Serious disease in pigs caused by S. suis have been reported in all countries where the swine industry is important. The natural habitat of S. suis is the upper respiratory tract, particularly the tonsils and nasal cavities, and the genital and alimentary tracts of pigs10. This bacterium has been increasingly isolated from a wide range of mammalian species (including human beings) and from birds, which suggests new concepts about some epidemiological aspects of the infection. In pigs, the most important clinical feature associated with S. suis is meningitis. However, other pathologies have also been described, such as arthritis, endocarditis, pneumonia, and septicaemia with sudden death10. It is considered one of the most important bacterial pathogens that affect the swine industry.
Knowledge on the virulence factors of S. suis is still limited. Most studies have been carried out with serotype 2 strains. Although there is confusion in the description of virulence, researchers agree at least on one point: the existence of virulent and non virulent strains of S. suis serotype 27. The capsule polysaccharide (CPS) is the only proven critical virulence factor since unencapsulated isogenic mutants were shown to be completely avirulent and rapidly cleared from circulation in pig and mouse models of infection1, 19. However, the presence of capsulated non virulent strains is frequently reported7. Other factors, such as cell-associated or extracellular proteins (muramidase-released protein or MRP, extracellular factor or EF and suilysin), have also been proposed as putative virulence factors/markers7. However, isogenic mutants lacking these factors presented a similar level of virulence that their respective wild type strains12, 20. In addition, European but not North American virulent strains seem to possess these proteins5. These two groups of strains are also genotypically different2. Other putative virulence factors, including adhesins, proteolytic enzymes, bacteriocins and fimbriae have also been proposed15.
The pathogenesis of S. suis infections is poorly understood. Piglets are contaminated during farrowing (vertical transmission of the infection). They also diversely and probably heterogeneously acquire the bacterium due to close contact with the sows, her faeces, and the surrounding structures (pen walls, dirty soil, etc.). Reasons that may explain why S. suis will successfully colonise only some piglets and not others are poorly known. Infection of newborn piglets may also take place through the respiratory route from sow to piglets and among piglets. Colonised animals will harbour the bacteria in their tonsils. Some animals will only be healthy carriers and will never develop disease, whereas others will, sooner or later, develop bacteremia, sometimes septicaemia and, meningitis. Hence, in these cases, bacteria should travel throughout the bloodstream (as free bacteria and/or in close contact with monocytes) and reach the central nervous system (CNS), where it causes inflammation and damage responsible for the signs of meningitis7.
Presently, there are more than 400 DNA sequence entries that are currently listed in GenBank. Among them, genes coding for polysaccharide production and capsule transport19, iron-restricted factors17, the suilysin8, virulence-associated proteins20, different enzymes3, 13, IgG binding proteins16, and also in vivo-expressed genes18. Techniques for gene transfer and mutagenesis have been developed21, 23, 24. The presence of plasmids and a phage have also been reported9, 22. Strain 89-1591 of S. suis serotype 2 was isolated from a diseased pig and showed to be virulent in an experimental infection model14. This is an interesting strain, since it represents a typical North American virulent strain that lacks the above mentioned virulence protein markers (MRP, EF and suilysin). Genome sequencing of S. suis strain 89-1591 will contribute to reveal important information since no critical virulence factor (other than the capsule polysaccharide) has been described for this representative North America strain. A complete sequence of the genome of this strain will help to pursue studies on the interaction of this pathogen with host cells which may lead to the understanding of mechanisms of disease progression. This would contribute to develop potential therapies and preventive measures to control S. suis infections.
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