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

A metalloprotease secreted by an environmentally acquired gut bacterium hinders Borrelia afzelii colonization in Ixodes ricinus

Although the importance of the microbiome in the context of tick biology and vector competence has recently come into a broader research focus, the field is still in its infancy and the complex ecological interactions between the tick residential bacteria and pathogens are obscure. Here, we show that an environmentally acquired gut bacterium has the potential to impair Borrelia afzelii colonization within the tick vector through a secreted metalloprotease. Oral introduction of either Bacillus cereus LTG-1 isolate or its purified enhancin (BcEnhancin) protein significantly reduces B. afzelii burden in the guts of Ixodes ricinus ticks. This effect is attributed to the ability of BcEnhancin to degrade a glycan-rich peritrophic matrix (PM), which is a gut protective barrier essential for Borrelia survival. Our study highlights the importance of the gut microbiome in determining tick vector competence and provides a deeper mechanistic insight into the complex network of interactions between Borrelia, the tick, and the tick microbiome.

Insecticidal action of mammalian galectin-1-transfected Arabidopsis thaliana

BACKGROUND

Galectins (GALs) are a family of mammalian sugar-binding proteins specific for β-galactosides. Our previous studies have shown that the larval development of the diamondback moth (Plutella xylostella) is significantly disturbed when fed with recombinant mammalian galectin 1 (GAL1) derived from Escherichia coli. To further explore its applicability, two GAL1-overexpressed Arabidopsis [GAL1-Arabidopsis (whole plant) and GAL1-Arabidopsis-vas (vascular bundle-specific)] lines were established for insecticidal activity and mechanism studies.

RESULTS

The expression level of GAL1 in transgenic Arabidopsis is 1-0.5% (GAL1-Arabidopsis) and 0.08-0.01% (GAL1-Arabidopsis-vas) of total leaf soluble protein. Survival, body weight, and food consumption significantly decreased in a time-dependent manner in P. xylostella larvae (with chewing mouthparts) fed on GAL1-Arabidopsis. The mortality of Kolla paulula (with piercing-sucking mouthparts and xylem feeder) fed on GAL1-Arabidopsis-vas was also significantly higher than that fed on wild-type Arabidopsis (WT-Arabidopsis), but was lower than that fed on GAL1-Arabidopsis. The histochemical structure and results of immunostaining suggested that the binding of GAL1 to the midgut epithelium of P. xylostella fed on GAL1-Arabidopsis was dose- and time-dependent. Ultrastructural studies further showed the disruption of microvilli, abnormalities in epithelial cells, and fragments of the peritrophic membrane (PM) in P. xylostella larvae fed on GAL1-Arabidopsis.

CONCLUSION

The insecticidal mechanism of GAL1 involves interference with PM integrity and suggests that GAL1 is a potential candidate for bioinsecticide development. © 2024 Society of Chemical Industry.

Quantitative tyramine analysis method for Apis mellifera larvae infected with Melissococcus plutonius, the causative agent of European foulbrood

Tyramine, a trace monoamine produced from tyrosine by decarboxylation and found naturally in foods, plants, and animals, is a suspected virulence factor of Melissococcus plutonius that causes European foulbrood in honey bee brood. In the present study, we developed a method for quantitative analysis of tyramine in culture medium and honey bee larvae with a limit of quantitation of 3 ng/mL and a recovery rate of >97% using Liquid Chromatography-Mass Spectrometry/Mass Spectrometry and deuterium-labeled tyramine, demonstrating for the first time that a highly virulent M. plutonius strain actually produces tyramine in infected larvae. This method will be an indispensable tool to elucidate the role of tyramine in European foulbrood pathogenesis in combination with exposure bioassays using artificially reared bee larvae.

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.

In the battle of the disease: a transcriptomic analysis of European foulbrood-diseased larvae of the Western honey bee (Apis mellifera)

BACKGROUND

European foulbrood is a significant bacterial brood disease of Apis sp. and can cause severe and devastating damages in beekeeping operations. Nevertheless, the epidemiology of its causative agent Melissococcus plutonius has been begun to uncover but the underlying mechanisms of infection and cause of disease still is not well understood. Here, we sought to provide insight into the infection mechanism of EFB employing RNAseq in in vitro reared Apis mellifera larvae of two developmental stages to trace transcriptional changes in the course of the disease, including Paenibacillus alvei secondary infected individuals.

RESULTS

In consideration of the progressing development of the larva, we show that infected individuals incur a shift in metabolic and structural protein-encoding genes, which are involved in metabolism of crucial compounds including all branches of macronutrient metabolism, transport protein genes and most strikingly chitin and cuticle associated genes. These changes underpin the frequently observed developmental retardation in EFB disease. Further, sets of expressed genes markedly differ in different stages of infection with almost no overlap. In an earlier stage of infection, a group of regulators of the melanization response cascade and complement component-like genes, predominantly C-type lectin genes, are up-regulated while a differential expression of immune effector genes is completely missing. In contrast, late-stage infected larvae up-regulated the expression of antimicrobial peptides, lysozymes and prominent bacteria-binding haemocyte receptor genes compared to controls. While we clearly show a significant effect of infection on expressed genes, these changes may partly result from a shift in expression timing due to developmental alterations of infection. A secondary infection with P. alvei elicits a specific response with most of the M. plutonius associated differential immune effector gene expression missing and several immune pathway genes even down-regulated.

CONCLUSION

We conclude that with progressing infection diseased individuals undergo a systemic response with a change of metabolism and their activated immune defence repertoire. Moreover, larvae are capable of adjusting their response to a secondary invasion in late stage infections.

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.

Antimicrobial Activity from Putative Probiotic Lactic Acid Bacteria for the Biological Control of American and European Foulbrood Diseases

The balance of the gut microbiome is important for the honey bee’s growth and development, immune function and defense against pathogens. The use of a beneficial bacteria-based strategy for the prevention and biocontrol of American foulbrood (AFB) and European foulbrood (EFB) diseases in honey bees offers interesting prospects. Lactic acid bacteria (LAB) are common inhabitants of the gastrointestinal tract of the honey bee. Among LABs associated with bee gut microbiota, Lactiplantibacillus plantarum (previously Lactobacillus plantarum) and Apilactobacillus kunkeei (formerly classified as Lactobacillus kunkeei) are two of the most abundant species. In this study, four Lactiplantibacillus plantarum strains and four Apilactobacillus kunkeei strains, isolated from the gastrointestinal tract of honey bee (Apis mellifera L.) were selected for their in vitro inhibition ability of Paenibacillus larvae ATCC 9545 and Melissococccus plutonius ATCC 35311. In addition, these LABs have been characterized through some biochemical and functional characteristics: cell surface properties (hydrophobicity and auto-aggregation), carbohydrates assimilation and enzymatic activities. The antimicrobial, biochemical and cell surface properties of these LABs have been functional to their candidature as potential probiotics in beekeeping and for the biocontrol of AFB and EFB diseases.

Virulence variations between clonal complexes of Melisococcus plutonius and the possible causes

Melissococcus plutonius is a pathogenic bacterium that affects honeybee brood triggering colony collapse in severe cases. The bacterium causes a European foulbrood (EFB) disease in the honeybee populations, impacting beekeeping and agricultural industries. The pathogenesis, epidemiology, and variants of M. plutonius have been studied, but the virulence factors involved in larval infection are still unknown. Recently, an in-silico study suggested putative genes that might play a role in the pathogenesis of EFB. However, studies are required to determine their function as virulence factors. In addition, the few studies of clonal complexes (CCs), virulence factors, and variation in the honeybee larvae mortality have interfered with the development of more efficient control methods. The research, development, and differences in virulence between genetic variants (CCs) of M. plutonius and potential virulence factors implicated in honeybee larval mortality are discussed in this review.