Predomination of hypervirulent ST283 and genetic diversity of levofloxacin resistance in multidrug-resistant, hypervirulent Streptococcus agalactiae in Thailand

<b>Introduction</b>. Group B <i>Streptococcus</i> (GBS) is a multi-host pathogen causing pneumonia and meningitis in humans as well as streptococcal diseases in tilapia and mastitis in cattle. Thailand has experienced a significant increase in GBS infections that greatly impact health and economics.<b>Gap statement</b>. The antimicrobial resistance (AMR) and genotype data of GBS in Thailand are still limited and require further study.<b>Aim</b>. This study aimed to describe AMR profiles and molecular characteristics, especially antimicrobial resistance genes (ARGs) and virulence factor (VF) genes of GBS in Thailand.<b>Methodology</b>. AMR profiles of 221 GBS isolates from humans, fish and freshwater were examined. Whole-genome sequencing of 41 representative isolates was used to investigate capsular genotypes and sequence types (STs), ARGs and VF genotypes.<b>Results</b>. All GBS isolates were susceptible to penicillin; the majority (99.1%) showed resistance to tetracycline. In addition, the rates of resistance to clindamycin, erythromycin and levofloxacin were 22.6%, 20.4% and 2.3%, respectively; multidrug-resistant (MDR) isolates (TE-E-CM and TE-E-CM-LVX) were 19.5%. Among 41 representative isolates, the dominant types were capsular genotype III (63.4%) and ST283 (43.9%). ARGs associated with resistance to tetracycline (<i>tetM</i>, <i>tetO</i> and <i>tetS</i>), erythromycin (<i>ermB</i>, <i>ermA</i>, <i>mefA</i> and <i>msrD</i>) and clindamycin (<i>lsaC</i>, <i>lsaE</i> and <i>lnuB</i>) were identified. Additionally, point mutations responsible for levofloxacin resistance, S81L in GyrA, S79F/Y in ParC and H221Y in ParE, were found. The MDR isolates belonged to various STs, predominantly clustering in capsular types III (60.0%) and Ib (30.0%). The MDR-hypervirulent ST17 and ST19 harboured multiple ARGs and mutations affecting quinolone resistance. Different VF gene patterns were found among hypervirulent STs (ST12, ST17, ST19 and ST283). Notably, a unique nt deletion [c.(1013_1020)delG] in <i>pilA</i> was found only in ST283.<b>Conclusion</b>. This study elucidated significant antimicrobial characteristics of a substantial number of GBS in Thailand. Moreover, the distribution of the hypervirulent ST283 and the genotypes of MDR-hypervirulent GBS were first described.