In August 2022, a large - scale mortality event of yellow catfish occurred in a fish farm in Yichang City, Hubei Province. Researchers isolated a pathogenic bacterium from the diseased yellow catfish. Through morphological, physiological, and biochemical characterizations, 16S rRNA gene sequencing, and phylogenetic analysis, the bacterium was identified as Streptococcus iniae.

By performing whole - genome sequencing of this Streptococcus iniae strain, researchers determined that it harbored a circular chromosome with a length of 1,776,777 bp and a GC content of 37.14%. Genome sequence analysis revealed that this bacterial strain contained 204 virulence - related genes and 127 antibiotic - resistance genes. Further experiments demonstrated that this strain of Streptococcus iniae was highly pathogenic, with a median lethal dose (LD50) of 9.53×10⁵ CFU/g. Histopathological examinations indicated that, following infection with this strain, multiple organs of the yellow catfish, including the skin, gills, fins, spleen, liver, kidneys, intestines, eyes, and brain, exhibited extensive cell degeneration, necrosis, hemorrhage, and inflammatory responses.

After infection, the activities of innate immune enzymes such as acid phosphatase and alkaline phosphatase in the serum of yellow catfish increased significantly at 24 and 48 hours post - infection, and then decreased at 168 hours. Additionally, the research team detected the transcription of immune - related genes in yellow catfish at different time points post - infection. They found that the transcription levels of these genes were significantly upregulated, suggesting that the host's immune system played a crucial role in resisting S. iniae infection.

This study not only elucidated that S. iniae was the direct cause of the large - scale death of yellow catfish but also enhanced the understanding of the pathogenic mechanism of S. iniae invasion and the host defense system. It provided an important theoretical foundation for subsequent disease prevention and control efforts.

Application of MGC Series Microbial Growth Curve Analyzer

To gain a better understanding of the growth characteristics of S. iniae, the research team employed the MGC Series Microbial Growth Curve Analyzer MGC - 200 for automated detection of the growth curve.

S. iniae was cultured in BHI medium until the optical density at 600 nm (OD600) reached 1.0. Subsequently, the bacterial suspension was inoculated into BHI media with different pH values (5.5, 6.0, 6.5, 7.0, 7.5, 8.0, and 8.5) and different NaCl concentrations (0.5%, 1.0%, 1.5%, 2.0%, 2.5%, and 3.0%) at a ratio of 1:100. The inoculated media were cultured in 96 - well plates (200 μL per well), and the OD600 was measured every 2 hours for a total of 60 hours.

As shown in Figure A, the isolated Streptococcus dolphinus grows best between pH 6.5 and 7.5, with the optimum pH being around 7. When the pH is 6.0, the bacterial growth hysteresis period is prolonged, when the pH is 5.5, the bacterial growth hysteresis period is significantly prolonged, and when the pH is 8.5, the bacterial growth is inhibited.

As shown in Figure B, the isolated Streptococcus dolphinus can grow normally at 0.5-1.5% NaCl concentration, and the optimal NaCl concentration is 1.0. When the NaCl concentration reaches 2.0%, the bacterial growth hysteresis period is significantly prolonged, and when the NaCl concentration reaches 3.0%, the bacterial growth stops.

Conclusion

The MGC series microbial growth curve analyzer has the functions of gas environment monitoring, high-throughput cultivation, fully automatic detection, and real-time display of growth curves. The whole process does not require frequent manual sampling or personnel on duty, which greatly reduces the risk of contamination, truly realizes the liberation of manpower and the improvement of research efficiency, and will become your right-hand man on the road of scientific research.

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