Interactions of Streptococcus suis serotype 9 with host cells and role of the capsular polysaccharide: Comparison with serotypes 2 and 14

Lipoteichoic acids influence cell shape and bacterial division of Streptococcus suis serotype 2, but play a limited role in the pathogenesis of the infection

Streptococcus suis serotype 2 is a major swine pathogen and a zoonotic agent, causing meningitis in both swine and humans, responsible for substantial economic losses to the swine industry worldwide. The pathogenesis of infection and the role of bacterial cell wall components in virulence have not been fully elucidated. Lipoproteins, peptidoglycan, as well as lipoteichoic acids (LTA) have all been proposed to contribute to virulence. In the present study, the role of the LTA in the pathogenesis of the infection was evaluated through the characterisation of a mutant of the S. suis serotype 2 strain P1/7 lacking the LtaS enzyme, which mediates the polymerization of the LTA poly-glycerolphosphate chain. The ltaS mutant was confirmed to completely lack LTA and displayed significant morphological defects. Although the bacterial growth of this mutant was not affected, further results showed that LTA is involved in maintaining S. suis bacterial fitness. However, its role in the pathogenesis of the infection appears limited. Indeed, LTA presence reduces self-agglutination, biofilm formation and even dendritic cell activation, which are important aspects of the pathogenesis of the infection caused by S. suis. In addition, it does not seem to play a critical role in virulence using a systemic mouse model of infection.

Chlorogenic acid protects against intestinal inflammation and injury by inactivating the mtDNA-cGAS-STING signaling pathway in broilers under necrotic enteritis challenge

This study aimed to determine the effects of chlorogenic acid (CGA) on the growth performance, intestinal health, immune response, and mitochondrial DNA (mtDNA)-cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING) signaling pathway in broilers under necrotic enteritis (NE) challenge. The 180 one-day-old male Cobb 500 broilers with similar body weight of 44.59 ± 1.39 g were randomly allocated into 3 groups. The groups were control diet (Control group), control diet + NE challenge (NE group), and control diet + 500 mg/kg CGA + NE challenge (NE + CGA group), with 6 replicates per treatment. All broilers except the Control group were given sporulated coccidian oocysts (d 14) and Clostridium perfringens (d 19-21) by oral gavage. Our findings showed that CGA improved the growth performance and intestinal morphology in broilers under NE challenge. CGA supplementation elevated the barrier function in broilers under NE challenge, which reflected in the decreased serum concentrations of D-lactate and diamine oxidase, and upregulated jejunal protein expression of occludin. CGA supplementation also improved the immune function, which reflected in the increased concentrations and gene expressions of anti-inflammatory factors, and decreased concentrations and gene expressions of proinflammatory factors. CGA supplementation further enhanced intestinal cell proliferation and differentiation, which manifested in the increased number of goblet cells and positive cells of proliferating cell nuclear antigen on d 28 and 42. Furthermore, CGA supplementation decreased the mtDNA (d 42) and mitochondrial reactive oxygen species levels (d 28 and 42), and increased the mitochondrial membrane potential (d 42) and mitochondrial complex I (d 28 and 42) or III (d 28) activity. Broilers challenged with NE had upregulated jejunal protein expressions of cGAS, phospho-TANK-binding kinase 1, and phospho-interferon regulatory factor 7 compared with the Control group, which were downregulated after CGA supplementation. In conclusion, dietary supplementation CGA could protect against intestinal inflammation and injury by reducing the leakage of mtDNA and inactivating the cGAS-STING signaling pathway in broilers under NE challenge.

Streptococcus suis Research Update: Serotype Prevalence and Antimicrobial Resistance Distribution in Swine Isolates Recovered in Spain from 2020 to 2022

This study aimed to update the Streptococcus suis serotype distribution in Spain by analysing 302 clinical isolates recovered from diseased pigs between 2020 and 2022. The main objectives were to identify prevalent serotypes, differentiate specific serotypes 1, 14, 2, and 1/2, investigate specific genotypic and phenotypic antimicrobial resistance features, and explore associations between resistance genes and phenotypic resistances. Serotypes 9 (21.2%), 1 (16.2%), 2 (15.6%), 3 (6%), and 7 (5.6%) were the most prevalent, whereas serotypes 14 and 1/2 corresponded with 4.3% and 0.7% of all isolates. Antimicrobial resistance genes, including tet(O), erm(B), lnu(B), lsa(E), tet(M), and mef(A/E), were analysed, which were present in 85.8%, 65.2%, 7%, 7%, 6.3%, and 1% of the samples, respectively. Susceptibility testing for 18 antimicrobials revealed high resistance levels, particularly for clindamycin (88.4%), chlortetracycline (89.4%), and sulfadimethoxine (94.4%). Notably, seven significant associations (p < 0.0001) were detected, correlating specific antimicrobial resistance genes to the observed phenotypic resistance. These findings contribute to understanding the S. suis serotype distribution and its antibiotic resistance profiles in Spain, offering valuable insights for veterinary and public health efforts in managing S. suis-associated infections.

Interactions of Mycoplasma hyopneumoniae and/or Mycoplasma hyorhinis with Streptococcus suis Serotype 2 Using In Vitro Co-Infection Models with Swine Cells

Bacterial and/or viral co-infections are very common in swine production and cause severe economic losses. Mycoplasma hyopneumoniae, Mycoplasma hyorhinis and Streptococcus suis are pathogenic bacteria that may be found simultaneously in the respiratory tracts of pigs. In the present study, the interactions of S. suis with epithelial and phagocytic cells in the presence or absence of a pre-infection with M. hyopneumoniae and/or M. hyorhinis were studied. Results showed relatively limited interactions between these pathogens. A previous infection with one or both mycoplasmas did not influence the adhesion or invasion properties of S. suis in epithelial cells or its resistance to phagocytosis (including intracellular survival) by macrophages and dendritic cells. The most important effect observed during the co-infection was a clear increment in toxicity for the cells. An increase in the relative expression of the pro-inflammatory cytokines IL-6 and CXCL8 was also observed; however, this was the consequence of an additive effect due to the presence of different pathogens rather than a synergic effect. It may be hypothesized that if one or both mycoplasmas are present along with S. suis in the lower respiratory tract at the same time, then increased damage to epithelial cells and phagocytes, as well as an increased release of pro-inflammatory cytokines, may eventually enhance the invasive properties of S. suis. However, more studies should be carried out to confirm this hypothesis.

False positive detection of serum cryptococcal antigens due to insufficient sample dilution: A case series

At present, with the development of technology, the detection of cryptococcal antigen (CRAG) plays an increasingly important role in the diagnosis of cryptococcosis. However, the three major CRAG detection technologies, latex agglutination test (LA), lateral flow assay (LFA) and Enzyme-linked Immunosorbent Assay, have certain limitations. Although these techniques do not often lead to false-positive results, once this result occurs in a particular group of patients (such as human immunodeficiency virus patients), it might lead to severe consequences.

Laminin-binding protein of Streptococcus suis serotype 2 influences zinc acquisition and cytokine responses

Streptococcus suis serotype 2 is an important bacterial pathogen of swine, responsible for substantial economic losses to the swine industry worldwide. The knowledge on the pathogenesis of the infection caused by S. suis is still poorly known. It has been previously described that S. suis possesses at least one lipoprotein with double laminin and zinc (Zn)-binding properties, which was described in the literature as either laminin-binding protein (Lmb, as in the current study), lipoprotein 103, CDS 0330 or AdcAII. In the present study, the role of the Lmb in the pathogenesis of the infection caused by S. suis serotype 2 was dissected. Using isogenic mutants, results showed that Lmb does not play an important role in the laminin-binding activity of S. suis, even when clearly exposed at the bacterial surface. In addition, the presence of this lipoprotein does not influence bacterial adhesion to and invasion of porcine respiratory epithelial and brain endothelial cells and it does not increase the susceptibility of S. suis to phagocytosis. On the other hand, the Lmb was shown to play an important role as cytokine activator when tested in vitro with dendritic cells. Finally, this lipoprotein plays a critical role in Zn acquisition from the host environment allowing bacteria to grow in vivo. The significant lower virulence of the Lmb defective mutant may be related to a combination of a lower bacterial survival due to the incapacity to acquire Zn from their surrounding milieu and a reduced cytokine activation.

How bacteria utilize sialic acid during interactions with the host: snip, snatch, dispatch, match and attach

N -glycolylneuraminic acid (Neu5Gc), and its precursor N-acetylneuraminic acid (Neu5Ac), commonly referred to as sialic acids, are two of the most common glycans found in mammals. Humans carry a mutation in the enzyme that converts Neu5Ac into Neu5Gc, and as such, expression of Neu5Ac can be thought of as a 'human specific' trait. Bacteria can utilize sialic acids as a carbon and energy source and have evolved multiple ways to take up sialic acids. In order to generate free sialic acid, many bacteria produce sialidases that cleave sialic acid residues from complex glycan structures. In addition, sialidases allow escape from innate immune mechanisms, and can synergize with other virulence factors such as toxins. Human-adapted pathogens have evolved a preference for Neu5Ac, with many bacterial adhesins, and major classes of toxin, specifically recognizing Neu5Ac containing glycans as receptors. The preference of human-adapted pathogens for Neu5Ac also occurs during biosynthesis of surface structures such as lipo-oligosaccharide (LOS), lipo-polysaccharide (LPS) and polysaccharide capsules, subverting the human host immune system by mimicking the host. This review aims to provide an update on the advances made in understanding the role of sialic acid in bacteria-host interactions made in the last 5-10 years, and put these findings into context by highlighting key historical discoveries. We provide a particular focus on 'molecular mimicry' and incorporation of sialic acid onto the bacterial outer-surface, and the role of sialic acid as a receptor for bacterial adhesins and toxins.

Neutrophils in Streptococcus suis Infection: From Host Defense to Pathology

Streptococcus suis is a swine pathogen and zoonotic agent responsible for economic losses to the porcine industry. Infected animals may develop meningitis, arthritis, endocarditis, sepsis and/or sudden death. The pathogenesis of the infection implies that bacteria breach mucosal host barriers and reach the bloodstream, where they escape immune-surveillance mechanisms and spread throughout the organism. The clinical manifestations are mainly the consequence of an exacerbated inflammation, defined by an exaggerated production of cytokines and recruitment of immune cells. Among them, neutrophils arrive first in contact with the pathogens to combat the infection. Neutrophils initiate and maintain inflammation, by producing cytokines and deploying their arsenal of antimicrobial mechanisms. Furthermore, neutrophilic leukocytosis characterizes S. suis infection, and lesions of infected subjects contain a large number of neutrophils. Therefore, this cell type may play a role in host defense and/or in the exacerbated inflammation. Nevertheless, a limited number of studies addressed the role or functions of neutrophils in the context of S. suis infection. In this review, we will explore the literature about S. suis and neutrophils, from their interaction at a cellular level, to the roles and behaviors of neutrophils in the infected host in vivo.

Role of Maturation of Lipoproteins in the Pathogenesis of the Infection Caused by Streptococcus suis Serotype 2

Streptococcus suis serotype 2 is an important porcine bacterial pathogen associated with multiple pathologies in piglets. Bacterial lipoproteins (LPPs) have been described as playing important roles in the pathogenesis of the infection of other Gram-positive bacteria as adhesins, pro-inflammatory cell activators and/or virulence factors. In the current study, we aimed to evaluate the role of the prolipoprotein diacylglyceryl transferase (Lgt) and lipoprotein signal peptidase (Lsp) enzymes, which are responsible for LPP maturation, on the pathogenesis of the infection caused by two different sequence types (STs) of S. suis serotype 2 strains (virulent ST1 and highly virulent ST7). Through the use of isogenic Δlgt, Δlsp and double Δlgt/Δlsp mutants, it was shown that lack of these enzymes did not influence S. suis adhesion/invasion to porcine respiratory epithelial cells. However, in the absence of the Lsp and/or Lgt, a significant reduction in the capacity of S. suis to activate phagocytic cells and induce pro-inflammatory mediators (in vitro and in vivo) was observed. In general, results obtained with the double mutant did not differ in comparison to single mutants, indicating lack of an additive effect. Finally, our data suggest that these enzymes play a differential role in virulence, depending on the genetic background of the strain and being more important for the highly virulent ST7 strain.

Capsular polysaccharide switching in Streptococcus suis modulates host cell interactions and virulence

The capsular polysaccharide (CPS) of Streptococcus suis defines various serotypes based on its composition and structure. Though serotype switching has been suggested to occur between S. suis strains, its impact on pathogenicity and virulence remains unknown. Herein, we experimentally generated S. suis serotype-switched mutants from a serotype 2 strain that express the serotype 3, 4, 7, 8, 9, or 14 CPS. The effects of serotype switching were then investigated with regards to classical properties conferred by presence of the serotype 2 CPS, including adhesion to/invasion of epithelial cells, resistance to phagocytosis by macrophages, killing by whole blood, dendritic cell-derived pro-inflammatory mediator production and virulence using mouse and porcine infection models. Results demonstrated that these properties on host cell interactions were differentially modulated depending on the switched serotypes, although some different mutations other than loci of CPS-related genes were found in each the serotype-switched mutant. Among the serotype-switched mutants, the mutant expressing the serotype 8 CPS was hyper-virulent, whereas mutants expressing the serotype 3 or 4 CPSs had reduced virulence. By contrast, switching to serotype 7, 9, or 14 CPSs had little to no effect. These findings suggest that serotype switching can drastically alter S. suis virulence and host cell interactions.

Streptococcus suis Induces Expression of Cyclooxygenase-2 in Porcine Lung Tissue

Streptococcus suis is a common pathogen colonising the respiratory tract of pigs. It can cause meningitis, sepsis and pneumonia leading to economic losses in the pig industry worldwide. Cyclooxygenase-2 (COX-2) and its metabolites play an important regulatory role in different biological processes like inflammation modulation and immune activation. In this report we analysed the induction of COX-2 and the production of its metabolite prostaglandin E₂ (PGE₂) in a porcine precision-cut lung slice (PCLS) model. Using Western blot analysis, we found a time-dependent induction of COX-2 in the infected tissue resulting in increased PGE₂ levels. Immunohistological analysis revealed a strong COX-2 expression in the proximity of the bronchioles between the ciliated epithelial cells and the adjacent alveolar tissue. The morphology, location and vimentin staining suggested that these cells are subepithelial bronchial fibroblasts. Furthermore, we showed that COX-2 expression as well as PGE₂ production was detected following infection with two prevalent S. suis serotypes and that the pore-forming toxin suilysin played an important role in this process. Therefore, this study provides new insights in the response of porcine lung cells to S. suis infections and serves as a basis for further studies to define the role of COX-2 and its metabolites in the inflammatory response in porcine lung tissue during infections with S. suis.

In vitro characterization of granulocyte-colony stimulating factor (G-CSF) production by dendritic cells and macrophages during Streptococcus suis infection

Streptococcus suis serotype 2 is an important porcine bacterial pathogen and emerging zoonotic agent. Infections induce an exacerbated inflammation that can result in sudden death (septic shock) and meningitis. Though neutrophilic leukocytosis characterizes S. suis infection, the mediators involved are poorly understood. Among them, granulocyte-colony stimulating factor (G-CSF), a pro-inflammatory cytokine, triggers proliferation of neutrophil progenitors and neutrophil mobilization. However, the systemic production of G-CSF induced during S. suis infection, the cell types involved, and the underlying mechanisms remain unknown. In a S. suis serotype 2 mouse model of systemic infection, plasma levels of G-CSF rapidly increased after infection. S. suis activation of DCs and macrophages resulted in high (> 1000 pg/mL) and comparable production levels of G-CSF, as measured by ELISA. By using mutant strains deficient in capsular polysaccharide (CPS) or lipoprotein maturation in combination with purified lipoteichoic acid (LTA) from the latter mutant strain, it was showed that G-CSF production is mainly mediated by S. suis lipoproteins. The Toll-like receptor (TLR) pathway via myeloid differentiation primary response 88 (MyD88) is required for G-CSF production by DCs and macrophages following S. suis activation, with a partial involvement of TLR2. On the other hand, TLR2-independant G-CSF production induced by S. suis requires internalization and bacterial DNA might play a role in this pathway. Finally, these signals activated nuclear factor-kappa B (NF-κB) and mitogen-activated protein kinase (MAPK) pathways leading to G-CSF production. In conclusion, this study demonstrated for the first time that S. suis induces G-CSF production in vivo and DCs and macrophages are key cellular sources of this cytokine mediator, mainly via the binding of lipoproteins to TLR2. The CPS significantly reduced this activation, confirming the powerful role of this component in S. suis virulence. As such, this study contributes to better understand how DCs and macrophages produce G-CSF in response to S. suis, and potentially to other streptococci.