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Abstract
Streptococcus parauberis is an important bacterial fish pathogen that causes streptococcosis in a variety of fish species including the olive flounder. Despite its importance in the aquaculture industry, little is known about the survival strategy of S. parauberis in the host. Therefore, the objective of this study was to produce genome-wide transcriptome data and identify key factors for the survival of S. parauberis SPOF3K in its host. To this end, S. parauberis SPOF3K was incubated in olive flounder serum and nutrient-enriched media as a control. Although S. parauberis SPOF3K proliferated in both culture conditions, the transcriptomic patterns of the two groups were very different. Interestingly, the expression levels of genes responsible for the replication of an S. parauberis plasmid in the presence of olive flounder serum were higher than those in the absence of olive flounder serum, indicating that this plasmid may play an important role in the survival and proliferation of S. parauberis in the host. Several ATP-binding cassette transporters known to transport organic substrates (e.g., biotin and osmoprotectants) that are vital for bacterial survival in the host were significantly up-regulated in S. parauberis cultured in serum. In addition, groEL, dnaK operon, and members of the clp protease family, which are known to play important roles in response to various stressors, were up-regulated in S. parauberis incubated in serum, thus limiting damage and facilitating cellular recovery. Moreover, important virulence factors including the hyaluronic acid capsule (has operon), sortase A (srtA), C5a peptidase (scp), and peptidoglycan O-acetyltransferase (oatA) were significantly upregulated in S. paraubers in serum. These results indicate that S. paraubers can resist and evade the humoral immune responses of fish. The transcriptomic data obtained in this study provide a better understanding of the mode of action of S. parauberis in fish.
Highlights
Streptococcosis is one of the most important bacterial diseases in a number of fish species worldwide [1]
While bacteria continued to grow in the culture medium (~ 2-fold increase at 4 hpe), the viability was maintained in the serum for 4 h. This result shows that S. parauberis SPOF3K is able to survive and persist in fish serum containing various antimicrobial components [17]
In the SPOF3K genome, we identified a seven-gene cluster including BC synthase, PNAG synthase, and diguanylate synthase/phosphodiesterase, which is known to modulate the concentration of cyclicdi-GMP, a positive regulator of bacterial cellulose (BC) synthesis [68]
Summary
Streptococcosis is one of the most important bacterial diseases in a number of fish species worldwide [1]. It has been revealed that the heart and brain are the major pathological target organs of S. parauberis; the microbe can cause pericarditis and/or meningitis, leading to mortality, in the olive flounder [7]. Despite these academic findings and the many practical applications of research in the field, Streptococcosis remains the most common and threatening bacterial disease in South Korean aquaculture farms [2]. Several genomic studies have been done on S. parauberis derived from a variety of fish species including the olive flounder and the data is being made publicly available [8, 9]. No efforts have yet been made to identify the pathogenic mechanism or key virulence factors of S. parauberis
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