Penicillin-nonsusceptible Streptococcus suis infections in humans: clinical characteristics and antimicrobial synergy testing

Antimicrobial resistance in Streptococcus suis represents an increasing global health concern, particularly in Thailand, where human and swine infections remain common and severe. A descriptive study was conducted in northeastern Thailand from January 2020 to December 2024 to examine the association between clinical characteristics and penicillin susceptibility (susceptible, intermediate, resistant) among S. suis isolates. In vitro antibiotic interactions were assessed by checkerboard microdilution in 23 drug-resistant isolates. Among 373 culture-confirmed S. suis cases, 161 isolates (43.2%) were analyzed for serotype and clonal complex (CC); serotype 2 predominated (95.0%), and CC1 was most common (30.4%). Most patients were male (70.2%), with a mean age of 56.9 ± 13.8 years and median illness duration of 3 days (IQR, 1–10). Bacteremia was the predominant clinical manifestation, followed by infective endocarditis (IE) and meningitis. Penicillin resistance increased from 7.1% in 2020 to 57.8% in 2024, whereas ceftriaxone resistance remained infrequent. Of 373 isolates, 62 were penicillin-susceptible, 225 intermediate, and 86 resistant. Meningitis was more frequently observed in penicillin-susceptible infections, whereas IE predominated in penicillin-nonsusceptible infections (P < 0.001). The overall case fatality rate was 28.4% (95% CI, 23.8–33.3) and did not differ by penicillin susceptibility group (P = 0.710). In vitro synergism was most frequently observed for ampicillin–ceftriaxone (39.1%) and penicillin–ceftriaxone (21.7%), whereas gentamicin combinations were mostly indifferent. Penicillin-nonsusceptible S. suis was frequently identified among infections in northeastern Thailand. In vitro synergistic activity was observed for the ampicillin–ceftriaxone combination in a subset of isolates, warranting further investigation.