Experimental Transmission of the Yeast, Metschnikowia bicuspidata, in the Chinese Mitten Crab, Eriocheir sinensis

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.

Development of a TaqMan real-time quantitative PCR assay to detect Metschnikowia bicuspidata in Chinese mitten crab Eriocheir sinensis

Milky disease of Chinese mitten crab Eriocheir sinensis caused by Metschnikowia bicuspidata is a novel disease with high mortality. No effective treatment is currently available, but a rapid, accurate detection method is required for the prevention and control of the disease. In this study, the genome-sequencing results of M. bicuspidata and similar species were used for comparative genomic analysis for genes specific to M. bicuspidata. A quantitative PCR (qPCR) detection method for M. bicuspidata was then established using the specific primers and probes designed according to the sequence of a hypothetical protein gene specific to M. bicuspidata. The assay was found to have a high degree of repeatability and reproducibility, with a linear dynamic range (R2 = 0.998) extending over 9 log10 dilutions and a high efficiency (100.7%). Furthermore, the method showed high sensitivity, being able to detect at least 11.3 copies μl-1 of recombinant plasmid, and strong specificity, without any cross-reaction with any of the 9 species of yeast that are closely related to M. bicuspidata or any of 16 species of pathogenic bacteria commonly observed in aquatic animals. The established method was used to examine 138 apparently healthy crabs collected from 22 farms, with 21 samples (15.2%) found to be M. bicuspidata-positive. Thus, the developed qPCR assay is a specific, sensitive, stable, and rapid diagnostic method for the detection and quantification of M. bicuspidata DNA from E. sinensis tissues.

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.