foulbrood - Recent Implications From Practical Applications

American foulbrood is one of the most serious and yet unsolved problems of beekeeping around the world, because it causes a disease leading to the weakening of the vitality of honey bee populations and huge economic losses both in agriculture and horticulture. The etiological agent of this dangerous disease is an extremely pathogenic spore-forming bacterium, Paenibacillus larvae, which makes treatment very difficult. What is more, the use of antibiotics in the European Union is forbidden due to restrictions related to the prevention of the presence of antibiotic residues in honey, as well as the global problem of spreading antibiotic resistance in case of bacterial strains. The only available solution is burning of entire bee colonies, which results in large economic losses. Therefore, bacteriophages and their lytic enzymes can be a real effective alternative in the treatment and prevention of this Apis mellifera disease. In this review, we summarize phage characteristics that make them a potentially useful tool in the fight against American foulbrood. In addition, we gathered data regarding phage application that have been described so far, and attempted to show practical implications and possible limitations of their usage.

Characterization of a new podovirus infecting Paenibacillus larvae

The Paenibacillus larvae infecting phage API480 (vB_PlaP_API480) is the first reported podovirus for this bacterial species, with an 58 nm icosahedral capsid and a 12 × 8 nm short, non-contractile tail. API480 encodes 77 coding sequences (CDSs) on its 45,026 bp dsDNA genome, of which 47 were confirmed using mass spectrometry. This phage has got very limited genomic and proteomic similarity to any other known ones registered in public databases, including P. larvae phages. Comparative genomics indicates API480 is a new species as it's a singleton with 28 unique proteins. Interestingly, the lysis module is highly conserved among P. larvae phages, containing a predicted endolysin and two putative holins. The well kept overall genomic organisation (from the structural and morphogenetic modules to the host lysis, DNA replication and metabolism related proteins) confirms a common evolutionary ancestor among P. larvae infecting phages. API480 is able to infect 69% of the 61 field strains with an ERIC I genotype, as well as ERIC II strains. Furthermore, this phage is very stable when exposed to high glucose concentrations and to larval gastrointestinal conditions. This highly-specific phage, with its broad lytic activity and stability in hive conditions, might potentially be used in the biocontrol of American Foulbrood (AFB).

Bacteriophage biodistribution and infectivity from honeybee to bee larvae using a T7 phage model

Bacteriophages (phages) or viruses that specifically infect bacteria have widely been studied as biocontrol agents against animal and plant bacterial diseases. They offer many advantages compared to antibiotics. The American Foulbrood (AFB) is a bacterial disease affecting honeybee larvae caused by Paenibacillus larvae. Phages can be very significant in fighting it mostly due to European restrictions to the use of antibiotics in beekeeping. New phages able to control P. larvae in hives have already been reported with satisfactory results. However, the efficacy and feasibility of administering phages indirectly to larvae through their adult workers only by providing phages in bees’ feeders has never been evaluated. This strategy is considered herein the most feasible as far as hive management is concerned. This in vivo study investigated the ability of a phage to reach larvae in an infective state after oral administration to honeybees. The screening (by direct PFU count) and quantification (by quantitative PCR) of the phage in bee organs and in larvae after ingestion allowed us to conclude that despite 10⁴ phages reaching larvae only an average of 32 were available to control the spread of the disease. The fast inactivation of many phages in royal jelly could compromise this therapeutic approach. The protection of phages from hive-derived conditions should be thus considered in further developments for AFB treatment.

Bystander Phage Therapy: Inducing Host-Associated Bacteria to Produce Antimicrobial Toxins against the Pathogen Using Phages

Brevibacillus laterosporus is often present in beehives, including presence in hives infected with the causative agent of American Foulbrood (AFB), Paenibacillus larvae. In this work, 12 B. laterosporus bacteriophages induced bactericidal products in their host. Results demonstrate that P. larvae is susceptible to antimicrobials induced from field isolates of the bystander, B. laterosporus. Bystander antimicrobial activity was specific against the pathogen and not other bacterial species, indicating that the production was likely due to natural competition between the two bacteria. Three B. laterosporus phages were combined in a cocktail to treat AFB. Healthy hives treated with B. laterosporus phages experienced no difference in brood generation compared to control hives over 8 weeks. Phage presence in bee larvae after treatment rose to 60.8 ± 3.6% and dropped to 0 ± 0.8% after 72 h. In infected hives the recovery rate was 75% when treated, however AFB spores were not susceptible to the antimicrobials as evidenced by recurrence of AFB. We posit that the effectiveness of this treatment is due to the production of the bactericidal products of B. laterosporus when infected with phages resulting in bystander-killing of P. larvae. Bystander phage therapy may provide a new avenue for antibacterial production and treatment of disease.

Genomic Analysis of 48 Paenibacillus larvae Bacteriophages

The antibiotic-resistant bacterium Paenibacillus larvae is the causative agent of American foulbrood (AFB), currently the most destructive bacterial disease in honeybees. Phages that infect P. larvae were isolated as early as the 1950s, but it is only in recent years that P. larvae phage genomes have been sequenced and annotated. In this study we analyze the genomes of all 48 currently sequenced P. larvae phage genomes and classify them into four clusters and a singleton. The majority of P. larvae phage genomes are in the 38⁻45 kbp range and use the cohesive ends (cos) DNA-packaging strategy, while a minority have genomes in the 50⁻55 kbp range that use the direct terminal repeat (DTR) DNA-packaging strategy. The DTR phages form a distinct cluster, while the cos phages form three clusters and a singleton. Putative functions were identified for about half of all phage proteins. Structural and assembly proteins are located at the front of the genome and tend to be conserved within clusters, whereas regulatory and replication proteins are located in the middle and rear of the genome and are not conserved, even within clusters. All P. larvae phage genomes contain a conserved N-acetylmuramoyl-l-alanine amidase that serves as an endolysin.

Bacteriophages as an alternative to conventional antibiotic use for the prevention or treatment of Paenibacillus larvae in honeybee hives

American Foulbrood (AFB) is an infectious disease caused by the bacteria, Paenibacillus larvae. P. larvae phages were isolated and tested to determine each phages' host range amongst 59 field isolate strains of P. larvae. Three phages were selected to create a phage cocktail for the treatment of AFB infections according to the combined phages' ability to lyse all tested strains of bacteria. Studies were performed to demonstrate the safety and efficacy of the phage cocktail treatment as a replacement for traditional antibiotics for the prevention of AFB and the treatment of active infections. Safety verification studies confirmed that the phage cocktail did not adversely affect the rate of bee death even when administered as an overdose. In a comparative study of healthy hives, traditional prophylactic antibiotic treatment experienced a 38±0.7% decrease in overall hive health, which was statistically lower than hive health observed in control hives. Hives treated with phage cocktail decreased 19±0.8%, which was not statistically different than control hives, which decreased by 10±1.0%. In a study of beehives at-risk for a natural infection, 100±0.5% of phage-treated hives were protected from AFB infection, while 80±0.5% of untreated controls became infected. AFB infected hives began with an average Hitchcock score of 2.25 out of 4 and 100±0.5% of the hives recovered completely within two weeks of treatment with phage cocktail. While the n numbers for the latter two studies are small, the results for both the phage protection rate and the phage cure rate were statistically significant (α=0.05). These studies demonstrate the powerful potential of using a phage cocktail against AFB and establish phage therapy as a feasible treatment.

Bacteriophage Clinical Use as Antibacterial "Drugs": Utility and Precedent

For phage therapy-the treatment of bacterial infections using bacterial viruses-a key issue is the conflict between apparent ease of clinical application, on the one hand, and on the other hand, numerous difficulties that can be associated with undertaking preclinical development. These conflicts between achieving efficacy in the real world versus rigorously understanding that efficacy should not be surprising because equivalent conflicts have been observed in applied biology for millennia: exploiting the inherent, holistic tendencies of useful systems, e.g., of dairy cows, inevitably is easier than modeling those systems or maintaining effectiveness while reducing such systems to isolated parts. Trial and error alone, in other words, can be a powerful means toward technological development. Undertaking trial and error-based programs, especially in the clinic, nonetheless is highly dependent on those technologies possessing both inherent safety and intrinsic tendencies toward effectiveness, but in this modern era we tend to forget that ideally there would exist antibacterials which could be thus developed, that is, with tendencies toward both safety and effectiveness, and which are even relatively inexpensive. Consequently, we tend to demand rigor as well as expense of development even to the point of potentially squandering such utility, were it to exist. In this review I lay out evidence that in phage therapy such potential, in fact, does exist. Advancement of phage therapy unquestionably requires effective regulation as well as rigorous demonstration of efficacy, but after nearly 100 years of clinical practice, perhaps not as much emphasis on strictly laboratory-based proof of principle.

Comparative genomics of 9 novel Paenibacillus larvae bacteriophages

American Foulbrood Disease, caused by the bacterium Paenibacillus larvae, is one of the most destructive diseases of the honeybee, Apis mellifera. Our group recently published the sequences of 9 new phages with the ability to infect and lyse P. larvae. Here, we characterize the genomes of these P. larvae phages, compare them to each other and to other sequenced P. larvae phages, and putatively identify protein function. The phage genomes are 38-45 kb in size and contain 68-86 genes, most of which appear to be unique to P. larvae phages. We classify P. larvae phages into 2 main clusters and one singleton based on nucleotide sequence identity. Three of the new phages show sequence similarity to other sequenced P. larvae phages, while the remaining 6 do not. We identified functions for roughly half of the P. larvae phage proteins, including structural, assembly, host lysis, DNA replication/metabolism, regulatory, and host-related functions. Structural and assembly proteins are highly conserved among our phages and are located at the start of the genome. DNA replication/metabolism, regulatory, and host-related proteins are located in the middle and end of the genome, and are not conserved, with many of these genes found in some of our phages but not others. All nine phages code for a conserved N-acetylmuramoyl-L-alanine amidase. Comparative analysis showed the phages use the "cohesive ends with 3' overhang" DNA packaging strategy. This work is the first in-depth study of P. larvae phage genomics, and serves as a marker for future work in this area.