Comparative evaluation of virulence and pathology of 1 Streptococcus suis serotypes 2 and 9 in 2 experimentally infected growers 3 4 5

27 28 Streptococcus (S.). suis is an invasive porcine pathogen causing meningitis, septicemia, 29 arthritis and other diseases. Studies on pathogenesis as well as vaccine trials have 30 focused on serotype 2 strains, which are worldwide the most prevalent among invasive 31 isolates. However, in Europe serotype 9 strains also contribute substantially to S. suis32 associated invasive diseases of piglets. The objective of this study was to determine the 33 virulence of an MRP* SLY+ serotype 9 S. suis strain in comparison to an MRP+ EF+ 34 SLY+ serotype 2 strain. Experimental intranasal and intravenous infections of 7-8 week 35 old SPF piglets were investigated with regard to clinic and pathology. In contrast to the 36 virulent serotype 2 strain, the serotype 9 strain did not cause disease with clinical 37 manifestations after intranasal administration. However, histological screenings of these 38 animals revealed pathological lesions, such as mild focal suppurative meningitis. Clinical 39 manifestations related to meningitis, arthritis and serositis could be induced by 40 intravenous application of this serotype 9 strain. Bacteriological culture, 41 immunohistochemistry of the brain and nested PCR of cerebrospinal fluid confirmed 42 association with the S. suis challenge strains in all cases with clinical manifestations. 43 Interestingly, expression of MRP within meningitis lesions was demonstrated for both 44 pathotypes via immunohistochemistry. In conclusion, this study demonstrates that MRP* 45 SLY+ serotype 9 strains are less virulent for growers than MRP+ EF+ SLY+ serotype 2 46 strains. Thus, intravenous application of this serotype 9 strain is required to evaluate 47 heterologous protection in the course of vaccine development based on serotype 2 48 strains in the future. 49 50