Streptococcal toxic shock syndrome caused by Streptococcus suis serotype 2

Abstract

Background: Streptococcus suis serotype 2 (S. suis 2, SS2) is a major zoonotic pathogen that causes only sporadic cases of meningitis and sepsis in humans. Most if not all cases of Streptococcal toxic shock syndrome (STSS) that have been well-documented to date were associated with the non-SS2 group A streptococcus (GAS). However, a recent large-scale outbreak of SS2 in Sichuan Province, China, appeared to be caused by more invasive deep-tissue infection with STSS, characterized by acute high fever, vascular collapse, hypotension, shock, and multiple organ failure.

Methods and findings: We investigated this outbreak of SS2 infections in both human and pigs, which took place from July to August, 2005, through clinical observation and laboratory experiments. Clinical and pathological characterization of the human patients revealed the hallmarks of typical STSS, which to date had only been associated with GAS infection. Retrospectively, we found that this outbreak was very similar to an earlier outbreak in Jiangsu Province, China, in 1998. We isolated and analyzed 37 bacterial strains from human specimens and eight from pig specimens of the recent outbreak, as well as three human isolates and two pig isolates from the 1998 outbreak we had kept in our laboratory. The bacterial isolates were examined using light microscopy observation, pig infection experiments, multiplex-PCR assay, as well as restriction fragment length polymorphisms (RFLP) and multiple sequence alignment analyses. Multiple lines of evidence confirmed that highly virulent strains of SS2 were the causative agents of both outbreaks.

Conclusions: We report, to our knowledge for the first time, two outbreaks of STSS caused by SS2, a non-GAS streptococcus. The 2005 outbreak was associated with 38 deaths out of 204 documented human cases; the 1998 outbreak with 14 deaths out of 25 reported human cases. Most of the fatal cases were characterized by STSS; some of them by meningitis or severe septicemia. The molecular mechanisms underlying these human STSS outbreaks in human beings remain unclear and an objective for further study.

Figure 1. Detection of the Pathogenic SS2 and Identification of Its Specific Genes

(A) Light microscopy image of the isolates cultured from autopsy specimens. GP ⁺ cocci (pointed to with black arrows) are arranged in various short chains (×100). (B) Qualitative PCR detection of isolates from the liver of fatal human cases with a set of primers specific for S. suis 2. M: 100bp DNA marker (Fermentas, Vilnius, Lithuania). CK: 16S rDNA PCR product from the R 735 standard strain of S. suis 2. Multi-PCR: performed with a set of unique primers specific for mrp, epf, suilysin, and cps-2J, respectively.

Figure 2. Microscopic Characterization of Sectioned Liver Tissue from Patients Who Had Died

(A) Light image of a liver tissue section (×100). The central vein is indicated with an arrow. (B) Light image of a liver tissue section (×200). (C) The convergent zone is indicated with an arrow (×100). (D) TEM image of a liver tissue section (×20,000). A bacterium found in the tissue is highlighted with an arrow.

Figure 3. Immunohistochemical Analysis of Liver Tissue Sections from Dead Patients Incubated with Normal and S. suis 2-Infected Swine Serum

(A) Staining with normal swine serum detected no S. suis 2 antigen. (B) Staining with the serum from the infected swine indicated the presence of S. suis 2 antigen.

Figure 4. RFLP Analysis of Different S. suis 2 Isolates

S. suis S10: a highly virulent strain from China; S. suis 9801: swine isolate from Jiangsu Province in 1998; S. suis Habb: human isolate from Jiangsu Province in 1998; S. suis ZYS3: swine isolate from Sichuan Province in 2005; S. suis ZYH13: human isolate from Sichuan Province in 2005; M: 1 kb DNA Ladder (MBI Ferments, Gdansk, Poland).

Figure 5. Comparison of the cps2 Gene Clusters Involved in CPS-2 Synthesis

Exactly matched genes (5/12) include cps2B, cps2D, cps2F, cps2K, and cps2L and are shown in pink. The seven other genes (which did not exactly match) include cps2A cps2C, cps2E, cps2G, cps2H, cps2I, and cps2J and were highlighted with green in S. suis P1/7 and with red in both S. suis 98 and S. suis ZY. Amino acid abbreviations used are: A, alanine; C, cysteine; E, glutamic acid; F, phenyalanine; G, glycine; I, isoleucine; K, lysine; L, leucine; M, methionine; Q, glutamine; R, arginine; S, serine; V, valine; W, tryptophan; Y, tyrosine.

Figure 6. Phylogenetic Trees of Six Representative Isolates Based on Comparison of 16S rDNA and Five Putative Virulence-Associated-Factor Genes with Known Sequences

Swine isolates from Sichuan ( S. suis ZYS3 and S. suis ZYS8) labeled in green, human isolates ( S. suis ZYH13 and S. suis ZYH14) from Sichuan labeled in red, Jiangsu isolates from 1998 ( S. suis 9801 and S. suis Habb) labeled in blue, and the standard highly virulent strain S. suis P1/7 labeled in pink. All representative strains from other streptococcus species or isolates of S. suis 2 are as indicated in the tree.