The Chinese mitten crab, Eriocheirsinensis, is an important farmed crustacean species in China, outranking other farmed crabs in yield and economic importance. An infection called “milky disease”, caused by the yeast, Metschnikowiabicuspidata, has emerged in E. sinensis farms in northeast China and has caused progressive economic losses. The diseased crabs present with opaque, whitish muscles and milky hemolymph. Currently, there are no effective drugs to treat the infection. Clarifying the transmission route of M. bicuspidata would help to treat and prevent the disease. We investigated the effects of three different M. bicuspidata infection methods (feeding, immersion, and cohabitation) on E. sinensis. All three infection methods led to a high infection rate in healthy crabs. After 35 d, the infection rate was 76.7%, 66.7%, and 53.3% in the feeding, immersion, and cohabitation groups, respectively. Diseased crabs exhibited the typical symptom of hemolymph emulsification, with a high pathogen load of M. bicuspidata. The yeast was not detected in the oocytes of infected crabs. Fertilized embryos, zoea larvae, and megalopae of infected ovigerous crabs tested negative for yeast, indicating that direct transmission from mother to offspring does not occur. Our results highlight avenues for the prevention and control of this yeast.

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.

Metschnikowia bicuspidata, a globally distributed opportunistic pathogenic fungus, poses a significant threat to crustaceans in diverse aquatic ecosystems, causing severe diseases. Iron, recognized as a virulence factor, plays a crucial role in successful infection with M. bicuspidata. Therefore, this study aims to investigate the transcriptome response of M. bicuspidata to low- and high-iron conditions. Overall, 1082 differentially expressed genes (DEGs) (FDR < 0.05, |log₂FC ≥ 1.5|) were identified, comprising 977 and 105 DEGs, in response to low- and high-iron conditions, respectively. These genes predominantly participate in altering metabolism, cell membranes, or cellular structure, allowing the organism to adapt to varying iron levels. Iron limitation-induced genes play crucial roles in energy metabolism, transport, and catabolism pathways. Moreover, 27 ortholog genes were associated with iron transport and homeostasis, with 7 of them participating in iron uptake and regulation under low-iron conditions. This study contributes to the comprehension of iron homeostasis in aquatic fungi. It may offer potential therapeutic strategies for managing M. bicuspidata diseases.

Pathogenic microorganisms, such as Metschnikowia bicuspidata, cause significant issues for the sustainable development of aquaculture. While natural antimicrobial agents show promise, their stability and target range are limited. For instance, berberine exhibits antibacterial activity by inhibiting ergosterol synthesis in fungi and inducing ROS production, but its poor solubility and low membrane penetration hinder its application. Carbon dots (CDs), with high surface area, tunable chemistry, and good biocompatibility, offer unique advantages for drug delivery. This study synthesized berberine CDs (B-CDs) and evaluated their antibacterial activity against M. bicuspidata. B-CDs were prepared via hydrothermal synthesis and exhibited excellent nanostructures, dispersibility, and biocompatibility. The minimum inhibitory concentration of B-CDs was 0.031 mg/mL, lower than berberine alone (1 mg/mL). Transcriptomic analysis revealed that B-CDs disrupted energy metabolism by downregulating key glycolytic enzymes. Gene ontology and KEGG enrichment analyses indicated that differentially expressed genes were involved in carbohydrate metabolism and signaling pathways. In conclusion, B-CDs improve solubility, cellular uptake, and ROS generation, enhancing antibacterial effects. This nanomaterial shows promise for sustainable pathogen control in aquaculture.