Streptococcus suis sequence type 7 outbreak, Sichuan, China

Abstract

An outbreak of Streptococcus suis serotype 2 emerged in the summer of 2005 in Sichuan Province, and sporadic infections occurred in 4 additional provinces of China. In total, 99 S. suis strains were isolated and analyzed in this study: 88 isolates from human patients and 11 from diseased pigs. We defined 98 of 99 isolates as pulse type I by using pulsed-field gel electrophoresis analysis of SmaI-digested chromosomal DNA. Furthermore, multilocus sequence typing classified 97 of 98 members of the pulse type I in the same sequence type (ST), ST-7. Isolates of ST-7 were more toxic to peripheral blood mononuclear cells than ST-1 strains. S. suis ST-7, the causative agent, was a single-locus variant of ST-1 with increased virulence. These findings strongly suggest that ST-7 is an emerging, highly virulent S. suis clone that caused the largest S. suis outbreak ever described.

Figure 1

Pulsed-field gel electrophoresis (PFGE) profiles of selected isolates from various parts of China. All isolates of Streptococcus suis were digested with SmaI. The molecular size of each restriction fragment was calculated with the Comparative Quantification/Polymorphism analysis feature of the Molecular Analyst Fingerprinting Plus software (Quantity One version 4.0, Bio-Rad, Beijing, People's Republic of China) based on mobility of S. suis isolates on the same gel with Salmonella enterica serovar Braenderup H9812 digested with XbaI, as universal standard of PulseNet International. Clustering of PFGE patterns was performed by an unweighted paired group with arithmetic averaging (UPGMA). The dendrogram of PFGE patterns of isolates tested were drawn with PulseNet software BioNumerics, with a 1.5% position tolerance and 1% optimization.

Figure 2

Population snapshot of Streptococcus suis. The entire S. suis multilocus sequence type database is displayed as a single eBURST diagram. The 6 major sequence type (ST) complexes are each denoted by a number. The patterns of descent within these ST complexes are discussed in the text. Primary founders (dark green) are positioned centrally in the cluster, and subgroup founders are shown in light green, except ST-7, which is shown in orange to emphasize its importance. The area of each circle in the diagram corresponds to the abundance of the isolates of the ST in the input data.

Figure 3

Cytotoxicity of Streptococcus suis sequence type (ST) 7 to human peripheral blood mononuclear cells. Data were collected from 3 experiments, which were performed in triplicate and expressed as the mean percentage of cytotoxicity, plus or minus standard deviations. *p<0.001 (compared to the value of S. suis ST-1 representative strain 31533) as determined by analysis of variance with SAS version 8 software (SAS Institute, Cary, NC, USA).

Figure 4

Unrooted Bayesian tree of the concatenated sequence of the 92 sequence types (STs) of Streptococcus suis. The tree was constructed by using MrBayes (version 3.1.1) according to the HKY85 model of DNA substitution with no rate variation across sites. Four Markov chains were run for a million generations, and the Markov chain is sampled every 100 generations (18). The sampled parameter values were summarized by discarding the first 2,000 samples as burn-in. On the basis of the last 9,000 samples taken from the posterior probability distribution, a 50% majority rule consensus tree was computed. The posterior probability given on each branch is a percentage of these trees supporting each node. The 6 lineages defined in this study are shadowed in light gray. The 6 major clonal complexes identified previously by King et al. (8) as well as in this study are shadowed in dark gray. The five STs containing isolates from human invasive disease are shown as red dots.