Abstract
In this study, whole-genome sequencing was performed on 139 S. suis isolates,
comprising 30 isolates from clonal complex (CC) 1, 40 from CC104, 40 from CC233, 15 from
CC25, 7 from CC28, and 7 from CC94. All isolates were successfully subjected to whole-genome
sequencing. Genomic analysis within each clonal complex revealed that CC1 consisted of 16
ST1 isolates, 2 ST11, 10 ST105, and 2 ST237. CC104 comprised 40 ST104 isolates. CC233 included
25 ST233, 6 ST379, 8 ST1656, and 1 ST1713. CC25 consisted of 11 ST25 and 4 ST103 isolates.
CC28 comprised 7 ST28 isolates, while CC94 included 4 ST94 and 3 ST1689 isolates.
Phylogenetic analysis indicated that CC233 evolved from CC104. Both clonal complexes
are more closely related to CC25, CC28, and CC94 than to CC1, which originated from a distinct
lineage. Notably, CC104 and CC233 appear to be unique to Thailand and likely emerged in the
20th century, around 1978 for CC104 and 1997 for CC233. Comparative genomic analysis of S.
suis serotype 2 across CC1, CC25, CC28, CC104, and CC233 identified 13, 194, 241, 208, and 214
clonal complex–specific genes, respectively. These genes may serve as potential markers for
predicting clonal complex classification in the future.
Among S. suis isolates from Thai patients, the dominant antimicrobial resistance genes
were tetO (tetracycline resistance) and ermB (macrolide resistance). Notably, CC104 and CC233
harbored the vgaLC gene, conferring resistance to pleuromutilins. In this study, 36.7% of isolates
were non-susceptible to penicillin, with this phenotype most strongly associated with CC233
and CC104. Furthermore, concatenated amino acid sequences of PBP2B, PBP2X, and MraY were
able to distinguish between beta-lactam non-susceptible and susceptible clusters.
