First report of Metschnikowia bicuspidata infection in Chinese grass shrimp (Palaemonetes sinensis) in China

Dynamic pathology and pathogen distribution of the yeast Metschnikowia bicuspidata in the Chinese mitten crab (Eriocheir sinensis)

In this study, the "milky disease" model of Eriocheir sinhensis was constructed via intramuscular injection with the pathogenic yeast Metschnikowia bicuspidata. The dynamic pathological changes of E. sinensis after injection were elucidated with two staining methods (haemotoxylin-eosin and alcian blue periodic acid-Schiff) and fluorescence in situ hybridization technology. Anatomical observation revealed three stages of the "milky disease": no clinical signs (1-4 days after infection), the appearance of signs of disease (5-7 days), and significant liquefaction (10 days). Histological observation also revealed three stages of the disease: yeast diffusion (1-2 days after infection), yeast slow development (3-4 days), and yeast rapid proliferation (5 days). And FISH technique was suitable for the early detection of infection with M. bicuspidata in E. sinensis. We found that M. bicuspidata spread to the whole body of the crab through the haemolymph and developed into fungal septicaemia. These results elucidate the systemic pathological characteristics of "milky disease" in E. sinensis and suggest the pathogenic mechanism of M. bicuspidata.

First report of Metschnikowia bicuspidata infection in the oriental river prawn (Macrobrachium nipponense, de Haan) in China

During breeding, some oriental river prawns (Macrobrachium nipponense, de Haan), an important aquaculture species in China, exhibit yellowish-brown body colouration, reduced appetite, and vitality. Diseased prawns revealed characteristic emulsifying disease signs, including whitened musculature, hepatopancreatic tissues, milky haemolymph, and non-coagulation. The present study investigated the causative agent of M. nipponense infection through isolation, histopathology, molecular sequencing, and infection experiments. The pathogenic strain exhibited distinctive white colonies on Bengal red medium, with microscopic examination confirming the presence of yeast cells. Histopathological analysis revealed prominent pathological alterations and yeast cell infiltration in muscles, hepatopancreas and gills. Additionally, 26S rDNA sequencing of the isolated yeast strain LNMN2022 revealed Metschnikowia bicuspidata (GenBank: OR518659) as the causative agent. This strain exhibited a 98.28% sequence homology with M. bicuspidata LNMB2021 (GenBank: OK094821) and 96.62% with M. bicuspidata LNES0119 (GenBank: OK073903). The pathogenicity test confirmed that M. bicuspidata elicited clinical signs in M. nipponense consistent with those observed in natural populations, and the median lethal concentration was determined to be 3.3 × 105  cfu/mL. This study establishes a foundation for further investigations into the host range and epidemiological characteristics of the pathogen M. bicuspidata in aquatic animals and provides an empirical basis for disease management in M. nipponense.

Establishment and Application of Real-Time Fluorescence Quantitative PCR Detection Technology for Metschnikowia bicuspidata Disease in Eriocheir sinensis

Metschnikowia bicuspidata causes a "milky disease" in Chinese mitten crab, Eriocheir sinensis, which inflicts significant damage on the breeding industry, but there are no effective drugs for this disease. Precise detection technologies and clarification of transmission routes are now essential to prevent its occurrence. A real-time fluorescent quantitative PCR (qPCR) detection method targeting the mitochondrial cytochrome c oxidase subunit VIA (COX6A) of M. bicuspidata was developed and its sensitivity, specificity, repeatability, and application effectiveness evaluated. There was a robust linear relationship between the qPCR threshold cycle value (Ct) and copy number of the standard with a wide dynamic range. The standard curve had a correlation coefficient (R2) of 0.996, amplification efficiency of 103.092%, and a lower limit of detection sensitivity of 7.6 × 101 copies/µL. The COX6A-qPCR method exhibited high specificity for the detection of M. bicuspidata, with no cross-reactivity. The intra- and inter-group variation coefficients were <1% and 2%, respectively. The qPCR exhibited superior sensitivity compared to existing detection methods, with a positivity rate of 76.67%. The M. bicuspidata content ranged from 1.0 × 101-2.7 × 106 copies/µL. The COX6A-qPCR detection technology exhibited high sensitivity, strong specificity, and excellent repeatability, enabling the accurate quantification of M. bicuspidata.

Transcriptome analysis of hepatopancreas of Chinese grass shrimp, Palaemonetes sinensis, infected by Enterocytospora artemiae

The emergence of the microsporidian, Enterocytospora artemiae, has caused serious economic losses to the aquaculture industry of Palaemonetes sinensis. The hepatopancreas is the main digestive and immune organ of P. sinensis, and the main site of E. artemiae infection. We used next-generation sequencing to determine the effects of E. artemiae parasitism on the hepatopancreas of P. sinensis at the transcriptome level. The hepatopancreas of P. sinensis was parasitized by E. artemiae, and 881 differentially expressed genes (DEGs) were obtained, of which 643 were upregulated and 238 were downregulated. These DEGs are mainly involved in DNA replication, transcription, translation, immunity, and metabolism. Among them, the cellular processes of DNA replication, transcription and translation are significantly strengthened, which may be related to the use of host ATP and nucleic acid by E. artemiae to achieve proliferation and damage to host cells to enhance DNA replication and repair. Moreover, to defend against E. artemiae, some immune genes related to antioxidation, such as glutathione metabolism, seleno compound metabolism, and cytochrome p450 2L1, were significantly upregulated, but simultaneously, tumor necrosis factor, NF-κB inhibitor α, and other immune-related genes were significantly down regulated, indicating that the parasitism of E. artemiae led to a significant decline in the immune defense ability of P. sinensis. From the perspective of metabolism, the metabolism-related DEGs of retinol, glycine, serine, and threonine metabolism, were significantly downregulated, resulting in insufficient nutrient absorption and decreased energy supply of the P. sinensis, which in turn affected their growth. The differential genes and pathways identified in this study can provide a reference basis to further elucidate the pathogenic mechanism of P. sinensis infected with E. artemiae and the prevention and control of microsporidia disease.

Metabolomic and metagenomic analyses of the Chinese mitten crab Eriocheir sinensis after challenge with Metschnikowia bicuspidata

Milky disease caused by Metschnikowia bicuspidata fungus has significantly harmed the Chinese mitten crab Eriocheir sinensis aquaculture industry. However, the effect of M. bicuspidata infection on the metabolism and intestinal flora of the crab remains unclear. In this study, we aimed to explore the changes in the metabolism and intestinal flora E. sinensis after 48 h of infection with M. bicuspidata, using metabolomic and metagenomic analyses. Metabolomic analysis results revealed 420 significantly different metabolites between the infected and control groups, and these metabolites were enriched in 58 metabolic pathways. M. bicuspidata infection decreased the levels of metabolites related to amino acid biosynthesis, the tricarboxylic acid cycle, as well as lysine, histidine, linolenic, arachidonic, and linoleic acid metabolism. These results indicated that M. bicuspidata infection significantly affected the energy metabolism, growth, and immunity of E. sinensis. The results of metagenomic analysis showed that the anaerobes and ascomycetes populations significantly increased and decreased, respectively, after M. bicuspidata infection. These changes in intestinal flora significantly upregulated metabolic and synthetic pathways while downregulating immunity-related pathways. The results of integrated metabolomic and metagenomic analyses showed that 55 differentially expressed genes and 28 operational taxonomic units were correlated with 420 differential metabolites. Thus, the intestinal flora changes caused by M. bicuspidata infection also affected the metabolites. This study provides novel insights into the metabolic-and intestinal microflora-based effects of M. bicuspidata infection in E. sinensis, as well as a theoretical basis for the interaction between fungi and crustaceans.

Development of a nested PCR assay for specific detection of Metschnikowia bicuspidata infecting Eriocheir sinensis

In recent years, the “milky disease” caused by Metschnikowia bicuspidata has seriously affected the Eriocheir sinensis culture industry. Discovering and blocking the transmission route has become the key to controlling this disease. The existing polymerase chain reaction (PCR) detection technology for M. bicuspidata uses the ribosomal DNA (rDNA) sequence, but low sensitivity and specificity lead to frequent false detections. We developed a highly specific and sensitive nested PCR method to detect M. bicuspidata, by targeting the hyphally regulated cell wall protein (HYR) gene. This nested HYR-PCR produced a single clear band, showed no cross-reaction with other pathogens, and was superior to rDNA-PCR in specificity and sensitivity. The sensitivity of nested HYR-PCR (6.10 × 10¹ copies/μL) was greater than those of the large subunit ribosomal RNA gene (LSU rRNA; 6.03 × 10⁴ copies/μL) and internal transcribed spacer (ITS; 6.74 × 10⁵ copies/μL) PCRs. The nested HYR-PCR also showed a higher positivity rate (71.1%) than those obtained with LSU rRNA (16.7%) and ITS rDNA (24.4%). In conclusion, we developed a new nested HYR-PCR method for the specific and sensitive detection of M. bicuspidata infection. This will help to elucidate the transmission route of M. bicuspidata and to design effective management and control measures for M. bicuspidata disease.

Comparative Genomic Analyses Provide Insight Into the Pathogenicity of Metschnikowia bicuspidata LNES0119

Metschnikowia bicuspidata is a globally distributed pathogenic yeast with a wide range of aquatic hosts. A new strain, M. bicuspidata LNES0119, isolated from the Chinese mitten crab Eriocheir sinensis, has caused a serious reduction in production and marked economic loss for the aquaculture industry in China. Therefore, the whole-genome sequence of M. bicuspidata LNES0119 was sequenced using Illumina and Oxford Nanopore technology; whole-genome annotation and comparative genomic analyses of this pathogen were performed as well. A high-quality genome of M. bicuspidata LNES0119 was 16.13 Mb in size, with six scaffolds and six contigs, and encoded 5,567 putative predicted genes. Of these, 1,467 genes shared substantial homology with genes in the pathogen–host interactions database. Comparative genomic analyses of three M. bicuspidata strains and one non-pathogenic yeast, M. aff. pulcherrima, showed 331 unique genes in M. bicuspidata LNES0119, 30 of which were putatively related to pathogenicity. Overall, we identified several meaningful characteristics related to pathogenicity and virulence that may play essential roles in the infection and pathogenicity of M. bicuspidata LNES0119. Our study will aid in identifying potential targets for further exploration of the molecular basis of the pathogenicity of M. bicuspidata as well as the therapeutic intervention of M. bicuspidata infection.