Serotype distribution and antibiogram of
Streptococcus parauberis
isolated from fish in South Korea
Authors: Lee Yoonhang , Kim Nameun , Roh HeyongJin , Ho Diem Tho , Park Jiyeon , Lee Ju Yeop , Kim Yoon-Jae , Kang Hyo-Young , Lee Jungmin , Song Jun-Young , Kim Ahran , Kim Myoung Sug , Cho Miyoung , Choi Hye Sung , Park Chan-Il , Kim Do-Hyung 
Published: August 2023

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.