Marcelo Gottschalk

The pathogenesis of the meningitis caused by Streptococcus suis : the unresolved questions

Streptococcus suis is one of the most important swine pathogens world-wide. Among the serotypes described, type 2 is the serotype most frequently associated with disease. Despite increasing research in recent years, knowledge of virulence factors and the pathogenesis of the infection remain limited. This review discusses the currently available information on S. suis serotype 2 virulence factors and the pathogenesis of the meningitis caused by this important bacterial species. In addition, some hypotheses on the critical steps of the infection, such as bacterial invasion from mucosal surfaces to the bloodstream, survival of bacteria in blood, and invasion from blood into the central nervous system, are presented. Finally, the role that the stimulation of the immune system of animals (inflammatory reaction) could play during infection is also discussed. A complete understanding of the cell-interacting pathways that S. suis may follow inside the host could give important insights into the progression of disease. Further studies to delineate the mechanisms through which S. suis induces meningitis will contribute to the development of potential therapies for S. suis infections.

Streptococcus suis: a new emerging or an old neglected zoonotic pathogen?

Infections caused by Streptococcus suis are considered a global and an economical problem in the swine industry. Moreover, S. suis is an agent of zoonosis that afflicts people in close contact with infected pigs or pork-derived products. Although sporadic cases of S. suis infections in humans (mainly meningitis) have been reported during the last 40 years, a large outbreak due to this pathogen emerged in the summer of 2005 in China. The severity of the infection in humans during the outbreak, such as a shorter incubation time, more rapid disease progression and higher rate of mortality, attracted a lot of attention from the scientific community and the general press. In fact, the number of publications on S. suis (including the number of reported human cases) has significantly increased during recent years. In this article we critically review the present knowledge on S. suis infection in humans, we discuss the hypotheses that may explain the 2005 outbreak and the repercussion of such an episode on the scientific community.

Streptococcus suis infections in humans: the Chinese experience and the situation in North America.

Abstract Infections caused by Streptococcus suis are considered a global problem in the swine industry. In this animal species, S. suis is associated with septicemia, meningitis, endocarditis, arthritis and, occasionally, other infections. Moreover, it is an agent of zoonosis that afflicts people in close contact with infected pigs or pork-derived products. Although sporadic cases of S. suis infection in humans have been reported, a large outbreak due to S. suis serotype 2 emerged in the summer of 2005 in Sichuan, China. A similar outbreak was observed in another Chinese province in 1998. Symptoms reported in these two outbreaks include high fever, malaise, nausea and vomiting, followed by nervous symptoms, subcutaneous hemorrhage, septic shock and coma in severe cases. The increased severity of S. suis infections in humans, such as a shorter incubation time, more rapid disease progression and higher rate of mortality, underscores the critical need to better understand the factors associated with pathogenesis of S. suis infection. From the 35 capsular serotypes currently known, serotype 2 is considered the most virulent and frequently isolated in both swine and humans. Here, we review the epidemiological, clinical and immunopathological features of S. suis infection in humans.

Virulence factors involved in the pathogenesis of the infection caused by the swine pathogen and zoonotic agent Streptococcus suis

Streptococcus suis is a major swine pathogen responsible for important economic losses to the swine industry worldwide. It is also an emerging zoonotic agent of meningitis and streptococcal toxic shock-like syndrome. Since the recent recognition of the high prevalence of S. suis human disease in southeast and east Asia, the interest of the scientific community in this pathogen has significantly increased. In the last few years, as a direct consequence of these intensified research efforts, large amounts of data on putative virulence factors have appeared in the literature. Although the presence of some proposed virulence factors does not necessarily define a S. suis strain as being virulent, several cell-associated or secreted factors are clearly important for the pathogenesis of the S. suis infection. In order to cause disease, S. suis must colonize the host, breach epithelial barriers, reach and survive in the bloodstream, invade different organs, and cause exaggerated inflammation. In this review, we discuss the potential contribution of different described S. suis virulence factors at each step of the pathogenesis of the infection. Finally, we briefly discuss other described virulence factors, virulence factor candidates and virulence markers for which a precise role at specific steps of the pathogenesis of the S. suis infection has not yet been clearly established.

Rapid Evolution of Virulence and Drug Resistance in the Emerging Zoonotic Pathogen Streptococcus suis

Background Streptococcus suis is a zoonotic pathogen that infects pigs and can occasionally cause serious infections in humans. S. suis infections occur sporadically in human Europe and North America, but a recent major outbreak has been described in China with high levels of mortality. The mechanisms of S. suis pathogenesis in humans and pigs are poorly understood. Methodology/Principal Findings The sequencing of whole genomes of S. suis isolates provides opportunities to investigate the genetic basis of infection. Here we describe whole genome sequences of three S. suis strains from the same lineage: one from European pigs, and two from human cases from China and Vietnam. Comparative genomic analysis was used to investigate the variability of these strains. S. suis is phylogenetically distinct from other Streptococcus species for which genome sequences are currently available. Accordingly, ∼40% of the ∼2 Mb genome is unique in comparison to other Streptococcus species. Finer genomic comparisons within the species showed a high level of sequence conservation; virtually all of the genome is common to the S. suis strains. The only exceptions are three ∼90 kb regions, present in the two isolates from humans, composed of integrative conjugative elements and transposons. Carried in these regions are coding sequences associated with drug resistance. In addition, small-scale sequence variation has generated pseudogenes in putative virulence and colonization factors. Conclusions/Significance The genomic inventories of genetically related S. suis strains, isolated from distinct hosts and diseases, exhibit high levels of conservation. However, the genomes provide evidence that horizontal gene transfer has contributed to the evolution of drug resistance.

Streptococcus suis, an important pig pathogen and emerging zoonotic agent-an update on the worldwide distribution based on serotyping and sequence typing.

Streptococcus suis is an important pathogen causing economic problems in the pig industry. Moreover, it is a zoonotic agent causing severe infections to people in close contact with infected pigs or pork-derived products. Although considered sporadic in the past, human S. suis infections have been reported during the last 45 years, with two large outbreaks recorded in China. In fact, the number of reported human cases has significantly increased in recent years. In this review, we present the worldwide distribution of serotypes and sequence types (STs), as determined by multilocus sequence typing, for pigs (between 2002 and 2013) and humans (between 1968 and 2013). The methods employed for S. suis identification and typing, the current epidemiological knowledge regarding serotypes and STs and the zoonotic potential of S. suis are discussed. Increased awareness of S. suis in both human and veterinary diagnostic laboratories and further establishment of typing methods will contribute to our knowledge of this pathogen, especially in regions where complete and/or recent data is lacking. More research is required to understand differences in virulence that occur among S. suis strains and if these differences can be associated with specific serotypes or STs.

An update on Streptococcus suis identification.

Streptococcus suis is a worldwide cause of a variety sular type 2 isolate was recovered in pure culture from of porcine infections. It has been isolated from cases lungs and kidneys of a 41⁄2-month-old aborted bovine of meningitis, bronchopneumonia, arthritis, pericarfetus and its placenta. This suggests that S. suis may ditis, endocarditis, polyserositis, septicemia, rhinitis, be pathogenic for more than 1 animal species. and abortion.20,22,24 Different alpha-hemolytic strepIn Canada, it was reported that 94% of 4-8-weektococci were ascribed to Lancefield groups R, S, RS, old clinically healthy piglets harbored S. suis in their and T in 1963.5 Other investigators working with capnasal cavities, and 79% of these isolates did not belong sulated streptococci similar to de Moor’s groups S and to the 9 capsular types.3 Recently, we demonstrated R realized that the polysaccharides involved in serothat almost 90% of these untypeable isolates belong to typing originated from the capsular material rather than only 4 of the new capsular types (types 17, 18, 19, and from the cell wall. They considered these isolates as a 21, unpublished data). Other investigators have atnew species (Streptococcus suis) within the Lancefield tributed the development of rhinitis4,7,22 to untypeable group D, with 2 capsular types: 1 and 2, respectively. S. suis isolates. At this time, it is not possible to know The de Moor’s group RS was also added as S. suis type whether any of the capsular types of S. suis belong to 1⁄2.26 In 1983, 6 new capsular types of S. suis origithe normal flora of the nasal cavity or whether they nating from diseased pigs were described. 20 In 1987 represent real pathogens. Of the new capsular types, the species was officially recognized, l4 but the authors types 9 and 22 are more frequently found in diseased demonstrated by DNA hybridization that S. suis was pigs than in healthy ones. An outbreak of S. suis type not closely related to group D streptococci. 9 infection recently occurred in swine in Canada? Despite the presence of 9 capsular types of S. suis, Although 22 of 23 capsular types of S. suis are present untypeable isolates were still frequently reported. These in North America, our studies revealed that almost isolates were recovered from diseased4,10,12,13,22,24,25 and 30% of isolates recovered from diseased pigs are still clinically healthy pigs.2,3,21 More recently, 14 new capuntypeable (unpublished data). sular types have been described.9 Some of the reference As the number of capsular types increases, serotypstrains originated from diseased pigs, whereas others ing becomes more complicated, and it could soon bewere from the nasal cavities of clinically healthy pigs. come limited to reference laboratories. Even biochemOne strain was isolated from a diseased calf (capsular ical identification, when used alone, can be misleading. type 20) and another from a human case of meningitis Thus, veterinary diagnostic laboratories face a difficult (capsular type 14). Isolates belonging to capsular type situation. There is an urgent need for a standardization 14 have recently been recovered from diseased pigs in of both the biochemical identification of S. suis and our laboratory as well as in Denmark and Belgium (Dr. the techniques for capsular typing. The purpose of this J. Henrichsen, personal communication, 1989). This paper is to summarize the knowledge of biochemical confirms that types other than capsular type 2 could characterization and serotyping of S. suis and to suggest be involved in a zoonosis.9 In addition to the bovine means to facilitate the proper identification of this incapsular type 20, several other S. suis isolates have fectious agent of growing importance. been recovered from ruminants.9,12 Capsular type 16 Streptococcus suis is a gram-positive coccus that ocwas recently isolated in our laboratory from lungs of curs singly, frequently in pairs, or occasionally in short a calf suffering from bronchopneumonia, and a capchains. The organism grows well in aerobiosis, but growth is often enhanced by microaerophilic condiFrom the Research Group in Swine Infectious Diseases, Faculty of Veterinary Medicine, University of Montreal, Saint-Hyacinthe, tions. The majority of strains are alpha-hemolytic on

Streptococcus suis serotype 2 mutants deficient in capsular expression.

Streptococcus suis serotype 2 is responsible for a wide variety of porcine infections. In addition, it is considered a zoonotic agent. Knowledge about the virulence factors for this bacterium is limited but its polysaccharide capsule is thought to be one of the most important. Transposon mutagenesis with the self-conjugative transposon Tn916 was used to obtain acapsular mutants from the virulent S. suis type 2 reference strain S735. Clones were screened by colony-dot ELISA with a monoclonal antibody specific for a type 2 capsular epitope and clones that failed to react with the antibody were characterized. Two mutants, 2A and 79, having one and two Tn916 insertions respectively, were chosen for further characterization. Absence of capsule was confirmed by coagglutination, capillary precipitation and capsular reaction tests and by transmission electron microscopy. Absence of capsular polysaccharides correlated with increased hydrophobicity and phagocytosis by both murine macrophages and porcine monocytes compared to the wild-type strain. Furthermore, both mutants were shown to be avirulent in murine and pig models of infection. Finally, mutant 2A was readily eliminated from circulation in mice compared to the wild-type strain, which persisted more than 48 h in blood. Thus, isogenic mutants defective in capsule production demonstrate the importance of capsular polysaccharides as a virulence factor for S. suis type 2.

Streptococcus suis serotype 2, an important swine and human pathogen, induces strong systemic and cerebral inflammatory responses in a mouse model of infection.

Streptococcus suis, an important swine and human pathogen, causes septic shock and meningitis. The pathogenesis of both systemic and CNS infections caused by S. suis is poorly understood. A hematogenous model of infection in CD1 mice was developed to study the systemic release of cytokines during the septic shock phase and the proinflammatory events in the CNS associated with this pathogen. Using a liquid array system, high levels of systemic TNF-α, IL-6, IL-12, IFN-γ, CCL2, CXCL1, and CCL5 were observed 24 h after infection and might be responsible for the sudden death of 20% of animals. Infected mice that survived the early sepsis later developed clinical signs of meningitis and exhibited lesions in the meninges and in numerous regions of the brain, such as the cortex, hippocampus, thalamus, hypothalamus, and corpus callosum. Bacterial Ags were found in association with microglia residing only in the affected zones. In situ hybridization combined with immunocytochemistry showed transcriptional activation of TLR2 and TLR3 as well as CD14, NF-κB, IL-1β, CCL2, and TNF-α, mainly in myeloid cells located in affected cerebral structures. Early transcriptional activation of TLR2, CD14, and inflammatory cytokines in the choroid plexus and cells lining the brain endothelium suggests that these structures are potential entry sites for the bacteria into the CNS. Our data indicate an important role of the inflammatory response in the pathogenesis of S. suis infection in mice. This experimental model may be useful for studying the mechanisms underlying sepsis and meningitis during bacterial infection.

Streptococcus suis sequence type 7 outbreak, Sichuan, China.

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.