ABSTRACT
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.
ABSTRACT
The oomycete Aphanomyces invadans causes epizootic ulcerative syndrome (EUS), a World Organization for Animal Health (WOAH)-listed disease that has seriously impacted a wide range of fish worldwide. Currently, only three conventional polymerase chain reaction (PCR) assays are recommended for the detection of A. invadans. The robust quantitative PCR (qPCR) assay has recently become more important due to its highly accurate nature and the applicability of qPCR-based environmental DNA (eDNA) detection in the monitoring of pathogens in aquatic environments. Therefore, in this study, we developed a novel TaqMan probe-based qPCR method to sensitively and quantitatively detect A. invadans. The assay limit of detection was determined using 10-fold serial dilutions of linearized A. invadans plasmid. Assay sensitivity was assessed in the presence of interfering substances and compared to three WOAH-listed primers using the mycelia and zoospores of A. invadans with and without fish muscle tissue. The assay specificity was also theoretically and experimentally assessed against other oomycetes, fish muscle tissue, and water samples. The assay's repeatability and reproducibility were determined. In this study, the limit of detection of the developed assay was 7.24 copies of A. invadans genomic DNA per reaction (95% confidence interval (CI): 2.75 to 19.05 copies/reaction). The assay showed the same sensitivity in the presence of other substances. Compared to the WOAH-recommended PCR assays, this assay had 10-times higher sensitivity for all tested samples. There were no cross-reactions with other closely related oomycetes, fish muscle, or water samples, indicating that the assay was highly specific for A. invadans. The repeatability and reproducibility tests showed little variation, ranging from 0.1-0.9% and 0.04-1.1%, respectively, indicating the high consistency, repeatability, and reliability of the developed assay. This highly rapid, sensitive, specific, and consistent EUS qPCR assay would be of importance in transboundary disease management and the monitoring of pathogens in aquatic environments.
Subject(s)
Aphanomyces , Fish Diseases , Oomycetes , Animals , Aphanomyces/genetics , Reproducibility of Results , Fishes , Water , Fish Diseases/diagnosisABSTRACT
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.
ABSTRACT
Aeromonas hydrophila is a Gram-negative pathogen that is associated with motile aeromonad septicemia in various fish species. Here, we report the complete genomic sequences of two A. hydrophila isolates derived from diseased fish in South Korea.
