Streptococcus suis : occurrence, identification and characteristics in Swedish pigs

Sammanfattning: Streptococcus suis is one of the most important bacterial pathogens in pigs globally, and it is also an emerging zoonotic agent. The aim of this thesis was to gain further knowledge of the occurrence and characteristics of S. suis in Swedish pigs.Samples were taken from the tonsils of healthy pigs, from necropsies of pigs suffering from meningitis, sepsis, and endocarditis, and from wild boar. S. suis was found in 95% of the pigs and was also isolated from wild boar. In total, 348 isolates were included in the studies presented in this thesis. Farm records on management practices, production data, and environmental parameters were gathered and analysed, but no potential risk factors were associated with disease. The commonly used MALDI-TOF MS method of species identification was evaluated, and it was concluded that additional confirmatory methods are needed, since closely related bacteria may be misidentified as S. suis. The isolates were tested for antibiotic susceptibility, and resistance to penicillin and tetracycline was identified. DNA was isolated and whole-genome sequencing was performed in order to study the population structure. Isolates from healthy and sick pigs exhibited genomic differences and clustered separately, and several new, possibly virulence-associated genes were identified.In conclusion, the results showed that S. suis was commonly found in Swedish pigs, including potentially zoonotic serotypes and sequence types. Isolates resistant to penicillin were identified for the first time in Sweden, and new virulenceassociated genes were proposed for further studies.