Streptococcous parauberis infection in starry flounder, Platichthys stellatus: characterization of innate immune responses following experimental infection

Serotype distribution and antibiogram of Streptococcus parauberis isolated from fish in South Korea

Streptococcus parauberis is the dominant etiological agent of streptococcosis, the most devastating bacterial disease in the olive flounder farming industry in South Korea. In this study, the distribution of serotypes, antimicrobial susceptibility, and presence of antimicrobial resistance genes (ARGs) in S. parauberis isolates obtained between 1999 and 2021 was thoroughly investigated to gain insight into the dynamics of their presence and the relationship between serotypes and antimicrobial resistance. Disk diffusion testing of 103 isolates against 10 antimicrobial agents was performed, and epidemiological cut-off values generated through normalized resistance interpretation analysis were used to classify wild-type (WT) and non-wild-type (NWT) populations. Principal component analysis and hierarchical clustering were implemented to achieve an understanding on the relationship between serotypes and antimicrobial resistance patterns. PCR-based serotyping showed that serotype Ia (67.1%) was the most prevalent in South Korea, followed by serotypes Ib/Ic (25.2%) and II (7.7%). The highest proportion of isolates was assigned to NWT against amoxicillin (80.6%), followed by oxytetracycline (77.7%) and erythromycin (48.5%). The time-scale data showed that recently obtained serotypes Ib/Ic and II isolates tended to be categorized as NWT populations resistant to more antibiotics, possibly due to microbial adaptation to antibiotic pressure. ARGs responsible for resistance to oxytetracycline and erythromycin were found only in NWT populations in serotype Ia [tet(S) and erm(B), respectively], and serotype II [tet(M) and mef(J)-msr(I), respectively]. We also found that the mef-msr gene pair in S. parauberis serotype II might be involved in low-level resistance to erythromycin. IMPORTANCE This study presents serotype distribution and antimicrobial susceptibility data along with the antimicrobial resistance genes (ARGs) of Streptococcus parauberis, which is an important bacterial fish pathogen worldwide. In particular, almost all oxytetracycline and erythromycin non-wild-type (NWT) populations harbored tet(S) or tet(M), and erm(B) or mef(J)-msr(I), respectively. Interestingly, these ARGs were distributed in a highly serotype-dependent manner, resulting in a clear correlation between the antibiogram and serotype distribution. Moreover, recent isolates belonging to serotypes Ib/Ic and II tended to be more frequently categorized as NWT against antimicrobials, including amoxicillin and cefalexin compared to old isolates, while a dramatic decrease in erythromycin and clindamycin NWT frequencies was observed in recent serotype Ia isolates, which lacked erm(B). These variations might be attributed to shifts in the antibiotics employed in South Korean aquaculture over time. The overall findings would provide important background knowledge for understanding the epidemiology of S. parauberis infection in aquaculture.

Complete Genome Sequences of Streptococcus parauberis Serotype Ia, Ib/Ic, and II Isolates from Diseased Olive Flounder in South Korea

We report the complete genome sequences of three isolates of Streptococcus parauberis, representing serotypes Ia, Ib/Ic, and II, which were isolated from diseased olive flounder (Paralichthys olivaceus) in South Korea.

Molecular insight into regulation of miRNAs in the spleen of zebrafish (Danio rerio) upon pathogenic Streptococcus parauberis infection

MicroRNAs (miRNAs) constitute a group of small non-coding RNAs (~22 nucleotides) and one of their main functions is to regulate the immune responses. Gram-positive bacterium, Streptococcus parauberis is the main causative agent of "Streptococcosis" in wide range of fish species. In this study, we performed high throughput sequencing analysis to identify the miRNA profile against S. parauberis infection in the spleen of zebrafish (Danio rerio). Overall, 349 known and 151 novel miRNAs were discovered. Among them, 12 known miRNAs (dre-miR-34b, dre-miR-135a, dre-miR-200b-5p, dre-miR-146b, dre-miR-31, dre-miR-17a-3p, dre-miR-222a-3p, dre-miR-731, dre-miR-301b-3p and dre-miR-30a-3p) and 9 novel miRNAs were differentially expressed (DE) in the spleen of S. parauberis challenged zebrafish. The identified 12 DE miRNAs were predicted to regulate 721 target genes. We confirmed the miRNA expression results by validating selected known and novel DE miRNAs using qRT-PCR. Gene Ontology (GO), Kyoto Encyclopedia of Genes (KEGG) pathway analysis and miRNA-mRNA interactions implies that specific target genes of DE miRNAs are associated with immune responses. The enriched pathways included Toll-like receptor (TLR), C-type lectin, NOD-like receptor, and RIG-I-like receptor signaling pathways, etc. Especially, dre-miR-200b-5p, dre-miR-146b, dre-miR-731, dre-miR-222a-3p, and dre-miR-34b were able to target potential immune-related genes such as il10, irak1, traf6, hspa8 and ikbke upon S. parauberis challenge. Thus, overall results could lay a foundation to understand the underlying immune regulatory role of miRNAs in response to pathogenic S. parauberis infection in teleosts.

Current use and management of commercial fish vaccines in Korea

The aquaculture industry in Korea has grown rapidly since the 1960s, and it is a major food source. However, the expansion of aquaculture systems has increased the chances of infectious disease outbreaks, and vaccination plays an important role in commercial fish farming. This is the first comprehensive review of commercial fish vaccines in Korea. It not only provides an overview of commercially available fish vaccines and their associated approval processes and laws, but also some perspectives on research advances regarding fish vaccines in Korea. In Korea, fish vaccines are approved only after their safety and effectiveness have been verified according to the Pharmaceutical Affairs Act, and after approval, each vaccine lot must pass the national evaluation criteria. As of the end of 2019, 29 vaccines were approved for 10 fish pathogens, including both single and combination vaccines containing more than two inactivated pathogens. The approved fish vaccines consist of 2 immersion vaccines, as well as 1 intramuscular and 26 intraperitoneal vaccines, which require syringe injection. All the 29 vaccines are manufactured as formalin-inactivated vaccines; 1 is an adjuvant vaccine and 28 are non-adjuvant vaccines; 25 are bacterial vaccines, 2 are viral vaccines, 1 is a parasite vaccine, and 1 is a parasite and bacterial vaccine. In terms of the target fish species, 27 vaccines are used in the olive flounder (Paralichthys olivaceus), 1 in the starry flounder (Platichthys stellatus), and 1 in the red seabream (Pagrus major), striped beakfish (Oplegnathus fasciatus), and amberjack (Seriola quinqueradiata). This imbalance exists mostly because the olive flounder is the main farmed fish species in Korea. In 2018, 67.71 million vaccine doses were distributed following satisfactory performance in the national evaluation. They were used to vaccinate approximately 80.6% of farmed olive flounders.

The correlations data between whether liposomes are coated or not and flatfish's (paralichthys olivaceus) Streptococcus parauberis antibody formation ability and survival rates

Streptococcus parauberis is a known etiologic agent that causes damage leading to death in flatfish (paralichthys olivaceus). Liposomes were used to deliver streptococcal oral vaccines to the intestinal mucous membranes of paralichthys olivaceus. The liposomes were coated for stabilization, and stability was measured with high performance liquid chromatography (HPLC). The liposomes were stable until day nine and were orally administered to flatfish as a vaccine. The resultant antibody titers were analyzed. The titers resulting from the uncoated liposomes were highest two weeks after the oral administration, and those resulting from the coated liposomes were highest one week after boosting. In addition, the bacteria were subcutaneously injected to artificially infect flatfish and the survival rates and relative survival rates were analyzed. The coated liposomes were found to yield the highest survival rate.

Molecular characterization of a hepcidin homologue in starry flounder (Platichthys stellatus) and its synergistic interaction with antibiotics

Hepcidins are small cysteine-rich antimicrobial peptides that play an important role in host immunity against pathogenic organisms. Most fish hepcidins exert bactericidal activities against a wide range of pathogens. In this study, we identified a cDNA sequence encoding a hepcidin homologue (PsHepcidin) in the starry flounder Platichthys stellatus. The predicted amino acid sequence of PsHepcidin comprises a signal peptide and a prodomain, which are followed by the mature peptide. Sequence analysis revealed that PsHepcidin belongs to the fish HAMP2 cluster and that it is closely related to mudskipper hepcidin-2. Expression of PsHepcidin mRNA was detected in all examined immune-related tissues, with the highest transcript levels being found in the liver. In response to lipopolysaccharide treatment, PsHepcidin was significantly up-regulated in the liver, kidney, and spleen in a time-dependent manner. Chemically synthesized mature peptides of PsHepcidin were found to exhibit broad antimicrobial activity in vitro. We also investigated the combined effect of PsHepcidin and conventional antibiotics and found that these combinations showed synergistic effects against most of the examined bacterial strains. Collectively, the results of this study indicate that PsHepcidin exhibits potent antibacterial activity both independently and when used in combination with conventional antibiotics.

Characterization and identification of streptococci from golden pompano in China

Streptococcal infections cause significant mortality and high economic losses in the fish farm industry worldwide, including in the culture of golden pompano Trachinotus ovatus L., a species gaining popularity in China. A total of 9 streptococcal strains were isolated from cage-cultured diseased golden pompano in Beihai, Zhanjing, and Shenzhen, China, between 2012 and 2014. Conventional and rapid identification systems were used to determine that the isolates were Streptococcus agalactiae, S. iniae, and S. dysgalactiae subsp. dysgalactiae. All isolates were gram-positive cocci cells in pairs or short-chain, non-motile, catalase negative, α or β hemolytic cocci. The results of multiplex PCR assays and 16S rRNA BLAST analysis also showed that the β hemolytic strains were S. agalactiae and S. iniae and the α hemolytic strain was S. dysgalactiae subsp. dysgalactiae, respectively. Pathogenicity assays revealed that S. agalactiae (lethal dose [LD50]: 6.38 × 10(4) CFU ml(-1)) was more virulent for golden pompano than S. iniae (LD50: 1.47 × 10(7) CFU ml(-1)) and S. dysgalactiae subsp. dysgalactiae (LD50: 2.57 × 10(6) CFU ml(-1)) when they were challenged by intraperiotoneal (i.p.) injection. The results of antibiotic susceptibility showed that all strains were extremely susceptible to cefradine, erythromycin, and cefotaxime but resistant to gentamicin, penicillin G, novobiocin, neomycin, ciprofloxacin, roxithromycin, furazolidone, enrofloxacin, norfloxacin, kanamycin, ampicillin, tetracycline, and vancomycin This is the first report of a phenomenon of golden pompano coinfection with S. agalactiae and S. iniae, which will contribute to the diagnosis and prevention of streptococcicosis.