Streptococcus suis infection: An emerging/reemerging challenge of bacterial infectious diseases?

Feng, Youjun; Zhang, Huimin; Wu, Zuowei; Wang, Shihua; Cao, Min; Hu, Dan; Wang, Changjun

General microbiology of S. suis S. suis is a group of heterogeneous gram-positive bacteria that were earlier classified into Lancefield groups R, S, and T (Fig. 3) . 1 These bacteria are facultative anaerobes with a spherical/ovoid shape which exist in pairs and/or short chains ( Fig. 3B and C) . Generally, these microorganisms show either α-hemolysis when growing on selective plates of horse blood agar 1 (Fig. 3A) . Given the variation in their CPS antigens, 35 serotypes have been proposed for S. suis population. 1 Very few studies of pathogenicity have been done for serotypes other than serotypes 2, 1, and 7. 1 Among them, SS2 is recognized as the most virulent species that is frequently associated with diseased pigs and often causes an opportunistic infection of adults having occupational contact with pig carcass or pork-related products. 2, 5 S. suis, an important animal pathogen, naturally inhabits in the upper respiratory (particularly the tonsils and nasal cavities), genital, and alimentary tracts of piglets. 1, 32 In addition to the natural host swine (Fig. 3D) , this pathogen has been suggested to be isolated from a wide range of other animals, such as horses, dogs, and cats. 1, 33 Of note, some variants of S. suis probably have evolved into highly infectious zoonotic agents that can cause meningitis, septicemia, arthritis, and even streptococcal toxic shocklike syndrome (which can cause rapid death) in humans. 9, 17, 18, 34 Soon after the big outbreak of human SS2 infections in China, in 2005, serious concerns from both the public health and scientific community have been raised. 18 Toward better understanding and prevention/control of SS2 infections, multiple lines of new bacterial virulence determinants (such as salK-salR two component system, 27 FeoB transporter, 35 and Rgg regulator 31 ) have been identified, and fast assays using PCR-based molecular detection 18 as well as ELISA (enzyme-linked immunosorbent assay)-guided diagnostics were established 36 (Table 1) . Most recently, epidemiological investigations conducted in Vietnam proposed that (1) pig population in slaughterhouses is a major reservoir of SS2 with a capacity to cause human infections 37 and (2) the most important risk factors of human S. suis infections are consecutively eating "high risk" pork-derived dishes, occupational exposure to pigs and pork-related products, and preparation of pork in the presence of skin lesions. 38 The availability of the epidemiological knowledge on human SS2 infections is critical to improve the current situation of public awareness of SS2 infections and to effectively prevent the potential occupational infections caused by SS2.