Genotypic profile of Streptococcus suis serotype 2 and clinical features of infection in humans, Thailand

Colloidal gold-based immunochromatographic strip test compromising optimised combinations of anti-S. suis capsular polysaccharide polyclonal antibodies for detection of Streptococcus suis

A rapid diagnosis kit that detects Streptococcus suis (S. suis) antigens from urine with an immunochromatographic stripe (ICS) test was developed in this study. The ICS test was produced using colloidal gold coated with polyclonal antibodies (pAbs) against S. suis. The pAbs were developed from rabbits immunised with S. suis serotype 2 capsular polysaccharides (CPS). Development of the pAbs was investigated to establish their binding to CPS and to determine the maximum sensitivity of two combination antibodies for the ICS test. The results of the ICS optimisation revealed that the combinations of pAb C-N1 and pAb C-N2 had the highest sensitivity to CPS. The minimum limitation of ICS sensitivity indicated 1.0 × 10(4) colony forming units (CFU) and a CPS concentration of 0.05 µg. The assay time for detection of S. suis antigens is less than 15 min, which is suitable for rapid detection. A cross-reactive test was also conducted, and it detected no other bacteria (Streptococcus pneumoniae, Streptococcus agalactiae, Staphylococcus aureus, Enterococcus faecalis, Escherichia coli, Pseudomonas aeruginosa, and Klebsiella pneumoniae). The cross-reactivity of other serotypes in S. suis was also investigated, and tests for serotypes of 1, 1/2, 3, 4, 5, 6, 7, 8, 9, 14, and 16 were positive. This study presents the first report of a development of an ICS that enables the quantitative detection of streptococcal antigens. The S. suis ICS provides several advantages over other methods, including the speed and simplicity of use.

Streptococcus suis serotyping by a new multiplex PCR

A multiplex PCR was developed to detect all true serotypes of Streptococcus suis. This multiplex PCR was composed of four reaction sets. The first set identified nine serotypes (serotypes 1/2, 1, 2, 3, 7, 9, 11, 14 and 16), the second set identified eight serotypes (serotypes 4, 5, 8, 12, 18, 19, 24 and 25), the third set identified seven serotypes (serotypes 6, 10, 13, 15, 17, 23 and 31), and the last set identified five serotypes (serotypes 21, 27, 28, 29 and 30). This assay correctly detected serotypes 2, 5, 14 and 24 in human isolates, and serotypes 1, 2, 1/2, 3, 4, 5, 6, 7, 8, 9, 11, 12, 14, 15, 16, 17, 19, 24, 28 and 31 in pig isolates from Thailand. No cross-reaction was observed with other bacterial species. Our multiplex PCR was able to simultaneously amplify a DNA mixture of reference Streptococcus suis serotypes. This assay should be useful for serotype surveillance of human and pig isolates of Streptococcus suis.

Latest developments on Streptococcus suis: an emerging zoonotic pathogen: part 1

First International Workshop on Streptococcus suis, Beijing, China, 12–13 August 2013

The first international workshop on Streptococcus suis, which is an important swine pathogen and emerging zoonotic agent, took place in Beijing, jointly organized by the Faculty of Veterinary Medicine, University of Montreal, Canada and the National Institute for Communicable Disease Control and Prevention, China CDC. The aim of the meeting was to gather together, for the first time, more than 80 researchers working on S. suis, from countries including China, Canada, Japan, The Netherlands, Germany, Thailand, the UK and Vietnam. This article, the first of a two-part report on this First International Workshop, reviews current aspects of the epidemiology and population genomics of S. suis, covers public health concerns and discusses questions about S. suis serotyping and molecular diagnostics.

Correlation between PFGE Groups and mrp/epf/sly Genotypes of Human Streptococcus suis Serotype 2 in Northern Thailand

Streptococcus suis infection is a severe zoonotic disease commonly found in Northern Thailand where people often consume raw pork and/or pig's blood. The most frequent clinical presentations are meningitis, sepsis, and endocarditis with higher rate of mortality and hearing loss sequelae. To clarify the correlation between pulsed-field gel electrophoresis (PFGE) groups and mrp/epf/sly genotypes of S. suis serotype 2, 62 patient and 4 healthy pig isolates from Northern Thailand were studied. By PFGE analysis, at 66% homology, most human isolates (69.4%) and 1 pig isolate were in group A, whereas 14.5% of human isolates and 3 out of 4 pig isolates were in group D. According to mrp/epf/sly genotypes, 80.6% of human isolates were identified in mrp⁺epf⁻sly⁻ and only 12.9% were in mrp⁻epf⁻sly⁺ genotypes; in contrast, 1 and 3 pig isolates were detected in these two genotypes, respectively. Interestingly, all isolates of S. suis serotype 2 classified in PFGE groups A, B, and E were set in mrp⁺epf⁻sly⁻ genotypes. These data show a close correlation between PFGE groups and mrp/epf/sly genotypes of human S. suis serotype 2.

The contribution of suilysin to the pathogenesis of Streptococcus suis meningitis

BACKGROUND

Streptococcus suis is an emerging zoonotic pathogen, and causes sepsis and meningitis in humans. Although sequence type (ST) 1 and ST104 strains are capable of causing sepsis, ST1 strains commonly cause meningitis. In this study, we investigated the role of suilysin, a member of cholesterol-dependent cytolysins, in differential pathogenicity between ST1 and ST104 strains.

METHODS

The levels of transcription and translation of the sly gene and messenger RNA of both ST strains were compared by means of quantitative polymerase chain reaction and Western blotting. Survival rates and bacterial densities in brain were compared between mice infected with wild-type and sly-knockout ST1 strain. ST104 infections with or without complementation of suilysin were also assessed.

RESULTS

The amounts of suilysin produced by ST1 strains were much higher than those produced by ST104 strains. Lower production of suilysin by ST104 strains were attributed to the attenuated sly gene expression, which seemed to be associated with 2 nucleotide insertions in sly promoter region. Furthermore, suilysin contributed to the higher bacterial density and enhanced inflammation in brain and increased mortality.

CONCLUSIONS

Our data may explain why ST1 strains, but not ST104 strains, commonly cause meningitis and also suggest the contribution of suilysin to the pathogenesis of meningitis in humans.

Alcohol consumption promotes the intestinal translocation of Streptococcus suis infections

Streptococcus suis is an emerging zoonotic agent. This study aimed to investigate whether S. suis is likely to translocate across the intestines of human hosts who have liver disease and/or consume alcohol. Both the alcoholism and cirrhosis models exhibited high mRNA expression of TGF and collagen1, but only the cirrhosis model had fibrosis in the liver. After both models were infected with S. suis, significantly different concentrations of S. suis were detected in the blood and brains of the alcoholism model (Blood: 36.4%; Brain: 31.8%) and the cirrhosis model (Blood: 62.5%; Brain: 62.5%) compared to the concentrations in the healthy mice (Blood: 15.4%; Brain: 0%). Trans-epithelial electrical resistance (TER) was used to examine the Caco-2 cells in the in vitro that had an S. suis infection combined with 1% ethanol. Although the ethanol did not influence the Caco-2 cells' barriers, it did rapidly decrease the barriers' TER value and then their E-cadherin compared to the infected Caco-2 cells without the ethanol treatment. Immunofluorescence also indicated that the barriers of the Caco-2 cells treated with ethanol were disrupted and that S. suis translocated from the apical to the basolateral side. This study demonstrated that alcohol consumption helped S. suis to translocate.

Toll-like receptor 2-independent host innate immune response against an epidemic strain of Streptococcus suis that causes a toxic shock-like syndrome in humans

Streptococcus suis is an emerging zoonotic agent causing meningitis and septicemia. Outbreaks in humans in China with atypical cases of streptococcal toxic shock-like syndrome have been described to be caused by a clonal epidemic S. suis strain characterized as sequence type (ST) 7 by multilocus sequence typing, different from the classical ST1 usually isolated in Europe. Previous in vitro studies showed that Toll-like receptor (TLR) 2 plays a major role in S. suis ST1 interactions with host cells. In the present study, the in vivo role of TLR2 in systemic infections caused by S. suis ST1 or ST7 strains using TLR2 deficient (TLR2(-/-)) mice was evaluated. TLR2-mediated recognition significantly contributes to the acute disease caused by the highly virulent S. suis ST1 strain, since the TLR2(-/-) mice remained unaffected when compared to wild type (WT) mice. The lack of mortality could not be associated with a lower bacterial burden; however, a significant decrease in the induction of pro-inflammatory mediators, as evaluated by microarray, real-time PCR and protein assays, was observed. On the other hand, TLR2(-/-) mice infected with the epidemic ST7 strain presented no significant differences regarding survival and expression of pro-inflammatory mediators when compared to the WT mice. Together, these results show a TLR2-independent host innate immune response to S. suis that depends on the strain.

Exacerbated type II interferon response drives hypervirulence and toxic shock by an emergent epidemic strain of Streptococcus suis

Streptococcus suis, a major porcine pathogen, can be transmitted to humans and cause severe symptoms. A large human outbreak associated with an unusual streptococcal toxic shock-like syndrome (STSLS) was described in China. Albeit an early burst of proinflammatory cytokines following Chinese S. suis infection was suggested to be responsible for STSLS case severity, the mechanisms involved are still poorly understood. Using a mouse model, the host response to S. suis infection with a North American intermediately pathogenic strain, a European highly pathogenic strain, and the Chinese epidemic strain was investigated by a whole-genome microarray approach. Proinflammatory genes were expressed at higher levels in mice infected with the Chinese strain than those infected with the European strain. The Chinese strain induced a fast and strong gamma interferon (IFN-γ) response by natural killer (NK) cells. In fact, IFN-γ-knockout mice infected with the Chinese strain showed significantly better survival than wild-type mice. Conversely, infection with the less virulent North American strain resulted in an IFN-β-subjugated, low inflammatory response that might be beneficial for the host to clear the infection. Overall, our data suggest that a highly virulent epidemic strain has evolved to massively activate IFN-γ production, mainly by NK cells, leading to a rapid and lethal STSLS.

Fifteen Streptococcus suis serotypes identified by multiplex PCR

A multiplex PCR was developed to detect 15 serotypes of Streptococcus suis. This multiplex PCR was separated into two reaction sets. The first set identified nine serotypes (serotypes 1/2, 1, 2, 3, 4, 7, 9, 14 and 16) and the second set identified six serotypes (serotypes 5, 8, 10, 19, 23 and 25). This assay correctly detected serotypes 2, 5 and 14 in human isolates, and serotypes 1, 2, 1/2, 3, 4, 5, 7, 9, 14, 16 and 19 in pig isolates from Thailand. No cross-reaction was observed with other streptococcal species. This assay may be useful for the serotype surveillance of human and pig isolates of S. suis.

Differences in the population structure of invasive Streptococcus suis strains isolated from pigs and from humans in The Netherlands

Streptococcus suis serotype 2 is the main cause of zoonotic S. suis infection despite the fact that other serotypes are frequently isolated from diseased pigs. Studies comparing concurrent invasive human and pig isolates from a single geographical location are lacking. We compared the population structures of invasive S. suis strains isolated between 1986 and 2008 from human patients (N = 24) and from pigs with invasive disease (N = 124) in the Netherlands by serotyping and multi locus sequence typing (MLST). Fifty-six percent of pig isolates were of serotype 9 belonging to 15 clonal complexes (CCs) or singleton sequence types (ST). In contrast, all human isolates were of serotype 2 and belonged to two non-overlapping clonal complexes CC1 (58%) and CC20 (42%). The proportion of serotype 2 isolates among S. suis strains isolated from humans was significantly higher than among strains isolated from pigs (24/24 vs. 29/124; P<0.0001). This difference remained significant when only strains within CC1 and CC20 were considered (24/24 vs. 27/37,P = 0.004). The Simpson diversity index of the S. suis population isolated from humans (0.598) was smaller than of the population isolated from pigs (0.765, P = 0.05) indicating that the S. suis population isolated from infected pigs was more diverse than the S. suis population isolated from human patients. S. suis serotype 2 strains of CC20 were all negative in a PCR for detection of genes encoding extracellular protein factor (EF) variants. These data indicate that the polysaccharide capsule is an important correlate of human S. suis infection, irrespective of the ST and EF encoding gene type of S. suis strains.

Lineage and virulence of Streptococcus suis serotype 2 isolates from North America

Two sequence types predominate and have lower virulence than other types.

We performed multilocus sequence typing of 64 North American Streptococcus suis serotype 2 porcine isolates. Strains were sequence type (ST) 28 (51%), ST25 (44%), and ST1 (5%). We identified nonrandom associations between STs and expression of the virulence markers suilysin (SLY), muramidase-relased protein (MRP), and extracellular factor (EF). Expression of pili encoded by the srtF and srtG pilus clusters was also nonrandomly associated with STs. ST1 strains were SLY+ EF+ MRP+ srtF pilus+ srtG pilus−. ST25 strains were SLY− EF− MRP− srtF pilus− srtG pilus+, and most ST28 strains were SLY− MRP+ EF− srtF pilus+ srtG pilus+. ST28 isolates proved essentially nonvirulent in a mouse infection model; ST25 strains showed moderate virulence and ST1 isolates were highly virulent. ST1 is responsible for a high proportion of S. suis disease in humans worldwide. Its presence in North America indicates that potential zoonotic S. suis outbreaks in this continent cannot be disregarded.

Population-based study of Streptococcus suis infection in humans in Phayao Province in northern Thailand

BACKGROUND

Streptococcus suis infection in humans has received increasing worldwide recognition.

METHODS AND FINDINGS

A prospective study of S. suis infection in humans was conducted in Phayao Province in northern Thailand to determine the incidence and the risk behaviors of the disease in this region in 2010. Thirty-one cases were confirmed. The case fatality rate was 16.1%, and the estimated incidence rate was 6.2 per 100,000 in the general population. The peak incidence occurred in May. The median age of the patients was 53 years and 64.5% were men. Consumption of raw pork products was confirmed in 22 cases and the median incubation period (range) was 2 days (0-11) after consumption of raw pork products. Isolates from 31 patients were confirmed as serotype 2 in 23 patients (74.2%) and serotype 14 in eight patients (25.8%). The major sequence types (STs) were ST1 (n = 20) for serotype 2 and ST105 (n = 8) for serotype 14. The epidemiological analysis suggested three possible clusters, which included 17 cases. In the largest possible cluster of 10 cases in Chiang Kham and its neighboring districts in May, the source of infection in four cases was identified as a raw pork dish served at the same restaurant in this district. Microbiological analysis confirmed that three of four cases associated with consumption of raw pork at this restaurant were attributable to an identical strain of serotype 2 with ST1 and pulsotype A2.

CONCLUSIONS

Our data suggest a high incidence rate of S. suis infection in the general population in Phayao Province in 2010 and confirm a cluster of three cases in 31 human cases. Food safety control should be strengthened especially for raw pork products in northern Thailand.

Fluoroquinolone efflux in Streptococcus suis is mediated by SatAB and not by SmrA

Streptococcus suis is an emerging zoonotic pathogen. With the lack of an effective vaccine, antibiotics remain the main tool to fight infections caused by this pathogen. We have previously observed a reserpine-sensitive fluoroquinolone (FQ) efflux phenotype in this species. Here, SatAB and SmrA, two pumps belonging to the ATP binding cassette (ABC) and the major facilitator superfamily (MFS), respectively, have been analyzed in the fluoroquinolone-resistant clinical isolate BB1013. Genes encoding these pumps were overexpressed either constitutively or in the presence of ciprofloxacin in this strain. These genes could not be cloned in plasmids in Escherichia coli despite strong expression repression. Finally, site-directed insertion of smrA and satAB in the amy locus of the Bacillus subtilis chromosome using ligated PCR amplicons allowed for the functional expression and study of both pumps. Results showed that SatAB is a narrow-spectrum fluoroquinolone exporter (norfloxacin and ciprofloxacin), susceptible to reserpine, whereas SmrA was not involved in fluoroquinolone resistance. Chromosomal integration in Bacillus is a novel method for studying efflux pumps from Gram-positive bacteria, which enabled us to demonstrate the possible role of SatAB, and not SmrA, in fluoroquinolone efflux in S. suis.