Understanding of bacterial virulence using the silkworm infection model

Antifungal Susceptibility and Biological Characteristics of Fonsecaea monophora Causing Cerebral Phaeohyphomycosis in Jinhua, China

Background

The management of cerebral abscesses caused by dark-pigmented Fonsecaea monophora in healthy individuals continues to be challenging due to no consensus on the therapeutic regimen. Due to the absence of an accurate identification method, Fonsecaea species are often misidentified due to indistinct morphology features.

Materials and Methods

An F. monophora strain from an immunocompetent host with cerebral abscess was collected and identified by ITS rDNA molecular sequencing. The ITS sequences of the isolate were compared with that of the other ten Chinese F. monophora isolates obtained from GenBank for difference comparison and phylogenetic analysis. Fluorescence, Gram stains, and medan lactate were used to observe the colonial morphology. Antifungal susceptibility testing was implemented to demonstrate the antibiotic susceptibility profile. Galleria mellonella larvae were used as a model to study virulence of F. monophora. Medical records and clinical data of the patient were collected and analyzed.

Results

Antifungal susceptibility testing indicated that triazole antifungal drugs possess remarkable antifungal effect against F. monophora, and satisfactory antifungal effect of itraconazole was corresponding to the drug susceptibility results. Compared with the GM test, the serum G test was found to be more sensitive. The virulence and melanization in G. mellonella models for F. monophora were observed, and the death rates of infected larvae were positively related to injected concentrations of fungus. The phylogenetic tree was constructed from the ITS sequences of the clinical isolate along with ten Chinese F. monophora isolates, revealing that there is high relatedness in F. monophora strains collected from China.

Conclusion

F. monophora is an important neurotropic dematiaceous fungus and increasingly causing disease in immunocompetent individuals by means of noninvasive ways. Fungal culture, stainings, and molecular methods could be utilized to identify the etiologic agent. Triazole antifungal drugs can be applied as empiric therapeutic agents for phaeohyphomycosis.

Temporal transcriptome reveals that circadian clock is involved in the dynamic regulation of immune response to bacterial infection in Bombyx mori

The circadian clock plays a critical role in the regulation of host immune defense. However, the mechanistic basis for this regulation is largely unknown. Herein, the core clock gene cryptochrome1 (cry1) knockout line in Bombyx mori, an invertebrate animal model, was constructed to obtain the silkworm with dysfunctional molecular clock, and the dynamic regulation of the circadian clock on the immune responsiveness within 24 h of Staphylococcus aureus infection was analyzed. We found that deletion of cry1 decreased viability of silkworms and significantly reduced resistance of larvae to S. aureus. Time series RNA-seq analysis identified thousands of rhythmically expressed genes, including immune response genes, in the larval immune tissue, fat bodies. Uninfected cry1 knockout silkworms exhibited expression patterns of rhythmically expressed genes similar to wild-type (WT) silkworms infected with S. aureus. However, cry1 knockout silkworms exhibited a seriously weakened response to S. aureus infection. The immune response peaked at 6 and 24 h after infection, during which "transcription storms" occurred, and the expression levels of the immune response genes, PGRP and antimicrobial peptides (AMPs), were significantly upregulated in WT. In contrast, cry1 knockout did not effectively activate Toll, Imd, or NF-κB signaling pathways during the immune adjustment period from 12 to 18 h after infection, resulting in failure to initiate the immune responsiveness peak at 24 h after infection. This may be related to inhibited silkworm fat body energy metabolism. These results demonstrated the dynamic regulation of circadian clock on silkworm immune response to bacterial infection and provided important insights into host antimicrobial defense mechanisms.

Molecular mechanism and potential application of bacterial infection in the silkworm, Bombyx mori

As a representative species of Lepidoptera, Bombyx mori has been widely studied and applied. However, bacterial infection has always been an important pathogen threatening the growth of silkworms. Bombyx mori can resist various pathogenic bacteria through their own physical barrier and innate immune system. However, compared with other insects, such as Drosophila melanogaster, research on the antibacterial mechanism of silkworms is still in its infancy. This review systematically summarized the routes of bacterial infection in silkworms, the antibacterial mechanism of silkworms after ingestion or wounding infection, and the intestinal bacteria and infection of silkworms. Finally, we will discuss silkworms as a model animal for studying bacterial infectious diseases and screening antibacterial drugs.

Genomics and proteomics combined analysis revealed the toxicity response of silkworm Bombyx mori to the environmental pathogen Bacillus cereus ZJ-4

Bacterial contamination has caused a major public health problem worldwide. Bacillus cereus is a conditional environmental pathogenic bacteria that can cause food poisoning. Whether environmental pathogens can cause widespread transmission in the insect kingdom is unclear. In this study, a Bacillus cereus ZJ-4 was isolated from the hospital environment of Zhenjiang City, Jiangsu Province, China. It was fatal by injection into the silkworm hemolymph. To investigated the potential toxic factors of ZJ-4 and clarified the toxicity response mechanism of silkworm by the ZJ-4 infection. Then, the whole genome of ZJ-4 was sequenced, and the immune mechanism of silkworm fat body to ZJ-4 pathogen was studied by HE pathological section and proteomics. Bacterial genome sequencing indicated that ZJ-4 had 352 drug resistance genes and 6 virulence genes. After 36 h of subcutaneous puncture with ZJ-4 suspension, the pathological changes were obviously found in HE pathological sections of fat body tissue. Comparative proteomic results indicated that differentially expressed proteins are mainly involved in stress reactions, biological regulation, and innate immunity. The qRT-PCR analysis showed that the expressions of β-GRP, Spaetzle, MyD88, Tube and Dorsal genes in Toll pathway were up-regulated, while Pell and Cactus genes were down-regulated; in the antimicrobial peptide pathway, Glv2, Lzm, Mor, and Leb3 genes were up-regulated, while attacin1 and defensin genes were down-regulated; Sod gene was up-regulated, while Cat gene was down-regulated in the antioxidant pathway; Ldh, Sdh, and Mdh genes were down-regulated in glucose metabolism pathway. These results indicated that ZJ-4 can damage the innate immune pathway of silkworm, and also affect the normal immune function of fat body cells.

Identification of Membrane-Bound Lytic Murein Transglycosylase A (MltA) as a Growth Factor for Francisella novicida in a Silkworm Infection Model

Francisella tularensis, the causative agent of tularemia, is transmitted by arthropod vectors within mammalian hosts. The detailed mechanisms contributing to growth and survival of Francisella within arthropod remain poorly understood. To identify novel factors supporting growth and survival of Francisella within arthropods, a transposon mutant library of F. tularensis subsp. novicida (F. novicida) was screened using an F. novicida-silkworm infection model. Among 750 transposon mutants screened, the mltA-encoding membrane-bound lytic murein transglycosylase A (MltA) was identified as a novel growth factor of F. novicida in silkworms. Silkworms infection with an mltA deletion mutant (ΔmltA) resulted in a reduction in the number of bacteria and prolonged survival. The ΔmltA strain exhibited limited intracellular growth and cytotoxicity in BmN4 silkworm ovary cells. Moreover, the ΔmltA strain induced higher expression of the antimicrobial peptide in silkworms compared to the wild-type strain. These results suggest that F. novicida MltA contributes to the survival of F. novicida in silkworms via immune suppression-related mechanisms. Intracellular growth of the ΔmltA strain was also reduced in human monocyte THP-1 cells. These results also suggest the contribution of MltA to pathogenicity in humans and utility of the F. novicida-silkworm infection model to explore Francisella infection.

Ingestion and effects of polystyrene nanoparticles in the silkworm Bombyx mori

Information on the occurrence and effects of nanoplastics in ecosystems worldwide currently represent one of the main challenges from the ecotoxicological point of view. This is particularly true for terrestrial environments, in which nanoplastics are released directly by human activities or derive from the fragmentation of larger plastic items incorrectly disposed. Since insects can represent a target for these emerging contaminants in land-based community, the aim of this study was the evaluation of ingestion of 0.5 μm polystyrene nanoplastics and their effects in silkworm (Bombyx mori) larvae, a useful and well-studied insect model. The ingestion of nanoplastics, the possible infiltration in the tissues and organ accumulation were checked by confocal microscopy, while we evaluated the effects due to the administered nanoplastics through a multi-tier approach based on insect development and behaviour assessment, as endpoints at organism level, and the measurements of some biochemical responses associated with the imbalance of the redox status (superoxide dismutase, catalase, glutathione s-transferase, reactive oxygen species evaluation, lipid peroxidation) to investigate the cellular and molecular effects. We observed the presence of microplastics in the intestinal lumen, but also inside the larvae, specifically into the midgut epithelium, the Malpighian tubules and in the haemocytes. The behavioural observations revealed a significant (p < 0.05) increase of erratic movements and chemotaxis defects, potentially reflecting negative indirect effects on B. mori survival and fitness, while neither effect on insect development nor redox status imbalance were measured, with the exception of the significant (p < 0.05) inhibition of superoxide dismutase activity.

Reactive oxygen species and antimicrobial peptides are sequentially produced in silkworm midgut in response to bacterial infection

The silkworm, Bombyx mori, is utilized as a research model in many aspects of biological studies, including genetics, development and immunology. Previous biochemical and genomic studies have elucidated the silkworm immunity in response to infections elicited by bacteria, fungi, microsporidia, and viruses. The intestine serves as the front line in the battle between insects and ingested harmful microorganisms. In this study, we performed RNA sequencing (RNA-seq) of the larval silkworm midgut after oral infection with the Gram-positive bacterium Bacillus bombysepticus and the Gram-negative bacterium Yersinia pseudotuberculosis. This enables us to get a comprehensive understanding of the midgut responses to bacterial infection. We found that B. bombysepticus induced much stronger immune responses than Y. pseudotuberculosis did. Bacterial infection resulted in more energy consumption including carbohydrates and fatty acids. The midgut immune system was characterized by the generation of reactive oxygen species and antimicrobial peptides. The former played a critical role in eliminating invading bacteria during early stage, while the latter executed during late stage. Our results provide an integrated insight into the midgut systematic responses to bacterial infection.

A novel silkworm infection model with fluorescence imaging using transgenic Trichosporon asahii expressing eGFP

Trichosporon asahii is a pathogenic fungus that causes deep mycosis in patients with neutropenia. Establishing an experimental animal model for quantitatively evaluating pathogenicity and developing a genetic recombination technology will help to elucidate the infection mechanism of T. asahii and promote the development of antifungal drugs. Here we established a silkworm infection model with a transgenic T. asahii strain expressing eGFP. Injecting T. asahii into silkworms eventually killed the silkworms. Moreover, the administration of antifungal agents, such as amphotericin B, fluconazole, and voriconazole, prolonged the survival time of silkworms infected with T. asahii. A transgenic T. asahii strain expressing eGFP was obtained using a gene recombination method with Agrobacterium tumefaciens. The T. asahii strain expressing eGFP showed hyphal formation in the silkworm hemolymph. Both hyphal growth and the inhibition of hyphal growth by the administration of antifungal agents were quantitatively estimated by monitoring fluorescence. Our findings suggest that a silkworm infection model using T. asahii expressing eGFP is useful for evaluating both the pathogenicity of T. asahii and the efficacy of antifungal drugs.

Staphylococcus aureus aggregation in the plasma fraction of silkworm hemolymph

Staphylococcus aureus formed bacterial aggregates in the plasma fraction of the hemolymph of silkworm, the larva of Bombyx mori, in a growth-dependent manner. The addition of arabinose or galactose inhibited the formation of S. aureus aggregates in the silkworm plasma. Formation of the bacterial aggregates depended on S. aureus genes required for the synthesis of bacterial surface polysaccharides–ypfP and ltaA, which are involved in lipoteichoic acid synthesis, and the tagO gene, which is involved in wall teichoic acid synthesis. These findings suggest that S. aureus forms bacterial aggregates in the silkworm plasma via bacterial surface teichoic acids.

Gene Expression Pattern and Regulatory Network of α-Toxin Treatment in Bombyx mori

Bacillus bombyseptieus is a pathogen of Bombyx mori; it can cause bacterial septicemia in silkworm. One of the components of the parasporal crystal toxin of B. bombyseptieus, α-toxin, plays an important role in the process of infection in silkworm. In this study, we investigated the immune response of silkworm induced by α-toxin by using RNA-seq. We compared the changes in gene expression in the midgut, fatbody, and hemocytes of silkworm and in the B. mori embryonic cell line (BmE) after treatment with α-toxin and identified 952 differentially expressed genes and 353 differentially expressed long noncoding RNAs (lncRNAs). These regulated genes in different tissues were found to be enriched in different pathways. The upregulated genes in the midgut were mainly involved in peptidoglycan catabolic process and tyrosine kinase signaling pathway, whereas the downregulated genes were mainly involved in chitin metabolic pathways. The upregulated genes in fatbody were also involved in peptidoglycan catabolic process, but they were for a different peptidoglycan subtype. Further, genes encoding cecropins were enriched in the fatbody. The downregulated genes were mainly involved in the metabolic pathways of fundamental substances such as cellular protein metabolic process and nucleobase-containing compound metabolic process. These results suggest that α-toxin can induce various immune responses in silkworm, and further studies are warranted to understand the mechanism of α-toxin action in silkworm. Further, lncRNAs and differentially expressed genes were correlated using coexpression network analysis. Our findings revealed potential candidate genes and lncRNAs that might play important physiological functions in the immune response to α-toxins in silkworm.

Bacterial diversity obtained by culturable approaches in the gut of Glossina pallidipes population from a non sleeping sickness focus in Tanzania: preliminary results

BACKGROUND

Glossina pallidipes is a haematophagous insect that serves as a cyclic transmitter of trypanosomes causing African Trypanosomiasis (AT). To fully assess the role of G. pallidipes in the epidemiology of AT, especially the human form of the disease (HAT), it is essential to know the microbial diversity inhabiting the gut of natural fly populations. This study aimed to examine the diversity of G. pallidipes fly gut bacteria by culture-dependent approaches.

RESULTS

113 bacterial isolates were obtained from aerobic and anaerobic microorganisms originating from the gut of G. pallidipes. 16S rDNA of each isolate was PCR amplified and sequenced. The overall majority of identified bacteria belonged in descending order to the Firmicutes (86.6%), Actinobacteria (7.6%), Proteobacteria (5.5%)and Bacteroidetes (0.3%). Diversity of Firmicutes was found higher when enrichments and isolation were performed under anaerobic conditions than aerobic ones. Experiments conducted in the absence of oxygen (anaerobiosis) led to the isolation of bacteria pertaining to four phyla (83% Firmicutes, 15% Actinobacteria, 1% Proteobacteria and 0.5% Bacteroidetes, whereas those conducted in the presence of oxygen (aerobiosis) led to the isolation of bacteria affiliated to two phyla only (90% Firmicutes and 10% Proteobacteria). Phylogenetic analyses placed these isolates into 11 genera namely Bacillus, Acinetobacter, Mesorhizobium, Paracoccus, Microbacterium, Micrococcus, Arthrobacter, Corynobacterium, Curtobacterium, Vagococcus and Dietzia spp.which are known to be either facultative anaerobes, aerobes, or even microaerobes.

CONCLUSION

This study shows that G. pallidipes fly gut is an environmental reservoir for a vast number of bacterial species, which are likely to be important for ecological microbial well being of the fly and possibly on differing vectorial competence and refractoriness against AT epidemiology.

Silkworm model for Francisella novicida infection

Understanding the virulence and pathogenesis of human pathogens using insect models is an increasingly popular method. Francisella novicida, which is virulent in mice but non-pathogenic to immunocompetent humans, is widely used as an ideal candidate for Francisella research. In this study, we developed a silkworm (Bombyx mori) infection model for F. novicida by inoculating the hemocoels of silkworms with F. novicida. We found that silkworms died within 3-7 days of F. novicida infection. However, the deletion mutant of DotU, the core part of type VI secretion systems, failed to kill silkworm. In whole silkworm bodies, the bacterial load of the DotU deletion mutant was significantly less than that of the wild-type strain. Approximately 10-fold increase in bacterial load was recorded in hemolymph and subcutaneous tissues compared with that in the silk gland, Malpighian tubule, and reproductive organs. The CFU count of the DotU deletion mutant in all organs was similar results to the whole body CFU count. Confocal microscopy further confirmed the arrested growth of the mutant strain within hemocytes. The intracellular growth of F. novicida strains was also analyzed using the silkworm ovary-derived cell line BmN4. In BmN4, both CFU count assay and confocal microscopy revealed extensive growth of the wild-type strain compared with that of the mutant strain. Francisella DotU has already been proven as a virulence factor in mammals, and it was also found to be an essential virulence factor in our silkworm infection model. Therefore, this silkworm infection model is suitable for identifying new virulence factors of Francisella.

Effect of Vitamin A on Listeria monocytogenes Infection in a Silkworm Model

Insect infection models have been used increasingly to study various pathogenic agents in evaluations of pathogenicity and drug efficacy. In this study, we demonstrated that larvae of the silkworm Bombyx mori are useful for studying Listeria monocytogenes infections in insects. Infection with the L. monocytogenes wild-type strain induced silkworm death. Infection by a listeriolysin O (LLO) deletion mutant also induced silkworm death, but the bacterial numbers in silkworms were lower than those of the wild-type strain. Intracellular growth was observed when the silkworm ovary-derived cell line BmN4 was infected with the wild-type strain. Explosive replication was not observed in BmN4 cells infected with the LLO mutant and the bacterial numbers of the LLO mutant were lower than those of the wild-type strain. Pretreatment with vitamin A did not affect silkworm mortality after bacterial infection, but the efficiency of infecting the hemocytes and BmN4 cells was decreased with vitamin A treatment. Our results indicate that silkworm larvae are a useful insect infection model for L. monocytogenes and that vitamin A has protective effects against bacterial infection in silkworms.

Symbiosis with Francisella tularensis provides resistance to pathogens in the silkworm

Francisella tularensis, the causative agent of tularemia, is a highly virulent facultative intracellular pathogen found in a wide range of animals, including arthropods, and environments. This bacterium has been known for over 100 years, but the lifestyle of F. tularensis in natural reservoirs remains largely unknown. Thus, we established a novel natural host model for F. tularensis using the silkworm (Bombyx mori), which is an insect model for infection by pathogens. F. tularensis established a symbiosis with silkworms, and bacteria were observed in the hemolymph. After infection with F. tularensis, the induction of melanization and nodulation, which are immune responses to bacterial infection, were inhibited in silkworms. Pre-inoculation of silkworms with F. tularensis enhanced the expression of antimicrobial peptides and resistance to infection by pathogenic bacteria. These results suggest that silkworms acquire host resistance via their symbiosis with F. tularensis, which may have important fitness benefits in natural reservoirs.