Streptococcus suis (S. suis) is a facultatively anaerobic Gram-positive ovoid or coccal bacterium surrounded by a polysaccharide capsule[1]. This bacterium is a zoonotic agent that causes sepsis and meningitis in pigs and humans. Human infections occur primarily through close contact with infected pigs or through the consumption of raw or undercooked contaminated pork products[1]. This bacterium was first reported by veterinarians in 1954, after outbreaks of meningitis, septicemia, and purulent arthritis occurred among piglets. Fourteen years later, the first human S. suis cases were diagnosed in Denmark[2].
About 1,000 different sequence types (STs) were identified in S. suis based on multi-locus sequence typing (MLST). Worldwide ST1, ST25, and ST28 are the most prevalent STs isolated from swine, the predominance of each ST tends to vary with geographic location[3]. MLST has revealed the presence of many clonal complexes (CCs) within the S. suis population. The most important CCs causing infections in pigs and humans are CC1, CC13/149, CC16, CC17, CC20, CC25, CC28, CC94, CC104, CC233, CC221/234, CC1109, CC1112, and CC1237[4,5]. In North America, CC25 (Canada) and CC28 (United States and Canada) are more commonly reported[6]. These latter CCs are also present in Australia and some parts of Asia[7], whereas CC1 strains are more prevalent in Europe, Asia, and South America[8]. CC20 is important in The Netherlands[9], whereas CC104 and CC233 (ST233, ST379, and ST1656) have caused outbreaks and are endemic to Thailand[10]. CC16 and CC94 predominate among swine isolates in Europe; however, human cases caused by isolates of the latter CCs have also been reported in Thailand[11].
Serotype identification is considered the typing gold standard for S. suis strains. The capsular polysaccharide (CPS) is the basis for the typing system of S. suis[3]. S. suis had originally been classified into 35 serotypes (1/2 and 1–34) based on the antigenicity of the capsular polysaccharide, which is suggested to be a major virulent factor. Because some described serotypes (serotypes 20, 22, 26, 32, 33, and 34) belong to other bacterial species, the number of official S. suis serotypes has been reduced to 29[12]. However, after 2010, serotypes 21/29, NCL21-NCL26 (novel cps loci), and Chz were described in China[13]. Among these, serotype, serotype 2 is regarded as the most virulent and the most frequently isolated serotype infecting humans, causing serious illness and streptococcal toxic shock syndrome (STSS), which was the main cause of death in the 2005 outbreak in Sichuan province of China[14]. STSS-causing S. suis has evolved to acquire, most likely through horizontal gene transfer, an 89 K pathogenicity island (89 K PaI) with multiple virulence genes. The most commonly described virulence markers for S. suis also include muramidase-released protein (MRP), extracellular protein factor (EF), and the hemolysin suilysin (SLY), which were mainly associated with a virulence potential of S. suis serotype 2 strains[5].
Genes encoding resistance against tetracycline, macrolides, aminoglycosides, chloramphenicol, and other antimicrobial drugs have been identified in the various sequenced S. suis genomes. As in other streptococci, many of the resistance genes identified in S. suis have been found to be carried by integrative and conjugative elements (ICEs), transposons, genomic islands, phages, and chimeric elements[8]. Resistance to macrolides, lincosamides, tetracyclines, and sulphonamides has been reported, with up to 85% of strains resistant. S. suis isolates are uniformly sensitive to penicillin and ampicillin[15]. Since 2010, the number of reported S. suis infections in humans has increased substantially; most cases have originated in Southeast Asia, where the density of pigs is high[16] .
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