Potentially virulence-related extracellular proteins of Streptococcus equi

Equine strangles, a disease of the upper respiratory tract caused by the bacterium Streptococcus equi subspecies equi, is one of the most commonly diagnosed and serious diseases in horses. However, the molecular basis of S. equi subsp. equi infection is poorly understood and there are no safe and effective vaccines on the market. The main objective of this study was to identify and characterise extracellular proteins used by S. equi subsp. equi to initiate infection and cause disease. Extracellular proteins, which can be secreted or cell surface-located, play an important role in the initiation of infection and in continued bacterial survival inside the host. Adhesins are a specific class of virulence-related proteins that are used by the bacteria to attach to host tissues. This study focused on a number of cell-surface anchored proteins that specifically adhere to collagen and fibronectin, two major extracellular matrix proteins of vertebrates. The binding characteristics of the fibronectin-binding protein FNEB were compared to two previously studied fibronectin-binding proteins, FNE and SFS. A follow-up investigation showed that FNE and FNEB are part of a family of six similar proteins encoded by S. equi subsp. equi. One of the four novel proteins identified (FNEE) was shown to bind to fibronectin and all four to collagen. In another study, a secreted immunoglobulin-specific protease was characterised. This enzyme could be used by the bacteria to interfere with the immune response of the infected horse. A useful tool in this research was the public S. equi subsp. equi genome database, which can be used to identify homologues to virulence-related proteins of other pathogens. An applied objective of the present study was to identify potential components for a future vaccine against strangles. One of the collagen-binding proteins characterised (CNE) is currently being used in vaccination trials as a component of a protein subunit vaccine for horses