Current Taxonomical Situation of Streptococcus suis

Genotypic diversity of Streptococcus suis strains isolated from humans in Thailand

The purpose of this study is to characterize Streptococcus suis isolates recovered from human infections regarding serotype distribution, genotypic profile, clinical manifestations, and epidemiology. A total of 668 S. suis isolates recovered from human infections in Thailand were characterized based on serotyping by multiplex PCR and co-agglutination, genotypic profiles by multilocus sequence typing, and PCR for virulence-associated genes, as well as review of medical records. Serotype 2 (94.6%) was predominant, followed by serotype 14 (4.5%), 24 (0.45%), 5 (0.3%), and 4 (0.15%). Multilocus sequence typing analyses revealed seven clonal complexes (CC): CC1 (56.43%), CC104 (31.74%), CC233/379 (5.4%), CC25 (4.5%), CC28 (0.9%), CC221/234 (0.6%), CC94 (0.15%), and two singletons. The CC1 group contained serotype 2 and 14 isolates, while CC25, 28, 104, and 233/379 consisted of serotype 2 isolates only. CC221/234 contained serotype 5 and 24 isolates, whereas the single serotype 4 isolate belonged to CC94. Two singletons contained serotype 5 (ST235) and 2 (ST236) isolates. Our data showed that ST1 isolates were more associated with meningitis than those of other STs (p < 0.001). The major route of infection was shown to be close contact with infected pigs or contaminated raw pork-derived products, including occupational exposure and recent consumption of raw pork products. This study revealed a relatively large number of CCs of S. suis causing human infection in Thailand. Among them, CC1 followed by CC104, with serotype 2 isolates, are predominant. Food safety campaigns and public health interventions would be important for controlling the S. suis infection in humans.

Limited Interactions between Streptococcus Suis and Haemophilus Parasuis in In Vitro Co-Infection Studies

Streptococcus suis and Haemophilus parasuis are normal inhabitants of the porcine upper respiratory tract but are also among the most frequent causes of disease in weaned piglets worldwide, causing inflammatory diseases such as septicemia, meningitis and pneumonia. Using an in vitro model of infection with tracheal epithelial cells or primary alveolar macrophages (PAMs), it was possible to determine the interaction between S. suis serotype 2 and H. parasuis strains with different level of virulence. Within H. parasuis strains, the low-virulence F9 strain showed higher adhesion levels to respiratory epithelial cells and greater association levels to PAMs than the high-virulence Nagasaki strain. Accordingly, the low-virulence F9 strain induced, in general, higher levels of pro-inflammatory cytokines than the virulent Nagasaki strain from both cell types. In general, S. suis adhesion levels to respiratory epithelial cells were similar to H. parasuis Nagasaki strain. Yet, S. suis strains induced a significantly lower level of pro-inflammatory cytokine expression from epithelial cells and PAMs than those observed with both H. parasuis strains. Finally, this study has shown that, overall and under the conditions used in the present study, S. suis and H. parasuis have limited in vitro interactions between them and use probably different host receptors, regardless to their level of virulence.

Csl2, a novel chimeric bacteriophage lysin to fight infections caused by Streptococcus suis, an emerging zoonotic pathogen

Streptococcus suis is a Gram-positive bacterium that infects humans and various animals, causing human mortality rates ranging from 5 to 20%, as well as important losses for the swine industry. In addition, there is no effective vaccine for S. suis and isolates with increasing antibiotic multiresistance are emerging worldwide. Facing this situation, wild type or engineered bacteriophage lysins constitute a promising alternative to conventional antibiotics. In this study, we have constructed a new chimeric lysin, Csl2, by fusing the catalytic domain of Cpl-7 lysozyme to the CW_7 repeats of LySMP lysin from an S. suis phage. Csl2 efficiently kills different S. suis strains and shows noticeable activity against a few streptococci of the mitis group. Specifically, 15 µg/ml Csl2 killed 4.3 logs of S. suis serotype 2 S735 strain in 60 min, in a buffer containing 150 mM NaCl and 10 mM CaCl₂, at pH 6.0. We have set up a protocol to form a good biofilm with the non-encapsulated S. suis mutant strain BD101, and the use of 30 µg/ml Csl2 was enough for dispersing such biofilms and reducing 1–2 logs the number of planktonic bacteria. In vitro results have been validated in an adult zebrafish model of infection.

A Locus Encoding Variable Defense Systems against Invading DNA Identified in Streptococcus suis

Streptococcus suis, an important zoonotic pathogen, is known to have an open pan-genome and to develop a competent state. In S. suis, limited genetic lineages are suggested to be associated with zoonosis. However, little is known about the evolution of diversified lineages and their respective phenotypic or ecological characteristics. In this study, we performed comparative genome analyses of S. suis, with a focus on the competence genes, mobile genetic elements, and genetic elements related to various defense systems against exogenous DNAs (defense elements) that are associated with gene gain/loss/exchange mediated by horizontal DNA movements and their restrictions. Our genome analyses revealed a conserved competence-inducing peptide type (pherotype) of the competence system and large-scale genome rearrangements in certain clusters based on the genome phylogeny of 58 S. suis strains. Moreover, the profiles of the defense elements were similar or identical to each other among the strains belonging to the same genomic clusters. Our findings suggest that these genetic characteristics of each cluster might exert specific effects on the phenotypic or ecological differences between the clusters. We also found certain loci that shift several types of defense elements in S. suis. Of note, one of these loci is a previously unrecognized variable region in bacteria, at which strains of distinct clusters code for different and various defense elements. This locus might represent a novel defense mechanism that has evolved through an arms race between bacteria and invading DNAs, mediated by mobile genetic elements and genetic competence.

Porcine Dendritic Cells as an In Vitro Model to Assess the Immunological Behaviour of Streptococcus suis Subunit Vaccine Formulations and the Polarizing Effect of Adjuvants

An in vitro porcine bone marrow-derived dendritic cell (DC) culture was developed as a model for evaluating immune polarization induced by adjuvants when administered with immunogens that may become vaccine candidates if appropriately formulated. The swine pathogen Streptococcus suis was chosen as a prototype to evaluate proposed S. suis vaccine candidates in combination with the adjuvants Poly I:C, Quil A ®, Alhydrogel ®, TiterMax Gold ® and Stimune ®. The toll-like receptor ligand Poly I:C and the saponin Quil A ® polarized swine DC cytokines towards a type 1 phenotype, with preferential production of IL-12 and TNF-α. The water-in-oil adjuvants TiterMax Gold ® and Stimune ® favoured a type 2 profile as suggested by a marked IL-6 release. In contrast, Alhydrogel ® induced a type 1/type 2 mixed cytokine profile. The antigen type differently modified the magnitude of the adjuvant effect, but overall polarization was preserved. This is the first comparative report on swine DC immune activation by different adjuvants. Although further swine immunization studies would be required to better characterize the induced responses, the herein proposed in vitro model is a promising approach that helps assessing behaviour of the vaccine formulation rapidly at the pre-screening stage and will certainly reduce numbers of animals used while advancing vaccinology science.

Morin Attenuates Streptococcus suis Pathogenicity in Mice by Neutralizing Suilysin Activity

Streptococcus suis, a Gram-positive pathogen, is widely recognized as an important agent of swine infection, and it is also known to cause a variety of zoonoses, such as meningitis, polyarthritis and pneumonia. Suilysin (SLY), an extracellular pore-forming toxin that belongs to the cholesterol-dependent cytolysin family, is an essential virulence factor of S. suis capsular type 2 (SS2). Here, we found that morin hydrate (morin), a natural flavonoid that lacks anti-SS2 activity, inhibits the hemolytic activity of SLY, protects J774 cells from SS2-induced injury and protects mice from SS2 infection. Further, by molecular modeling and mutational analysis, we found that morin binds to the "stem" domain 2 in SLY and hinders its transformation from the monomer form to the oligomer form, which causes the loss of SLY activity. Our study demonstrates that morin hinders the cell lysis activity of SLY through a novel mechanism of interrupting the heptamer formation. These findings may lead to the development of promising therapeutic candidates for the treatment of SS2 infections.

Critical Streptococcus suis Virulence Factors: Are They All Really Critical?

Streptococcus suis is an important swine pathogen that can be transmitted to humans by contact with diseased animals or contaminated raw pork products. This pathogen possesses a coat of capsular polysaccharide (CPS) that confers protection against the immune system. Yet, the CPS is not the only virulence factor enabling this bacterium to successfully colonize, invade, and disseminate in its host leading to severe systemic diseases such as meningitis and toxic shock-like syndrome. Indeed, recent research developments, cautiously inventoried in this review, have revealed over 100 'putative virulence factors or traits' (surface-associated or secreted components, regulatory genes or metabolic pathways), of which at least 37 have been claimed as being 'critical' for virulence. In this review we discuss the current contradictions and controversies raised by this explosion of virulence factors and the future directions that may be conceived to advance and enlighten research on S. suis pathogenesis.

Capsular serotypes, virulence-associated genes and antimicrobial susceptibility of Streptococcus suis isolates from pigs in Korea

Streptococcus suis is an important pig pathogen with potential for human transmission. The serotype distributions and phenotypic characteristics vary over time and among regions; however, little is known about the characteristics of S. suis isolates in Korea. In this study, 240 S. suis isolates collected from pigs in Korea in 2009–2010 were serotyped by coagglutination tests, subsequently screened for three virulence-associated genes (mrp, epf and sly) and tested for antimicrobial susceptibility. As for 80 isolates, the serotypes of which were relevant to human infections, clonal complexes (CCs) were further identified by PCR. Serotype 3 was the most prevalent (15.8%), followed by serotype 2 (15.0%), with geographical variation for each serotype. Overall, 55.4% of the isolates carried mrp, whereas only 3.8% carried epf. CC25 was the most prevalent (41.3%) and was related to serotypes 2 and 9. The isolates showed higher susceptibility to ampicillin (93.4%) and ceftiofur (90.8%) than to the other antimicrobial agents tested. The highest resistance rate was observed to tetracycline (98.0%), followed by erythromycin (88.8%). In addition, the resistance to certain antimicrobials was significantly associated, in part, with virulence-associated genes or serotypes. Therefore, continuous characterization of S. suis is essential for the benefit of veterinary and human medicine.

Taxonomic update on proposed nomenclature and classification changes for bacteria of medical importance, 2016

A key aspect of medical, public health, and diagnostic microbiology laboratories is the accurate identification and rapid reporting and communication to medical staff regarding patients with infectious agents of clinical importance. Microbial taxonomy in the age of molecular diagnostics and phylogenetics creates changes in taxonomy at a logarithmic rate further complicating this process. This update focuses on the description of new species and classification changes proposed in 2016.

Initial steps of the pathogenesis of the infection caused by Streptococcus suis: fighting against nonspecific defenses

Interactions between a bacterial pathogen and its potentially susceptible host are initiated with the colonization step. During respiratory/oral infection, the pathogens must compete with the normal microflora, resist defense mechanisms of the local mucosal immunity, and finally reach, adhere, and breach the mucosal epithelial cell barrier in order to induce invasive disease. This is the case during infection by the swine and zoonotic pathogen Streptococcus suis, which is able to counteract mucosal barriers to induce severe meningitis and sepsis in swine and in humans. The initial steps of the pathogenesis of S. suis infection has been a neglected area of research, overshadowed by studies on the systemic and central nervous phases of the disease. In this Review article, we provide for the first time, an exclusive focus on S. suis colonization and the potential mechanisms involved in S. suis establishment at the mucosa, as well as the mechanisms regulating mucosal barrier breakdown. The role of mucosal immunity is also addressed. Finally, we demystify the extensive list of putative adhesins and virulence factors reported to be involved in the initial steps of pathogenesis by S. suis.