How to kill the honey bee larva: genomic potential and virulence mechanisms of Paenibacillus larvae

Paenibacillus larvae, a Gram positive bacterial pathogen, causes American Foulbrood (AFB), which is the most serious infectious disease of honey bees. In order to investigate the genomic potential of P. larvae, two strains belonging to two different genotypes were sequenced and used for comparative genome analysis. The complete genome sequence of P. larvae strain DSM 25430 (genotype ERIC II) consisted of 4,056,006 bp and harbored 3,928 predicted protein-encoding genes. The draft genome sequence of P. larvae strain DSM 25719 (genotype ERIC I) comprised 4,579,589 bp and contained 4,868 protein-encoding genes. Both strains harbored a 9.7 kb plasmid and encoded a large number of virulence-associated proteins such as toxins and collagenases. In addition, genes encoding large multimodular enzymes producing nonribosomally peptides or polyketides were identified. In the genome of strain DSM 25719 seven toxin associated loci were identified and analyzed. Five of them encoded putatively functional toxins. The genome of strain DSM 25430 harbored several toxin loci that showed similarity to corresponding loci in the genome of strain DSM 25719, but were non-functional due to point mutations or disruption by transposases. Although both strains cause AFB, significant differences between the genomes were observed including genome size, number and composition of transposases, insertion elements, predicted phage regions, and strain-specific island-like regions. Transposases, integrases and recombinases are important drivers for genome plasticity. A total of 390 and 273 mobile elements were found in strain DSM 25430 and strain DSM 25719, respectively. Comparative genomics of both strains revealed acquisition of virulence factors by horizontal gene transfer and provided insights into evolution and pathogenicity.

Identification and functional analysis of the S-layer protein SplA of Paenibacillus larvae, the causative agent of American Foulbrood of honey bees

The Gram-positive, spore-forming bacterium Paenibacillus larvae is the etiological agent of American Foulbrood (AFB), a globally occurring, deathly epizootic of honey bee brood. AFB outbreaks are predominantly caused by two genotypes of P. larvae, ERIC I and ERIC II, with P. larvae ERIC II being the more virulent genotype on larval level. Recently, comparative proteome analyses have revealed that P. larvae ERIC II but not ERIC I might harbour a functional S-layer protein, named SplA. We here determine the genomic sequence of splA in both genotypes and demonstrate by in vitro self-assembly studies of recombinant and purified SplA protein in combination with electron-microscopy that SplA is a true S-layer protein self-assembling into a square 2D lattice. The existence of a functional S-layer protein is novel for this bacterial species. For elucidating the biological function of P. larvae SplA, a genetic system for disruption of gene expression in this important honey bee pathogen was developed. Subsequent analyses of in vivo biological functions of SplA were based on comparing a wild-type strain of P. larvae ERIC II with the newly constructed splA-knockout mutant of this strain. Differences in cell and colony morphology suggest that SplA is a shape-determining factor. Marked differences between P. larvae ERIC II wild-type and mutant cells with regard to (i) adhesion to primary pupal midgut cells and (ii) larval mortality as measured in exposure bioassays corroborate the assumption that the S-layer of P. larvae ERIC II is an important virulence factor. Since SplA is the first functionally proven virulence factor for this species, our data extend the knowledge of the molecular differences between these two genotypes of P. larvae and contribute to explaining the observed differences in virulence. These results present an immense advancement in our understanding of P. larvae pathogenesis.

Paenibacillus larvae is the most devastating bacterial pathogen of honey bees. However, the molecular interactions between infected larvae and P. larvae are poorly understood and little more than speculation exist concerning virulence factors. Recently, a putative S-layer protein has been identified in P. larvae. We here demonstrate that only representatives of P. larvae genotype ERIC II harbor a functional splA-gene and that SplA is a true S-layer protein with self-assembly capability. The presence of a functional S-layer protein is novel for P. larvae. When elucidating the biological function of SplA we broke new ground by establishing primary cell culture for pupal gut cells and by developing a genetic system for disruption of gene expression in this important honey bee pathogen. By using these novel methods we were able to prove that SplA serves as a shape-determining factor, mediates adhesion to host cells, and is a key virulence factor of P. larvae ERIC II. These results present an immense advancement in our understanding of P. larvae pathogenesis. Furthermore, we propose P. larvae as a model system for the analysis of the in vivo functions of S-layer proteins because P. larvae SlpA knockout-mutants retain viability and are thus suitable for functional studies.

Production of the catechol type siderophore bacillibactin by the honey bee pathogen Paenibacillus larvae

The Gram-positive bacterium Paenibacillus larvae is the etiological agent of American Foulbrood. This bacterial infection of honey bee brood is a notifiable epizootic posing a serious threat to global honey bee health because not only individual larvae but also entire colonies succumb to the disease. In the recent past considerable progress has been made in elucidating molecular aspects of host pathogen interactions during pathogenesis of P. larvae infections. Especially the sequencing and annotation of the complete genome of P. larvae was a major step forward and revealed the existence of several giant gene clusters coding for non-ribosomal peptide synthetases which might act as putative virulence factors. We here present the detailed analysis of one of these clusters which we demonstrated to be responsible for the biosynthesis of bacillibactin, a P. larvae siderophore. We first established culture conditions allowing the growth of P. larvae under iron-limited conditions and triggering siderophore production by P. larvae. Using a gene disruption strategy we linked siderophore production to the expression of an uninterrupted bacillibactin gene cluster. In silico analysis predicted the structure of a trimeric trithreonyl lactone (DHB-Gly-Thr)3 similar to the structure of bacillibactin produced by several Bacillus species. Mass spectrometric analysis unambiguously confirmed that the siderophore produced by P. larvae is identical to bacillibactin. Exposure bioassays demonstrated that P. larvae bacillibactin is not required for full virulence of P. larvae in laboratory exposure bioassays. This observation is consistent with results obtained for bacillibactin in other pathogenic bacteria.

Transcriptional response of honey bee larvae infected with the bacterial pathogen Paenibacillus larvae

American foulbrood disease of honey bees is caused by the bacterium Paenibacillus larvae. Infection occurs per os in larvae and systemic infection requires a breaching of the host peritrophic matrix and midgut epithelium. Genetic variation exists for both bacterial virulence and host resistance, and a general immunity is achieved by larvae as they age, the basis of which has not been identified. To quickly identify a pool of candidate genes responsive to P. larvae infection, we sequenced transcripts from larvae inoculated with P. larvae at 12 hours post-emergence and incubated for 72 hours, and compared expression levels to a control cohort. We identified 75 genes with significantly higher expression and six genes with significantly lower expression. In addition to several antimicrobial peptides, two genes encoding peritrophic-matrix domains were also up-regulated. Extracellular matrix proteins, proteases/protease inhibitors, and members of the Osiris gene family were prevalent among differentially regulated genes. However, analysis of Drosophila homologs of differentially expressed genes revealed spatial and temporal patterns consistent with developmental asynchrony as a likely confounder of our results. We therefore used qPCR to measure the consistency of gene expression changes for a subset of differentially expressed genes. A replicate experiment sampled at both 48 and 72 hours post infection allowed further discrimination of genes likely to be involved in host response. The consistently responsive genes in our test set included a hymenopteran-specific protein tyrosine kinase, a hymenopteran specific serine endopeptidase, a cytochrome P450 (CYP9Q1), and a homolog of trynity, a zona pellucida domain protein. Of the known honey bee antimicrobial peptides, apidaecin was responsive at both time-points studied whereas hymenoptaecin was more consistent in its level of change between biological replicates and had the greatest increase in expression by RNA-seq analysis.

Introduction of Non-Native Pollinators Can Lead to Trans-Continental Movement of Bee-Associated Fungi

Bees are essential pollinators for many flowering plants, including agriculturally important crops such as apple. As geographic ranges of bees or their host plants change as a result of human activities, we need to identify pathogens that could be transmitted among newly sympatric species to evaluate and anticipate their effects on bee communities. We used PCR screening and DNA sequencing to evaluate exposure to potentially disease-causing microorganisms in a pollinator of apple, the horned mason bee (Osmia cornifrons). We did not detect microsporidia, Wolbachia, or trypanosomes, which are common pathogens of bees, in any of the hundreds of mason bees screened. We did detect both pathogenic and apathogenic (saprophytic) fungal species in the genus Ascosphaera (chalkbrood), an unidentified species of Aspergillus fungus, and a strain of bacteria in the genus Paenibacillus that is probably apathogenic. We detected pathogenic fungal strains in asymptomatic adult bees that therefore may be carriers of disease. We demonstrate that fungi from the genus Ascosphaera have been transported to North America along with the bee from its native range in Japan, and that O. cornifrons is exposed to fungi previously only identified from nests of other related bee species. Further study will be required to quantify pathogenicity and health effects of these different microbial species on O. cornifrons and on closely-related native North American mason bees that may now be exposed to novel pathogens. A global perspective is required for pathogen research as geographic ranges of insects and microorganisms shift due to intentional or accidental introductions.

A novel bacterial pathogen of Biomphalaria glabrata: a potential weapon for schistosomiasis control?

BACKGROUND

Schistosomiasis is the second-most widespread tropical parasitic disease after malaria. Various research strategies and treatment programs for achieving the objective of eradicating schistosomiasis within a decade have been recommended and supported by the World Health Organization. One of these approaches is based on the control of snail vectors in endemic areas. Previous field studies have shown that competitor or predator introduction can reduce snail numbers, but no systematic investigation has ever been conducted to identify snail microbial pathogens and evaluate their molluscicidal effects.

METHODOLOGY/PRINCIPAL FINDINGS

In populations of Biomphalaria glabrata snails experiencing high mortalities, white nodules were visible on snail bodies. Infectious agents were isolated from such nodules. Only one type of bacteria, identified as a new species of Paenibacillus named Candidatus Paenibacillus glabratella, was found, and was shown to be closely related to P. alvei through 16S and Rpob DNA analysis. Histopathological examination showed extensive bacterial infiltration leading to overall tissue disorganization. Exposure of healthy snails to Paenibacillus-infected snails caused massive mortality. Moreover, eggs laid by infected snails were also infected, decreasing hatching but without apparent effects on spawning. Embryonic lethality was correlated with the presence of pathogenic bacteria in eggs.

CONCLUSIONS/SIGNIFICANCE

This is the first account of a novel Paenibacillus strain, Ca. Paenibacillus glabratella, as a snail microbial pathogen. Since this strain affects both adult and embryonic stages and causes significant mortality, it may hold promise as a biocontrol agent to limit schistosomiasis transmission in the field.

Analogous wheat root rhizosphere microbial successions in field and greenhouse trials in the presence of biocontrol agents Paenibacillus peoriae SP9 and Streptomyces fulvissimus FU14

Two Pythium-infested soils were used to compare the wheat root and rhizosphere soil microbial communities from plants grown in the field or in greenhouse trials and their stability in the presence of biocontrol agents. Bacteria showed the highest diversity at early stages of wheat growth in both field and greenhouse trials, while fungal diversity increased later on, at 12 weeks of the crop cycle. The microbial communities were stable in roots and rhizosphere samples across both soil types used in this study. Such stability was also observed irrespective of the cultivation system (field or greenhouse) or addition of biocontrol coatings to wheat seeds to control Pythium disease (in this study soil infected with Pythium sp. clade F was tested). In greenhouse plant roots, Archaeorhizomyces, Debaryomyces, Delftia, and unclassified Pseudeurotiaceae were significantly reduced when compared to plant roots obtained from the field trials. Some operational taxonomic units (OTUs) represented genetic determinants clearly transmitted vertically by seed endophytes (specific OTUs were found in plant roots) and the plant microbiota was enriched over time by OTUs from the rhizosphere soil. This study provided key information regarding the microbial communities associated with wheat roots and rhizosphere soils at different stages of plant growth and the role that Paenibacillus and Streptomyces strains play as biocontrol agents in supporting plant growth in infested soils.

Low-molecular-weight metabolites secreted by Paenibacillus larvae as potential virulence factors of American foulbrood

The spore-forming bacterium Paenibacillus larvae causes a severe and highly infective bee disease, American foulbrood (AFB). Despite the large economic losses induced by AFB, the virulence factors produced by P. larvae are as yet unknown. To identify such virulence factors, we experimentally infected young, susceptible larvae of the honeybee, Apis mellifera carnica, with different P. larvae isolates. Honeybee larvae were reared in vitro in 24-well plates in the laboratory after isolation from the brood comb. We identified genotype-specific differences in the etiopathology of AFB between the tested isolates of P. larvae, which were revealed by differences in the median lethal times. Furthermore, we confirmed that extracts of P. larvae cultures contain low-molecular-weight compounds, which are toxic to honeybee larvae. Our data indicate that P. larvae secretes metabolites into the medium with a potent honeybee toxic activity pointing to a novel pathogenic factor(s) of P. larvae. Genome mining of P. larvae subsp. larvae BRL-230010 led to the identification of several biosynthesis gene clusters putatively involved in natural product biosynthesis, highlighting the potential of P. larvae to produce such compounds.

Effect of high hydrostatic pressure processing on microbiological shelf-life and quality of fruits pretreated with ascorbic acid or SnCl2

In the current study, the processing conditions required for the inactivation of Paenibacillus polymyxa and relevant spoilage microorganisms by high hydrostatic pressure (HHP) treatment on apricot, peach, and pear pieces in sucrose (22°Brix) solution were assessed. Accordingly, the shelf-life was determined by evaluating both the microbiological quality and the sensory characteristics (taste, odor, color, and texture) during refrigerated storage after HHP treatment. The microbiological shelf-life of apricots, peaches, and pears was prolonged in the HHP-treated products in comparison with the untreated ones. In all HHP-treated packages for apricots, peaches, and pears, all populations were below the detection limit of the method (1 log CFU/g) and no growth of microorganisms was observed until the end of storage. Overall, no differences of the L*, a*, or b* value among the untreated and the HHP-treated fruit products were observed up to the time at which the unpressurized product was characterized as spoiled. HHP treatment had no remarkable effect on the firmness of the apricots, peaches, and pears. With regard to the sensory assessment, the panelists marked better scores to HHP-treated products compared to their respective controls, according to taste and total evaluation during storage of fruit products.

[Isolation, identification and insecticidal activity of endophyte from Achnatherum inebrians]

OBJECTIVE

To study endophyte species of Achnatherum inebrians and to screen strains with insecticidal activity against cotton insect.

METHODS

We isolated endophytic from roots,stems,leaves and seeds of health A. inebrians by grinding separation method and identified by a dual approach of morphological and physiological observation and 16S rDNA gene (for bacteria) and ITS sequence (for fungi) based molecular identification. Then,those endophytes were inoculated into liquid media for fermentation and the crude extracts were used to test insecticidal activities by slide disc immersion and nebulization methods.

RESULTS

We isolated bacteria species classified into 8 genera of Bacillus, Streptomyces, Corynebacterium, Phyllobacterium, sphingomonnas, Paenibacillus, Pseudomonas, Acinetobacter and 2 fungi of Claviceps purpure and Claviceps Chaetomium. Of them, the strain Streptomyces rochei (GA) and Claviceps purpurea (PF-2) had more than 85% of mortality to cotton aphis.

CONCLUSION

Two strains of PF-2 and GA associated within the A. inebrians had significant insecticidal activity to cotton aphis (Aphis gossypii), which may provide a new biological resource to explore new microbial insecticide.

Ecology and Pathogenicity for Honey Bee Brood of Recently Described Paenibacillus melissococcoides and Comparison With Paenibacillus dendritiformis, Paenibacillus thiaminolyticus

Honey bee colonies contain thousands of individuals living in close proximity in a thermally homeostatic nest, creating ideal conditions for the thriving of numerous pathogens. Among the bacterial pathogens, Paenibacillus larvae infects larvae via the nutritive jelly that adult workers feed them, causing the highly contagious American foulbrood disease. Further Paenibacillus species were anecdotally found in association with honey bees, including when affected by another disease, European foulbrood (EFB). However, their pathogenicity remains largely unknown. Our results indicate that Paenibacillus dendritiformis, Paenibacillus thiaminolyticus and newly described Paenibacillus melissococcoides are pathogenic towards honey bee brood and that their virulence correlates with their sporulation ability, which confers them resistance to the bactericidal properties of the nutritive jelly. Our survey occasionally but increasingly detected P. melissococcoides in confirmed and idiopathic cases of EFB but never in healthy colonies, suggesting that this bacterium is an emerging pathogen of honey bee brood. Overall, our results suggest that virulence traits allowing a pathogenic or opportunistically pathogenic habit towards honey bee brood are frequent in Paenibacillus spp., but that their degree of adaptation to this host varies. Our study clarifies the ecology of this ubiquitous genus, especially when infecting honey bees.

Honey-derived Paenibacillus spp. with potential to affect bee brood development in Apis mellifera: Are they a new threat to honey bees?

Honey bees are important pollinators in both agriculture and ecosystems, and their health is essential for sustainable human development. Although only two bacteria, Paenibacillus larvae and Melissococcus plutonius, have been identified as bacterial pathogens in honey bee brood for over 100 years, we found three additional Paenibacillus strains (Paenibacillus sp. J27TS7, Paenibacillus azoreducens J34TS1, and Paenibacillus melissococcoides J46TS7) in honey that harmed honey bee brood development. In particular, Paenibacillus sp. J27TS7 was highly virulent in bee larvae (the median lethal dose [LD50] = 12.7 spores/larva) and was comparable to P. larvae (LD50 = 2.3-11.5 spores/larva). Paenibacillus azoreducens J34TS1 showed the second-highest virulence (LD50 = 45.9 spores/larva), and P. melissococcoides J46TS7 was the least virulent (LD50 = 469.0 spores/larva). However, P. melissococcoides was most frequently detected in Japanese honey among the three species, with the highest concentration being 1.8 × 106 spores/mL honey, suggesting its wide distribution in Japanese apiaries. The novel pathogenic Paenibacillus species were categorized into the fast killer (Paenibacillus sp. J27TS7), medium-fast killer (P. melissococcoides), and slow killer (P. azoreducens) like P. larvae strains in terms of the time to kill infected brood; however, histopathological and genome analyses indicated that their pathogenic mechanisms were different from those of P. larvae strains. Moreover, P. melissococcoides showed differences in virulence depending on the lineage of the strain. These findings represent the first discovery of honey bee brood pathogens in more than 100 years and indicate the need to look beyond known pathogens for a comprehensive understanding of honey bee diseases.