Abstract

Streptococcus equi subspecies equi is the causative agent of strangles, a respiratory disease that affects horses worldwide. Like other pathogenic bacteria, S. equi subspecies equi expresses a number of extracellular proteins important for the interaction with the host and thus for the pathogenesis. In this thesis, one aim was to identify and characterise novel extracellular proteins in this subspecies. Signal phage display was used to screen the genome of S. equi subsp. equi strain 1866 for signal sequences thereby identifying extracellular proteins. A novel collagen-like protein, SclC, was found and further characterised. The SclC protein, similar to Scl proteins in S. pyogenes, has an N-terminal signal sequence, a unique region, a collagen-like region and a wall-spanning region ending with an LPXTG-motif preceding the membrane-bound region in the C-terminal end. Using bioinformatic tools, the collagen-like protein was found to be a member of a family consisting of seven collagen-like proteins, called SclC-SclI, in subspecies equi. In contrast to mammalian collagen, all these proteins have collagen-like regions that consist of KGD/KGE triplets, which are suggested to be of importance for the structure of the proteins. When sera from horses previously infected with subsp. equi and sera from healthy horses were analysed, antibodies against all seven proteins were detected. Furthermore, antibodies against SclC recognised all members of the family of collagen-like proteins. In an immunisation study in a strangles model in mice, immunisation with SclC resulted in protection against infection by S. equi subsp. equi. The biological role of SclC was further investigated in a collagen lattice gel contraction study to investigate whether mammalian integrins would recognise SclC, which turned out not to be the case. The biological role of the collagen-like proteins identified here is not fully comprehended. However, since the vaccination studies in mice resulted in protection against infection, these proteins have an interesting potential for further vaccine trials in horses.