Paenibacillus larvae chitin-degrading protein PlCBP49 is a key virulence factor in American Foulbrood of honey bees

Active roles of lytic polysaccharide monooxygenases in human pathogenicity

Lytic polysaccharide monooxygenases (LPMOs) are redox enzymes widely studied for their involvement in microbial and fungal biomass degradation. The catalytic versatility of these enzymes is demonstrated by the recent discovery of LPMOs in arthropods, viruses, insects and ferns, where they fulfill diverse functions beyond biomass conversion. This mini-review puts a spotlight on a recently recognized aspect of LPMOs: their role in infectious processes in human pathogens. It discusses the occurrence and potential biological mechanisms of LPMOs associated with human pathogens and provides an outlook on future avenues in this emerging and exciting research field.

Aspartyl protease in the secretome of honey bee trypanosomatid parasite contributes to infection of bees

BACKGROUND

The exoproteome, which consists of both secreted proteins and those originating from cell surfaces and lysed cells, is a critical component of trypanosomatid parasites, facilitating interactions with host cells and gut microbiota. However, its specific roles in the insect hosts of these parasites remain poorly understood.

METHODS

We conducted a comprehensive characterization of the exoproteome in Lotmaria passim, a trypanosomatid parasite infecting honey bees, under culture conditions. We further investigated the functions of two conventionally secreted proteins, aspartyl protease (LpAsp) and chitinase (LpCht), as representative models to elucidate the role of the secretome in L. passim infection of honey bees.

RESULTS

Approximately 48% of L. passim exoproteome proteins were found to share homologs with those found in seven Leishmania spp., suggesting the existence of a core exoproteome with conserved functions in the Leishmaniinae lineage. Bioinformatics analyses suggested that the L. passim exoproteome may play a pivotal role in interactions with both the host and its microbiota. Notably, the deletion of genes encoding two secretome proteins revealed the important role of LpAsp, but not LpCht, in L. passim development under culture conditions and its efficiency in infecting the honey bee gut.

CONCLUSIONS

Our results highlight the exoproteome as a valuable resource for unraveling the mechanisms employed by trypanosomatid parasites to infect insect hosts by interacting with the gut environment.

The Ustilago maydis AA10 LPMO is active on fungal cell wall chitin

Lytic polysaccharide monooxygenases (LPMOs) can perform oxidative cleavage of glycosidic bonds in carbohydrate polymers (e.g., cellulose, chitin), making them more accessible to hydrolytic enzymes. While most studies have so far mainly explored the role of LPMOs in a (plant) biomass conversion context, alternative roles and paradigms begin to emerge. The AA10 LPMOs are active on chitin and/or cellulose and mostly found in bacteria and in some viruses and archaea. Interestingly, AA10-encoding genes are also encountered in some pathogenic fungi of the Ustilaginomycetes class, such as Ustilago maydis, responsible for corn smut disease. Transcriptomic studies have shown the overexpression of the AA10 gene during the infectious cycle of U. maydis. In fact, U. maydis has a unique AA10 gene that codes for a catalytic domain appended with a C-terminal disordered region. To date, there is no public report on fungal AA10 LPMOs. In this study, we successfully produced the catalytic domain of this LPMO (UmAA10_cd) in Pichia pastoris and carried out its biochemical characterization. Our results show that UmAA10_cd oxidatively cleaves α- and β-chitin with C1 regioselectivity and boosts chitin hydrolysis by a GH18 chitinase from U. maydis (UmGH18A). Using a biologically relevant substrate, we show that UmAA10_cd exhibits enzymatic activity on U. maydis fungal cell wall chitin and promotes its hydrolysis by UmGH18A. These results represent an important step toward the understanding of the role of LPMOs in the fungal cell wall remodeling process during the fungal life cycle.IMPORTANCELytic polysaccharide monooxygenases (LPMOs) have been mainly studied in a biotechnological context for the efficient degradation of recalcitrant polysaccharides. Only recently, alternative roles and paradigms begin to emerge. In this study, we provide evidence that the AA10 LPMO from the phytopathogen Ustilago maydis is active against fungal cell wall chitin. Given that chitin-active LPMOs are commonly found in microbes, it is important to consider fungal cell wall as a potential target for this enigmatic class of enzymes.

Lytic Polysaccharide Monooxygenase Activity of Tma12 Is Critical for Its Toxicity to Whitefly

Lytic polysaccharide monooxygenases (LPMOs) are powerful redox enzymes that transform complex carbohydrates through oxidation and make them suitable for saccharification by canonical hydrolases. Due to this property, LPMOs are considered to be a valuable component of enzymatic consortia for industrial biorefineries. Tma12 is a fern entomotoxic protein that kills whitefly and has structural similarities with chitinolytic LPMO. However, its enzymatic activity is poorly understood. Studying the role of the LPMO-like activity in the insecticidal function of Tma12 can be of considerable importance. Our results show that Tma12 preferentially binds and digests β-chitin. Liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis shows that the digestion of chitin produces chitin oligosaccharides of various lengths (DP2-DP7). The Michaelis constant (km) and catalytic constant (kcat) for hydrocoerulignone are 0.022 mM and 0.044 s-1, respectively. The attenuation of catalytic activity through diethylpyrocarbonate modification abolishes the insecticidal activity of the protein. Our findings reveal that (a) Tma12 is an active LPMO and (b) LPMO activity is indispensable for its function as a bioinsecticide.

Atypical Melissococcus plutonius strains: their characteristics, virulence, epidemiology, and mysteries

Melissococcus plutonius is a Gram-positive lanceolate coccus that is the causative agent of European foulbrood, an important bacterial disease of honey bee brood. Although this bacterium was originally described in the early 20th century, a culture method for this bacterium was not established until more than 40 years after its discovery due to its fastidious characteristics, including the requirement for high potassium and anaerobic/microaerophilic conditions. These characteristics were considered to be common to the majority of M. plutonius strains isolated worldwide, and M. plutonius was also thought to be genetically homologous or clonal for years. However, non-fastidious variants of this species (designated as atypical M. plutonius) were very recently identified in Japan. Although the morphology of these unusual strains was similar to that of traditionally well-known M. plutonius strains, atypical strains were genetically very different from most of the M. plutonius strains previously isolated and were highly virulent to individual bee larva. These atypical variants were initially considered to be unique to Japan, but were subsequently found worldwide; however, the frequency of isolation varied from country to country. The background of the discovery of atypical M. plutonius in Japan and current knowledge on atypical strains, including their biochemical and culture characteristics, virulence, detection methods, and global distribution, are described in this review. Remaining mysteries related to atypical M. plutonius and directions for future research are also discussed.

Proteomic insight into the interaction of Paenibacillus larvae with honey bee larvae before capping collected from an American foulbrood outbreak: Pathogen proteins within the host, lysis signatures and interaction markers

American foulbrood (AFB) is a devastating disease of honey bees. There remains a gap in the understanding of the interactions between the causative agent and host, so we used shotgun proteomics to gain new insights. Nano-LC-MS/MS analysis preceded visual description and Paenibacillus larvae identification in the same individual sample. A further critical part of our methodology was that larvae before capping were used as the model stage. The identification of the virulence factors SplA, PlCBP49, enolase, and DnaK in all P. larvae-positive samples was consistent with previous studies. Furthermore, the results were consistent with the array of virulence factors identified in an in vitro study of P. larvae exoprotein fractions. Although an S-layer protein and a putative bacteriocin were highlighted as important, the microbial collagenase ColA and InhA were not found in our samples. The most important virulence factor identified was isoform of neutral metalloproteinase (UniProt: V9WB82), a major protein marker responsible for the shift in the PCA biplot. This protein is associated with larval decay and together with other virulence factors (bacteriocin) can play a key role in protection against secondary invaders. Overall, this study provides new knowledge on host-pathogen interactions and a new methodical approach to study the disease.

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.

AA15 lytic polysaccharide monooxygenase is required for efficient chitinous cuticle turnover during insect molting

Microbial lytic polysaccharide monooxygenases (LPMOs) catalyze the oxidative cleavage of crystalline polysaccharides including chitin and cellulose. The discovery of a large assortment of LPMO-like proteins widely distributed in insect genomes suggests that they could be involved in assisting chitin degradation in the exoskeleton, tracheae and peritrophic matrix during development. However, the physiological functions of insect LPMO-like proteins are still undetermined. To investigate the functions of insect LPMO15 subgroup I-like proteins (LPMO15-1s), two evolutionarily distant species, Tribolium castaneum and Locusta migratoria, were chosen. Depletion by RNAi of T. castaneum TcLPMO15-1 caused molting arrest at all developmental stages, whereas depletion of the L. migratoria LmLPMO15-1, prevented only adult eclosion. In both species, LPMO15-1-deficient animals were unable to shed their exuviae and died. TEM analysis revealed failure of turnover of the chitinous cuticle, which is critical for completion of molting. Purified recombinant LPMO15-1-like protein from Ostrinia furnacalis (rOfLPMO15-1) exhibited oxidative cleavage activity and substrate preference for chitin. These results reveal the physiological importance of catalytically active LPMO15-1-like proteins from distant insect species and provide new insight into the enzymatic mechanism of cuticular chitin turnover during molting.

Molecular basis of antibiotic self-resistance in a bee larvae pathogen

Paenibacillus larvae, the causative agent of the devastating honey-bee disease American Foulbrood, produces the cationic polyketide-peptide hybrid paenilamicin that displays antibacterial and antifungal activity. Its biosynthetic gene cluster contains a gene coding for the N-acetyltransferase PamZ. We show that PamZ acts as self-resistance factor in Paenibacillus larvae by deactivation of paenilamicin. Using tandem mass spectrometry, nuclear magnetic resonance spectroscopy and synthetic diastereomers, we identified the N-terminal amino group of the agmatinamic acid as the N-acetylation site. These findings highlight the pharmacophore region of paenilamicin, which we very recently identified as a ribosome inhibitor. Here, we further determined the crystal structure of PamZ:acetyl-CoA complex at 1.34 Å resolution. An unusual tandem-domain architecture provides a well-defined substrate-binding groove decorated with negatively-charged residues to specifically attract the cationic paenilamicin. Our results will help to understand the mode of action of paenilamicin and its role in pathogenicity of Paenibacillus larvae to fight American Foulbrood.

Functional Properties and Antimicrobial Activity from Lactic Acid Bacteria as Resources to Improve the Health and Welfare of Honey Bees

Simple Summary

Honey bees play a pivotal role in the sustainability of ecosystems and biodiversity. Many factors including parasites, pathogens, pesticide residues, forage losses, and poor nutrition have been proposed to explain honey bee colony losses. Lactic acid bacteria (LAB) are normal inhabitants of the gastrointestinal tract of honey bees and their role has been consistently reported in the literature. In recent years, there have been numerous scientific evidence that the intestinal microbiota plays an essential role in honey bee health. Management strategies, based on supplementation of the gut microbiota with probiotics, may be important to increase stress tolerance and disease resistance. In this review, recent scientific advances on the use of LABs as microbial supplements in the diet of honey bees are summarized and discussed.

Abstract

Honey bees (Apis mellifera) are agriculturally important pollinators. Over the past decades, significant losses of wild and domestic bees have been reported in many parts of the world. Several biotic and abiotic factors, such as change in land use over time, intensive land management, use of pesticides, climate change, beekeeper’s management practices, lack of forage (nectar and pollen), and infection by parasites and pathogens, negatively affect the honey bee’s well-being and survival. The gut microbiota is important for honey bee growth and development, immune function, protection against pathogen invasion; moreover, a well-balanced microbiota is fundamental to support honey bee health and vigor. In fact, the structure of the bee’s intestinal bacterial community can become an indicator of the honey bee’s health status. Lactic acid bacteria are normal inhabitants of the gastrointestinal tract of many insects, and their presence in the honey bee intestinal tract has been consistently reported in the literature. In the first section of this review, recent scientific advances in the use of LABs as probiotic supplements in the diet of honey bees are summarized and discussed. The second section discusses some of the mechanisms by which LABs carry out their antimicrobial activity against pathogens. Afterward, individual paragraphs are dedicated to Chalkbrood, American foulbrood, European foulbrood, Nosemosis, and Varroosis as well as to the potentiality of LABs for their biological control.

On the expansion of biological functions of lytic polysaccharide monooxygenases

Lytic polysaccharide monooxygenases (LPMOs) constitute an enigmatic class of enzymes, the discovery of which has opened up a new arena of riveting research. LPMOs can oxidatively cleave the glycosidic bonds found in carbohydrate polymers enabling the depolymerisation of recalcitrant biomasses, such as cellulose or chitin. While most studies have so far mainly explored the role of LPMOs in a (plant) biomass conversion context, alternative roles and paradigms begin to emerge. In the present review, we propose a historical perspective of LPMO research providing a succinct overview of the major achievements of LPMO research over the past decade. This journey through LPMOs landscape leads us to dive into the emerging biological functions of LPMOs and LPMO-like proteins. We notably highlight roles in fungal and oomycete plant pathogenesis (e.g. potato late blight), but also in mutualistic/commensalism symbiosis (e.g. ectomycorrhizae). We further present the potential importance of LPMOs in other microbial pathogenesis including diseases caused by bacteria (e.g. pneumonia), fungi (e.g. human meningitis), oomycetes and viruses (e.g. entomopox), as well as in (micro)organism development (including several plant pests). Our assessment of the literature leads to the formulation of outstanding questions, promising for the coming years exciting research and discoveries on these moonlighting proteins.

Isolation, characterization, and comparative genomic analysis of vB_PlaP_SV21, new bacteriophage of Paenibacillus larvae

Paenibacillus larvae cause an American foulbrood disease (AFB) that is responsible for the extinction of honeybee colonies and is a honeybee bacterial disease that has to be obligatory notified worldwide. Recently, bacteriophage studies targeting Paenibacillus larvae have emerged as a promising alternative treatment method. The inability of bacteria to create resistance against bacteriophages makes this method advantageous. As a consequence, this study was conducted to describe the genome and biological characteristics of a novel phage capable of lysing Paenibacillus larvae samples isolated from honeybee larva samples in Turkey. The Paenibacillus phage SV21 (vB_PlaP_SV21) was isolated by inducing Paenibacillus larvae strain SV21 with Mitomycin-C. Whole-genome sequencing, comparative genomics, and phylogenetic analysis of vB_PlaP_SV21 were performed. Transmission electron microscopy images showed that vB_PlaP_SV21 phage was a Podovirus morphology. The vB_PlaP_SV21 phage specific for Paenibacillus larvae was determined to belong to the Podoviridae family. Host range and specificity, burst size, lytic activity, and morphological characteristics of the phage were determined. Bioinformatic analysis of the Paenibacillus phage SV21 showed 77 coding sequences in its linear 44,949 bp dsDNA genome with a GC content of 39.33%. In this study, we analysed the genomes of all of the currently sequenced P. larvae phage genomes and classified them into five clusters and a singleton. According to molecular, morphological, and bioinformatics results, ıt was observed that API480 (podovirus), which was reported as a singleton in previous studies and public databases, and Paenibacillus phage SV21 phage could form a new cluster together.

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.

Effects of Insecticides and Microbiological Contaminants on Apis mellifera Health

Over the past two decades, there has been an alarming decline in the number of honey bee colonies. This phenomenon is called Colony Collapse Disorder (CCD). Bee products play a significant role in human life and have a huge impact on agriculture, therefore bees are an economically important species. Honey has found its healing application in various sectors of human life, as well as other bee products such as royal jelly, propolis, and bee pollen. There are many putative factors of CCD, such as air pollution, GMO, viruses, or predators (such as wasps and hornets). It is, however, believed that pesticides and microorganisms play a huge role in the mass extinction of bee colonies. Insecticides are chemicals that are dangerous to both humans and the environment. They can cause enormous damage to bees' nervous system and permanently weaken their immune system, making them vulnerable to other factors. Some of the insecticides that negatively affect bees are, for example, neonicotinoids, coumaphos, and chlorpyrifos. Microorganisms can cause various diseases in bees, weakening the health of the colony and often resulting in its extinction. Infection with microorganisms may result in the need to dispose of the entire hive to prevent the spread of pathogens to other hives. Many aspects of the impact of pesticides and microorganisms on bees are still unclear. The need to deepen knowledge in this matter is crucial, bearing in mind how important these animals are for human life.

Comparative transcriptome analysis reveals a potential mechanism for host nutritional manipulation after parasitization by Leptopilina boulardi

Parasitoids have been extensively found to manipulate nutrient amounts of their hosts to benefit their own development and survival, but the underlying mechanisms are largely unknown. Leptopilina boulardi (Hymenoptera: Figitidae) is a larval-pupal endoparasitoid wasp of Drosophila melanogaster whose survival relies on the nutrients provided by its Drosophila host. Here, we used RNA-seq to compare the gene expression levels of the host midgut at 24 h and 48 h post L. boulardi parasitization. We obtained 95 and 191 differentially expressed genes (DEGs) in the parasitized host midgut at 24 h and 48 h post L. boulardi parasitization, respectively. A KEGG analysis revealed that several metabolic pathways were significantly enriched in the upregulated DEGs, and these pathways included "starch and sucrose metabolism" and "galactose metabolism". A functional annotation analysis showed that four classes of genes involved in carbohydrate digestion process had increased expression levels in the midgut post L.boulardi parasitization than nonparasitized groups: glucosidase, mannosidase, chitinase and amylase. Genes involved in protein digestion process were also found among the DEGs, and most of these genes, which belonged to the metallopeptidase and serine-type endopeptidase families, were found at higher expression levels in the parasitized host midgut comparing with nonparasitized hosts. Moreover, some immune genes, particularly those involved in the Toll and Imd pathways, also exhibited high expression levels after L.boulardi parasitization. Our study provides large-scale transcriptome data and identifies sets of DEGs between parasitized and nonparasitized host midgut tissues at 24 h and 48 h post L. boulardi parasitization. These resources help improve our understanding of how parasitoid infection affects the nutrient components in the hosts.

Peritrophic matrix-degrading proteins are dispensable virulence factors in a virulent Melissococcus plutonius strain

European foulbrood (EFB) caused by Melissococcus plutonius is a major bacterial disease of honey bees. Strains of the causative agent exhibit genetic heterogeneity, and the degree of virulence varies among strains. In bee larvae orally infected with the highly virulent strains, ingested bacterial cells colonize the larval midgut and proliferate within the sac of the peritrophic matrix (PM), a barrier lining the midgut epithelium. However, the barrier is degraded during the course of infection, and M. plutonius cells eventually directly interact with the midgut epithelium. As M. plutonius possesses genes encoding putative PM-degrading proteins (enhancin, a chitin-binding domain-containing protein and endo-α-N-acetylgalactosaminidase), we constructed PM-degrading protein gene-knockout mutants from a highly virulent M. plutonius strain and investigated their role in the pathogenesis of EFB. In larvae infected with the triple-knockout mutant, which has no PM-degrading protein genes, M. plutonius that proliferated in the larval midguts was confined to the sac of the PM. However, the midgut epithelial cells degenerated over time, and the mutant killed approximately 70-80% of bee brood, suggesting that although the PM-degrading proteins are involved in the penetration of the PM by M. plutonius, they are not indispensable virulence factors in the highly virulent M. plutonius strain.

The Buzz about ADP-Ribosylation Toxins from Paenibacillus larvae, the Causative Agent of American Foulbrood in Honey Bees

The Gram-positive, spore-forming bacterium Paenibacillus larvae is the etiological agent of American Foulbrood, a highly contagious and often fatal honey bee brood disease. The species P. larvae comprises five so-called ERIC-genotypes which differ in virulence and pathogenesis strategies. In the past two decades, the identification and characterization of several P. larvae virulence factors have led to considerable progress in understanding the molecular basis of pathogen-host-interactions during P. larvae infections. Among these virulence factors are three ADP-ribosylating AB-toxins, Plx1, Plx2, and C3larvin. Plx1 is a phage-born toxin highly homologous to the pierisin-like AB-toxins expressed by the whites-and-yellows family Pieridae (Lepidoptera, Insecta) and to scabin expressed by the plant pathogen Streptomyces scabiei. These toxins ADP-ribosylate DNA and thus induce apoptosis. While the presumed cellular target of Plx1 still awaits final experimental proof, the classification of the A subunits of the binary AB-toxins Plx2 and C3larvin as typical C3-like toxins, which ADP-ribosylate Rho-proteins, has been confirmed experimentally. Normally, C3-exoenzymes do not occur together with a B subunit partner, but as single domain toxins. Interestingly, the B subunits of the two P. larvae C3-like toxins are homologous to the B-subunits of C2-like toxins with striking structural similarity to the PA-63 protomer of Bacillus anthracis.

Putative determinants of virulence in Melissococcus plutonius, the bacterial agent causing European foulbrood in honey bees

Melissococcus plutonius

is a bacterial pathogen that causes epidemic outbreaks of European foulbrood (EFB) in honey bee populations. The pathogenicity of a bacterium depends on its virulence, and understanding the mechanisms influencing virulence may allow for improved disease control and containment. Using a standardized in vitro assay, we demonstrate that virulence varies greatly among sixteen M. plutonius isolates from five European countries. Additionally, we explore the causes of this variation. In this study, virulence was independent of the multilocus sequence type of the tested pathogen, and was not affected by experimental co-infection with Paenibacillus alvei, a bacterium often associated with EFB outbreaks. Virulence in vitro was correlated with the growth dynamics of M. plutonius isolates in artificial medium, and with the presence of a plasmid carrying a gene coding for the putative toxin melissotoxin A. Our results suggest that some M. plutonius strains showed an increased virulence due to the acquisition of a toxin-carrying mobile genetic element. We discuss whether strains with increased virulence play a role in recent EFB outbreaks.

Fructose and Trehalose Selectively Enhance In Vitro Sporulation of Paenibacillus larvae ERIC I and ERIC II Strains

Paenibacillus larvae is a Gram-positive bacterium, the spores of which are the causative agent of the most destructive brood disease of honeybees, American foulbrood (AFB). Obtaining viable spores of pathogen strains is requisite for different studies concerning AFB. The aim of this work was to investigate the effects of five saccharides that may naturally occur in higher amounts in bee larvae on in vitro sporulation of P. larvae. The effect of individual saccharides at different concentrations on spore yields of P. larvae strains of epidemiologically important ERIC genotypes was examined in Columbia sheep blood agar (CSA) and MYPGP agar media. It was found that fructose in ERIC I and trehalose in ERIC II strains at concentrations in the range of 0.5-2% represent new sporulation factors that significantly enhanced the yields of viable spores in both media, mostly in a concentration-dependent manner. The enhancements in spore yield were mainly caused by improvements of the germination ability of the spores produced. Glucose, maltose and sucrose at 1% or 0.5% concentrations also supported sporulation but to a lower extent and not in all strains and media. Based on the knowledge gained, a novel procedure was proposed for the preparation of viable P. larvae spores with supposed improved quality for AFB research.

Characterization of C3larvinA, a novel RhoA-targeting ADP-ribosyltransferase toxin produced by the honey bee pathogen, Paenibacillus larvae

C3larvinA is a putative virulence factor produced by Paenibacillus larvae enterobacterial-repetitive-intergenic-consensus (ERIC) III/IV (strain 11-8051). Biochemical, functional and structural analyses of C3larvinA revealed that it belongs to the C3-like mono-ADP-ribosylating toxin subgroup. Mammalian RhoA was the target substrate for its transferase activity suggesting that it may be the biological target of C3larvinA. The kinetic parameters of the NAD+ substrate for the transferase (KM = 75 ± 10 µM) and glycohydrolase (GH) (KM = 107 ± 20 µM) reactions were typical for a C3-like bacterial toxin, including the Plx2A virulence factor from Paenibacillus larvae ERIC I. Upon cytoplasmic expression in yeast, C3larvinA caused a growth-defective phenotype indicating that it is an active C3-like toxin and is cytotoxic to eukaryotic cells. The catalytic variant of the Q187-X-E189 motif in C3larvinA showed no cytotoxicity toward yeast confirming that the cytotoxicity of this factor depends on its enzymatic activity. A homology consensus model of C3larvinA with NAD+ substrate was built on the structure of Plx2A, provided additional confirmation that C3larvinA is a member of the C3-like mono-ADP-ribosylating toxin subgroup. A homology model of C3larvinA with NADH and RhoA was built on the structure of the C3cer-NADH-RhoA complex which provided further evidence that C3larvinA is a C3-like toxin that shares an identical catalytic mechanism with C3cer from Bacillus cereus. C3larvinA induced actin cytoskeleton reorganization in murine macrophages, whereas in insect cells, vacuolization and bi-nucleated cells were observed. These cellular effects are consistent with C3larvinA disrupting RhoA function by covalent modification that is shared among C3-like bacterial toxins.

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).

Paenibacillus lutrae sp. nov., A Chitinolytic Species Isolated from A River Otter in Castril Natural Park, Granada, Spain

A highly chitinolytic facultative anaerobic, chemoheterotrophic, endospore-forming, Gram-stain-positive, rod-shaped bacterial strain N10^T was isolated from the feces of a river otter in the Castril Natural Park (Granada, Spain). It is a slightly halophilic, motile, catalase-, oxidase-, ACC deaminase- and C4 and C8 lipase-positive strain. It is aerobic, respiratory and has a fermentative metabolism using oxygen as an electron acceptor, produces acids from glucose and can fix nitrogen. Phylogenetic analysis of the 16S rRNA gene sequence, multilocus sequence analysis (MLSA) of 16S rRNA, gyrB, recA and rpoB, as well as phylogenomic analyses indicate that strain N10^T is a novel species of the genus Paenibacillus, with the highest 16S rRNA sequence similarity (95.4%) to P. chitinolyticus LMG 18047^T and <95% similarity to other species of the genus Paenibacillus. Digital DNA–DNA hybridization (dDDH) and average nucleotide identity (ANIb) were 21.1% and <75%, respectively. Its major cellular fatty acids were anteiso-C_15:0, C_16:0, and iso-C_15:0. G + C content ranged between 45%–50%. Using 16S rRNA phylogenetic and in silico phylogenomic analyses, together with chemotaxonomic and phenotypic data, we demonstrate that type strain N10^T (= CECT 9541^T =LMG 30535^T) is a novel species of genus Paenibacillus and the name Paenibacillus lutrae sp. nov. is proposed.

Comprehensive proteomic analysis of exoproteins expressed by ERIC I, II, III and IV Paenibacillus larvae genotypes reveals a wide range of virulence factors

American foulbrood is a quarantine disease of the honeybee Apis mellifera L. in many countries and contributes greatly to colony losses. We performed a label-free proteomics study of exoprotein fractions produced in vitro by Paenibacillus larvae reference strains of the ERIC I-IV genotypes. A quantitative comparison was performed of previous studied protein-based virulence factors and many newly identified putative virulence factors. Among the multiple proteases identified, key virulence factors included the microbial collagenase ColA and immune inhibitor A (InhA, an analog of the Bacillus thuringiensis protein InhA). Both of these virulence factors were detected in ERICs II-IV but were absent from ERIC I. Furthermore, the different S-layer proteins and polysaccharide deacetylases prevailed in ERICs II-IV. Thus, the expression patterns of these virulence factors corresponded with the different speeds at which honeybee larvae are known to be killed by ERICs II-IV compared to ERIC I. In addition, putative novel toxin-like proteins were identified, including vegetative insecticidal protein Vip1, a mosquitocidal toxin, and epsilon-toxin type B, which exhibit similarity to homologs present in Bacillus thuringiensis or Lysinibacillus sphaericus. Furthermore, a putative bacteriocin similar to Lactococcin 972 was identified in all assayed genotypes. It appears that P. larvae shares virulence factors similar to those of the Bacillus cereus group. Overall, the results provide novel information regarding P. larvae virulence potential, and a comprehensive exoprotein comparison of all four ERICs was performed for the first time. The identification of novel virulence factors can explain differences in the virulence of isolates.

Fly foregut and transmission of microbes

Two areas of research that have greatly increased in attention are: dipterans as vectors and the microbes they are capable of vectoring. Because it is the front-end of the fly that first encounters these microbes, this review focuses on the legs, mouthparts, and foregut, which includes the crop as major structures involved in dipteran vectoring ability. The legs and mouthparts are generally involved in mechanical transmission of microbes. However, the crop is involved in more than just mechanical transmission, for it is within the lumen of the crop that microbes are taken up with the meal of the fly, stored, and it is within the lumen that horizontal transmission of bacterial resistance has been demonstrated. In addition to storage of microbes, the crop is also involved in depositing the microbes via a process known as regurgitation. Various aspects of crop regulation are discussed and specific examples of crop involvement with microorganisms are discussed. The importance of biofilm and biofilm formation are presented, as well as, some physical parameters of the crop that might either facilitate or inhibit biofilm formation. Finally, there is a brief discussion of dipteran model systems for studying crop microbe interactions.

The biological role of the enigmatic C3larvinAB toxin of the honey bee pathogenic bacterium Paenibacillus larvae

Paenibacillus larvae is the causative agent of the notifiable epizootic American foulbrood, a fatal bacterial disease of honey bee larvae. The species P. larvae has been classified into four differentially virulent and prevalent genotypes (ERIC I-IV), which also differ in their virulence factor equipment. Recently, a novel P. larvae toxin, the C3-like C3larvin, has been described. Genome analysis now revealed that the C3larvin gene is actually a part of a toxin locus encompassing two genes encoding a binary AB toxin with the A subunit being C3larvin (C3larvinA) and a putative B subunit (C3larvinB) encoded by the second gene. Sequence and structural analyses demonstrated that C3larvinB is a homologue of the Bacillus anthracis protective antigen (PA), the B subunit of anthrax toxin. The C3larvinAB toxin locus was interrupted by point mutations in all analysed P. larvae ERIC I and ERIC II strains. Only one P. larvae ERIC III/IV strain harboured an uninterrupted toxin locus comprising full-length genes for C3larvinA and B. Exposure bioassays did not substantiate a role as virulence factor for C3larvinAB in P. larvae ERIC I/II. However, the PA homologue C3larvinB had an influence on the virulence of the unique P. larvae strain expressing the functional C3larvinAB locus.

Production of a Thermostable Chitosanase from Shrimp Heads via Paenibacillus mucilaginosus TKU032 Conversion and its Application in the Preparation of Bioactive Chitosan Oligosaccharides

Chitosanase has attracted great attention due to its potential applications in medicine, agriculture, and nutraceuticals. In this study, P. mucilaginosus TKU032, a bacterial strain isolated from Taiwanese soil, exhibited the highest chitosanase activity (0.53 U/mL) on medium containing shrimp heads as the sole carbon and nitrogen (C/N) source. Using sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis, a chitosanase isolated from P. mucilaginosus TKU032 cultured on shrimp head medium was determined at approximately 59 kDa. The characterized chitosanase showed interesting properties with optimal temperature and thermal stability up to 70 °C. Three chitosan oligosaccharide (COS) fractions were isolated from hydrolyzed colloidal chitosan that was catalyzed by TKU032 chitosanase. Of these, fraction I showed the highest α-glucosidase inhibitor (aGI) activity (65.86% at 20 mg/mL); its inhibitory mechanism followed the mixed noncompetitive inhibition model. Fractions II and III exhibited strong 2,2-diphenyl1-picrylhydrazyl (DPPH) radical scavenging activity (79.00% at 12 mg/mL and 73.29% at 16 mg/mL, respectively). In summary, the COS fractions obtained by hydrolyzing colloidal chitosan with TKU032 chitosanase may have potential use in medical or nutraceutical fields due to their aGI and antioxidant activities.

Wild bees and their nests host Paenibacillus bacteria with functional potential of avail

BACKGROUND

In previous studies, the gram-positive firmicute genus Paenibacillus was found with significant abundances in nests of wild solitary bees. Paenibacillus larvae is well-known for beekeepers as a severe pathogen causing the fatal honey bee disease American foulbrood, and other members of the genus are either secondary invaders of European foulbrood or considered a threat to honey bees. We thus investigated whether Paenibacillus is a common bacterium associated with various wild bees and hence poses a latent threat to honey bees visiting the same flowers.

RESULTS

We collected 202 samples from 82 individuals or nests of 13 bee species at the same location and screened each for Paenibacillus using high-throughput sequencing-based 16S metabarcoding. We then isolated the identified strain Paenibacillus MBD-MB06 from a solitary bee nest and sequenced its genome. We did find conserved toxin genes and such encoding for chitin-binding proteins, yet none specifically related to foulbrood virulence or chitinases. Phylogenomic analysis revealed a closer relationship to strains of root-associated Paenibacillus rather than strains causing foulbrood or other accompanying diseases. We found anti-microbial evidence within the genome, confirmed by experimental bioassays with strong growth inhibition of selected fungi as well as gram-positive and gram-negative bacteria.

CONCLUSIONS

The isolated wild bee associate Paenibacillus MBD-MB06 is a common, but irregularly occurring part of wild bee microbiomes, present on adult body surfaces and guts and within nests especially in megachilids. It was phylogenetically and functionally distinct from harmful members causing honey bee colony diseases, although it shared few conserved proteins putatively toxic to insects that might indicate ancestral predisposition for the evolution of insect pathogens within the group. By contrast, our strain showed anti-microbial capabilities and the genome further indicates abilities for chitin-binding and biofilm-forming, suggesting it is likely a useful associate to avoid fungal penetration of the bee cuticula and a beneficial inhabitant of nests to repress fungal threats in humid and nutrient-rich environments of wild bee nests.

10-HDA, A Major Fatty Acid of Royal Jelly, Exhibits pH Dependent Growth-Inhibitory Activity Against Different Strains of Paenibacillus larvae

Paenibacillus larvae (P. larvae) is a bacterial pathogen causing American foulbrood (AFB), the most serious disease of honeybee larvae. The food of young larvae could play an important role in the resistance of larvae against AFB. It contains antibacterial substances produced by honeybees that may inhibit the propagation of the pathogen in larval midguts. In this study, we identified and investigated the antibacterial effects of one of these substances, trans-10-hydroxy-2-decenoic acid (10-HDA), against P. larvae strains including all Enterobacterial Repetitive Intergenic Consensus (ERIC) genotypes. Its inhibitory activities were studied by determining the minimum inhibitory concentrations (MICs). It was found that 10-HDA efficacy increases substantially with decreasing pH; up to 12-fold differences in efficacy were observed between pH = 5.5 and pH = 7.2. P. larvae strains showed different susceptibility to 10-HDA; up to 2.97-fold differences existed among various strains with environmentally important ERIC I and ERIC II genotypes. Germinating spores of the pathogen were generally more susceptible to 10-HDA than vegetative cells. Our findings suggest that 10-HDA could play significant role in conferring antipathogenic activity to larval food in the midguts of young larvae and contribute to the resistance of individual larvae to P. larvae.

Reclamation of Marine Chitinous Materials for Chitosanase Production via Microbial Conversion by Paenibacillus macerans

Chitinous materials from marine byproducts elicit great interest among biotechnologists for their potential biomedical or agricultural applications. In this study, four kinds of marine chitinous materials (squid pens, shrimp heads, demineralized shrimp shells, and demineralized crab shells) were used to screen the best source for producing chitosanase by Paenibacillus macerans TKU029. Among them, the chitosanase activity was found to be highest in the culture using the medium containing squid pens as the sole carbon/nitrogen (C/N) source. A chitosanase which showed molecular weights at 63 kDa was isolated from P. macerans cultured on a squid pens medium. The purified TKU029 chitosanase exhibited optimum activity at 60 °C and pH 7, and was stable at temperatures under 50 °C and pH 3-8. An analysis by MALDI-TOF MS revealed that the chitosan oligosaccharides (COS) obtained from the hydrolysis of water-soluble chitosan by TKU029 crude enzyme showed various degrees of polymerization (DP), varying from 3⁻6. The obtained COS enhanced the growth of four lactic acid bacteria strains but exhibited no effect on the growth of E. coli. By specialized growth enhancing effects, the COS produced from hydrolyzing water soluble chitosan with TKU029 chitinolytic enzymes could have potential for use in medicine or nutraceuticals.

Comparative Genomics and Description of Putative Virulence Factors of Melissococcus plutonius, the Causative Agent of European Foulbrood Disease in Honey Bees

In Europe, approximately 84% of cultivated crop species depend on insect pollinators, mainly bees. Apis mellifera (the Western honey bee) is the most important commercial pollinator worldwide. The Gram-positive bacterium Melissococcus plutonius is the causative agent of European foulbrood (EFB), a global honey bee brood disease. In order to detect putative virulence factors, we sequenced and analyzed the genomes of 14 M. plutonius strains, including two reference isolates. The isolates do not show a high diversity in genome size or number of predicted protein-encoding genes, ranging from 2.021 to 2.101 Mbp and 1589 to 1686, respectively. Comparative genomics detected genes that might play a role in EFB pathogenesis and ultimately in the death of the honey bee larvae. These include bacteriocins, bacteria cell surface- and host cell adhesion-associated proteins, an enterococcal polysaccharide antigen, an epsilon toxin, proteolytic enzymes, and capsule-associated proteins. In vivo expression of three putative virulence factors (endo-alpha-N-acetylgalactosaminidase, enhancin and epsilon toxin) was verified using naturally infected larvae. With our strain collection, we show for the first time that genomic differences exist between non-virulent and virulent typical strains, as well as a highly virulent atypical strain, that may contribute to the virulence of M. plutonius. Finally, we also detected a high number of conserved pseudogenes (75 to 156) per genome, which indicates genomic reduction during evolutionary host adaptation.

Swarming motility and biofilm formation of Paenibacillus larvae, the etiological agent of American Foulbrood of honey bees (Apis mellifera)

American Foulbrood is a worldwide distributed, fatal disease of the brood of the Western honey bee (Apis mellifera). The causative agent of this fatal brood disease is the Gram-positive, spore-forming bacterium Paenibacillus larvae, which can be classified into four different genotypes (ERIC I-IV), with ERIC I and II being the ones isolated from contemporary AFB outbreaks. P. larvae is a peritrichously flagellated bacterium and, hence, we hypothesized that P. larvae is capable of coordinated and cooperative multicellular behaviors like swarming motility and biofilm formation. In order to analyze these behaviors of P. larvae, we firstly established appropriate functional assays. Using these assays we demonstrated that P. larvae ERIC II, but not P. larvae ERIC I, was capable of swarming. Swarming motility was hampered in a P. larvae ERIC II-mutant lacking production of paenilarvin, an iturin-like lipopeptide exclusively expressed by this genotype. Both genotypes were able to form free floating biofilm aggregates loosely attached to the walls of the culture wells. Visualizing the biofilms by Congo red and thioflavin S staining suggested structural differences between the biofilms formed. Biofilm formation was shown to be independent from paenilarvin production because the paenilarvin deficient mutant was comparably able to form a biofilm.

Genomics analysis of Aphanomyces spp. identifies a new class of oomycete effector associated with host adaptation

Background

Oomycetes are a group of filamentous eukaryotic microorganisms that have colonized all terrestrial and oceanic ecosystems, and they include prominent plant pathogens. The Aphanomyces genus is unique in its ability to infect both plant and animal species, and as such exemplifies oomycete versatility in adapting to different hosts and environments. Dissecting the underpinnings of oomycete diversity provides insights into their specificity and pathogenic mechanisms.

Results

By carrying out genomic analyses of the plant pathogen A. euteiches and the crustacean pathogen A. astaci, we show that host specialization is correlated with specialized secretomes that are adapted to the deconstruction of the plant cell wall in A. euteiches and protein degradation in A. astaci. The A. euteiches genome is characterized by a large repertoire of small secreted protein (SSP)-encoding genes that are highly induced during plant infection, and are not detected in other oomycetes. Functional analysis revealed an SSP from A. euteiches containing a predicted nuclear-localization signal which shuttles to the plant nucleus and increases plant susceptibility to infection.

Conclusion

Collectively, our results show that Aphanomyces host adaptation is associated with evolution of specialized secretomes and identify SSPs as a new class of putative oomycete effectors.

Electronic supplementary material

The online version of this article (10.1186/s12915-018-0508-5) contains supplementary material, which is available to authorized users.

Bacterial pathogens of bees

Pollination is an indispensable ecosystem service provided by many insects, especially by wild and managed bee species. Hence, reports on large scale honey bee colony losses and on population declines of many wild bees were alarming and resulted in increased awareness of the importance of bee health and increased interest in bee pathogens. To serve this interest, this review will give a comprehensive overview on bacterial bee pathogens by covering not only the famous pathogens (Paenibacillus larvae, Melissococcus plutonius), but also the orphan pathogens which have largely been neglected by the scientific community so far (spiroplasmas) and the pathogens which were only recently discovered as being pathogenic to bees (Serratia marcescens, Lysinibacillus sphaericus).

Prevalence of American foulbrood in asymptomatic apiaries of Kurdistan, Iran

AIM

Paenibacillus larvae subsp. larvae is the etiological agent of American foulbrood (AFB), the most virulent bacterial disease of honey bee brood worldwide. In many countries, AFB is a notifiable disease since it is highly contagious, in most cases incurable, and able to kill affected colonies. The aim of this study was to determine the prevalence of P. larvae subsp. larvae in Kurdistan province apiaries by polymerase chain reaction (PCR) technique.

MATERIALS AND METHODS

A total of 100 samples were randomly purchased from apiaries in Kurdistan, Iran. Apiaries were randomly sampled in accordance with the instructions of the veterinary organization from different provinces and were tested using PCR method and an exclusive primer of 16S rRNA for the presence of P. larvae subsp. larvae.

RESULTS

The results of this study indicated a low level of contamination with P. larvae subsp. larvae in the Kurdistan province. The number of positive samples obtained by PCR was 2%.

CONCLUSION

Therefore, monitoring programs for this honeybee disease in Kurdistan should be developed and implemented to ensure that it is detected early and managed.

In silico analysis of ChtBD3 domain to find its role in bacterial pathogenesis and beyond

Chitin binding domain 3, known by the acronym ChtBD3, is a domain in the enzymes and proteins of several pathogenic virus, bacteria and fungi. As this domain is evolutionarily-conserved in virulence factors of these infectious agents, its detailed investigation is of clinical interest. In this regard, the current in silico study analyzed ChtBD3 domain distribution in bacterial proteins present in publicly-available SMART (simple modular architecture research tool) database. Also, the co-occurring domains of ChtBD3 in the studied proteins were mapped to understand positional rearrangement of the domain and consequent functional diversity. Custom-made scripts were used to interpret the data and to derive patterns. As expected, interesting results were obtained. ChtBD3 domain co-occurred with other critical domains like peptidase, glycol_hydrolase, kinase, hemagglutinin-acting, collagen-binding, among others. The findings are expected to be of clinical relevance.

Physiological and Molecular Understanding of Bacterial Polysaccharide Monooxygenases

Bacteria have long been known to secrete enzymes that degrade cellulose and chitin. The degradation of these two polymers predominantly involves two enzyme families that work synergistically with one another: glycoside hydrolases (GHs) and polysaccharide monooxygenases (PMOs). Although bacterial PMOs are a relatively recent addition to the known biopolymer degradation machinery, there is an extensive amount of literature implicating PMO in numerous physiological roles. This review focuses on these diverse and physiological aspects of bacterial PMOs, including facilitating endosymbiosis, conferring a nutritional advantage, and enhancing virulence in pathogenic organisms. We also discuss the correlation between the presence of PMOs and bacterial lifestyle and speculate on the advantages conferred by PMOs under these conditions. In addition, the molecular aspects of bacterial PMOs, as well as the mechanisms regulating PMO expression and the function of additional domains associated with PMOs, are described. We anticipate that increasing research efforts in this field will continue to expand our understanding of the molecular and physiological roles of bacterial PMOs.

Chitin and chitinase: Role in pathogenicity, allergenicity and health

Chitin, a polysaccharide with particular abundance in fungi, nematodes and arthropods is immunogenic. It acts as a threat to other organisms, to tackle which they have been endowed with chitinase enzyme. Even if this enzyme is not present in all organisms, they possess proteins having chitin-binding domain(s) (ChtBD). Many lethal viruses like Ebola, and HCV (Hepatitis C virus) have these domains to manipulate their carriers and target organisms. In keeping with the basic rule of survival, the self-origin (own body component) chitins and chitinases are protective, but that of non-self origin (from other organisms) are detrimental to health. The exogenous chitins and chitinases provoke human innate immunity to generate a deluge of inflammatory cytokines, which injure organs (leading to asthma, atopic dermatitis etc.), and in persistent situations lead to death (multiple sclerosis, systemic lupus erythromatosus (SLE), cancer, etc.). Unfortunately, chitin-chitinase-stimulated hypersensitivity is a common cause of occupational allergy. On the other hand, chitin, and its deacetylated derivative chitosan are increasingly proving useful in pharmaceutical, agriculture, and biocontrol applications. This critical review discusses the complex nexus of chitin and chitinase and assesses both their pathogenic as well as utilitarian aspects.

Functionality of Tn916 in Paenibacillus larvae

The conjugative transposon Tn916 was determined to be functional in Paenibacillus larvae in regard to expression of tetracycline resistance and conjugative transfer. Expression of erythromycin resistance, using Tn916ΔE, was also observed. Conjugative transfer experiments employing Paenibacillus popilliae strains Tc1001 and Em1001 as transposon donors and experiments using different P. larvae subspecies or different transposon-containing strains demonstrated interspecies and intraspecies transfer occurred for Tn916 and Tn916ΔE. Southern hybridization analysis of several Tn916-containing P. larvae isolates showed that the transposon randomly inserted into the bacterial chromosome with an indication that hot spot insertion had occurred. Hybridization analysis indicated single-copy insertion of Tn916 into the genome predominated. However, selection of multiple-resistant isolates (i.e., isolates containing Tn916 and Tn916ΔE) demonstrated that multiple copies of the transposon could coexist in the bacterial genome. Growth of transposon-containing isolates in broth medium in the absence of selective antibiotic pressure showed that Tn916 and Tn916ΔE were stably maintained in the bacterium.

Biological Role of Paenilarvins, Iturin-Like Lipopeptide Secondary Metabolites Produced by the Honey Bee Pathogen Paenibacillus larvae

The Gram-positive bacterium Paenibacillus larvae (P. larvae) is the causative agent of a deadly honey bee brood disease called American Foulbrood (AFB). AFB is a notifiable epizootic in most countries and, hence, P. larvae is of considerable relevance for veterinarians and apiculturists alike. Over the last decade, much progress has been made in the understanding of the (patho)biology of P. larvae. Recently, several non-ribosomally produced peptides (NRP) and peptide/polyketide (NRP/PK) hybrids produced by P. larvae were identified. Among these NRPs were iturin-like lipopeptides, the paenilarvins A-C. Iturins are known to exhibit strong anti-fungal activity; for some iturins, cytotoxic activity towards mammalian erythrocytes and human cancer cell lines are described. We here present our results on the analysis of the natural function of the paenilarvins during pathogenesis of P. larvae infections. We demonstrated production of paenilarvins in infected larvae. However, we could neither demonstrate cytotoxicity of paenilarvins towards cultured insect cells nor towards larvae in feeding assays. Accordingly, exposure bioassays performed with larvae infected by wild-type P. larvae and a knockout mutant of P. larvae lacking production of paenilarvins did not substantiate a role for the paenilarvins as virulence factor. Further experiments are necessary to analyze the relevance of the paenilarvins' anti-fungal activity for P. larvae infections in the presence of fungal competitors in the larval midgut or cadaver.

Biology of Paenibacillus larvae, a deadly pathogen of honey bee larvae

The gram-positive bacterium Paenibacillus larvae is the etiological agent of American Foulbrood of honey bees, a notifiable disease in many countries. Hence, P. larvae can be considered as an entomopathogen of considerable relevance in veterinary medicine. P. larvae is a highly specialized pathogen with only one established host, the honey bee larva. No other natural environment supporting germination and proliferation of P. larvae is known. Over the last decade, tremendous progress in the understanding of P. larvae and its interactions with honey bee larvae at a molecular level has been made. In this review, we will present the recent highlights and developments in P. larvae research and discuss the impact of some of the findings in a broader context to demonstrate what we can learn from studying "exotic" pathogens.

Rhizoxin analogs, orfamide A and chitinase production contribute to the toxicity of Pseudomonas protegens strain Pf-5 to Drosophila melanogaster

Pseudomonas protegens strain Pf-5 is a soil bacterium that was first described for its capacity to suppress plant diseases and has since been shown to be lethal to certain insects. Among these is the common fruit fly Drosophila melanogaster, a well-established model organism for studies evaluating the molecular and cellular basis of the immune response to bacterial challenge. Pf-5 produces the insect toxin FitD, but a ΔfitD mutant of Pf-5 retained full toxicity against D. melanogaster in a noninvasive feeding assay, indicating that FitD is not a major determinant of Pf-5's oral toxicity against this insect. Pf-5 also produces a broad spectrum of exoenzymes and natural products with antibiotic activity, whereas a mutant with a deletion in the global regulatory gene gacA produces none of these exoproducts and also lacks toxicity to D. melanogaster. In this study, we made use of a panel of Pf-5 mutants having single or multiple mutations in the biosynthetic gene clusters for seven natural products and two exoenzymes that are produced by the bacterium under the control of gacA. Our results demonstrate that the production of rhizoxin analogs, orfamide A, and chitinase are required for full oral toxicity of Pf-5 against D. melanogaster, with rhizoxins being the primary determinant.

Microbial ecology-based methods to characterize the bacterial communities of non-model insects

Among the animals of the Kingdom Animalia, insects are unparalleled for their widespread diffusion, diversity and number of occupied ecological niches. In recent years they have raised researcher interest not only because of their importance as human and agricultural pests, disease vectors and as useful breeding species (e.g. honeybee and silkworm), but also because of their suitability as animal models. It is now fully recognized that microorganisms form symbiotic relationships with insects, influencing their survival, fitness, development, mating habits and the immune system and other aspects of the biology and ecology of the insect host. Thus, any research aimed at deepening the knowledge of any given insect species (perhaps species of applied interest or species emerging as novel pests or vectors) must consider the characterization of the associated microbiome. The present review critically examines the microbiology and molecular ecology techniques that can be applied to the taxonomical and functional analysis of the microbiome of non-model insects. Our goal is to provide an overview of current approaches and methods addressing the ecology and functions of microorganisms and microbiomes associated with insects. Our focus is on operational details, aiming to provide a concise guide to currently available advanced techniques, in an effort to extend insect microbiome research beyond simple descriptions of microbial communities.

Molecular pathogenesis of American Foulbrood: how Paenibacillus larvae kills honey bee larvae

American Foulbrood caused by Paenibacillus larvae is one of the unsolved health problems honey bee colonies are suffering from. In the recent past, considerable progress has been achieved in understanding molecular details of P. larvae infections of honey bee larvae. This was facilitated by the development of molecular tools for manipulating P. larvae and by the availability of complete genome sequences of different P. larvae genotypes. We here report on several peptides and proteins that have recently been identified, biochemically analyzed, and proposed to act as virulence factors of P. larvae. For some of them, experimental proof for their role as virulence factor has been provided allowing presenting a preliminary model for the molecular pathogenesis of American Foulbrood.

The Apis mellifera Filamentous Virus Genome

A complete reference genome of the Apis mellifera Filamentous virus (AmFV) was determined using Illumina Hiseq sequencing. The AmFV genome is a double stranded DNA molecule of approximately 498,500 nucleotides with a GC content of 50.8%. It encompasses 247 non-overlapping open reading frames (ORFs), equally distributed on both strands, which cover 65% of the genome. While most of the ORFs lacked threshold sequence alignments to reference protein databases, twenty-eight were found to display significant homologies with proteins present in other large double stranded DNA viruses. Remarkably, 13 ORFs had strong similarity with typical baculovirus domains such as PIFs (per os infectivity factor genes: pif-1, pif-2, pif-3 and p74) and BRO (Baculovirus Repeated Open Reading Frame). The putative AmFV DNA polymerase is of type B, but is only distantly related to those of the baculoviruses. The ORFs encoding proteins involved in nucleotide metabolism had the highest percent identity to viral proteins in GenBank. Other notable features include the presence of several collagen-like, chitin-binding, kinesin and pacifastin domains. Due to the large size of the AmFV genome and the inconsistent affiliation with other large double stranded DNA virus families infecting invertebrates, AmFV may belong to a new virus family.