Identification, differentiation, and evaluation of Streptococcus suis isolates obtained from clinical cases for autogenous vaccine production

摘要

Streptococcus suis is a major porcine pathogen that causes meningitis, endocarditis, septicemia, arthritis, bronchopneumonia, polyserositis, and sudden death in pigs. Substantial economic losses in the pig industry worldwide have been attributed to Ssuis. Many attempts to control S suis infection in pig herds are hampered due to lack of effective vaccines and sensitive tests. Knowledge about the pathogenesis and epidemiology of S suis remains limited. During the past few years, phenotypic markers were used to distinguish highly virulent and avirulent strains of S suis by some research scientists. These phenotypic markers include the presence of serotype-specific capsular polysaccharide, the expression of the muraminidasereleased protein (MRP), extracellular factor (EF), and a thiol-activated haemolysin (sly). Other factors such as fibrinogen-bindingprotein (FBPS), hyaluronate lyase and serum opacity factor (OFS) were also included. However, the role of these factors as virulence markers is still not clear. Capsular polysaccharide is the only one that has been shown to prevent phagocytosis of S suis by host macrophages and thus been proposed as an important virulence trait. MRP and EF had been found to be associated with highly virulent isolates of S suis in Europe, yet they do not appear to confer virulence directly as demonstrated by the use of gene knockout technology. Suilysin (sly) has been considered to play a certain role in the pathogenesis of S suis infection due to its toxic effect on endothelial, epithelial, and phagocytic cells. However, a non-hemolytic knock-out mutant was shown to be only slightly attenuated in a porcine model of systemic infection with S suis. These reports suggest that the presence of these virulence-related markers should not be used as a single condition for classifying the virulence of a field strain.