Sessions

In pig production, Streptococcus species—particularly Streptococcus suis, which also has zoonotic potential — are frequent pathogens in the post-weaning period, while selection pressure from antimicrobial use can shape farm- and age group–specific resistance patterns. The aim of this study was to assess the antimicrobial susceptibility of Streptococcus isolates from large-scale pig farms in Hungary and to interpret the resistance burden by species group, age group, and farm level, complemented by multivariate pattern analyses. Between 2023 and 2025, we included 332 Streptococcus strains isolated from routine diagnostic nasal swabs collected from 14 farms. Species identification was performed by MALDI-TOF MS. Antimicrobial susceptibility was determined by broth microdilution for 15 agents.Among the isolates, S. suis predominated (60.5%), followed by S. hyovaginalis (28.3%), while the remaining species occurred at low frequency. Overall, the highest resistance rates were observed for lincomycin (91.6%), tilmicosin (86.4%), oxytetracycline (78.3%), tiamulin (78.3%), and doxycycline (76.2%). In contrast, resistance to ceftiofur, cefquinome, and florfenicol remained low (3.6%), and relatively low rates were also recorded for enrofloxacin (10.2%) and imipenem (6.9%). Resistance patterns were species dependent: β-lactam resistance was more pronounced in S. hyovaginalis than in S. suis. By age group, the most marked differences were observed for β-lactams: at 6 weeks of age, resistance to penicillin, amoxicillin, and amoxicillin–clavulanic acid was significantly higher, and a high MDR burden (≥5 resistant categories) was more frequent (59.6% vs 42.6% and 38.3%), supported by a Monte Carlo permutation test (p = 0.0031). At the farm level, several agents showed marked heterogeneity and clustered resistance patterns. Resistance profiles grouped into 126 unique resistance types; clustering and PCoA indicated partial separation by species group. In the co-resistance network, the strongest associations were observed between tetracyclines and among several co-occurring therapeutic groups. Overall, Streptococcus isolates from large-scale pig production in Hungary showed a notably high tetracycline resistance burden, while age- and farm-level heterogeneity was evident for β-lactams. These findings support the value of species identification combined with MIC-based susceptibility testing for farm-level AMR surveillance and antimicrobial stewardship.