Development of an Indirect Dot-PPA-ELISA using glutamate dehydrogenase as a diagnostic antigen for the rapid and specific detection of Streptococcus suis and its application to clinical specimens

A CRISPR-Cas12a-based platform facilitates the detection and serotyping of Streptococcus suis serotype 2

Streptococcus suis serotype 2 is an economically important zoonotic pathogen that causes septicemia, arthritis, and meningitis in pigs and humans. S. suis serotype 2 is responsible for substantial economic losses to the swine industry and poses a serious threat to public health, and accurate and rapid detection is important for the prevention and control of epidemic disease. In this study, we developed a high-fidelity detection and serotyping platform for S. suis serotype 2 based on recombinase polymerase amplification (RPA) and a clustered regularly interspaced short palindromic repeat (CRISPR)-Cas12a system called Cards-SSJ/K. Cards-SSJ had a detection limit of 10 CFU, takes <60 min, and no cross-reaction was found with other S. suis serotypes, closely related Streptococcus spp., or common pig pathogens, and Cards-SSK could differentiate serotype 2 from serotype 1/2. Results from Cards-SSJ and qPCR were equivalent in detecting S. suis serotype 2 in tissue samples. Analysis indicated that despite a relatively high reagent cost compared to PCR and qPCR, Cards-SSJ was less time-consuming and had low requirements for equipment and personnel. Thus, it is an excellent method for point-of-care detection for S. suis serotype 2.

Ratiometric electrochemical lateral flow immunoassay for the detection of Streptococcus suis serotype 2

An electrochemical lateral flow immunoassay (eLFIA) strip with high reproducibility was developed to rapidly and accurately detect Streptococcus suis serotype 2. This proposed strip was fabricated by integrating ratiometric electrochemical detection and LFIA (R-eLFIA). The R-eLFIA exhibited excellent reproducibility, which was improved by 3.8 times compared to a single electrode. A dual-working screen-printed graphene electrode (SPGE) was designed by tuning the working electrode with electroactive species in the biosensing system. Ferrocene carboxylic acid (Fc) was used as a signal probe, and sunset yellow (SY) at one working electrode was used as an internal reference signal to provide a built-in correction for reducing the effects of inherent background current. S. suis serotype 2-specific antibodies were immobilized on a nitrocellulose membrane of LFIA, which is located on the position of Fc-SPGE. In the presence of the analyte, an immunocomplex formed on the region of Fc-SPGE, causing a decrease in Fc current while SY current remained constant. The current ratio's decrease was proportional to S. suis serotype 2's concentration. Under optimization, this biosensor showed good linearity in the range of 102-1010 CFU/mL with a limit of detection of 10 CFU/mL and achieved a rapid detection time (15 min). Moreover, the R-eLFIA biosensor exhibited excellent reproducibility and high selectivity and was applied in human serum samples. Thus, this study successfully matched the advantages of the ratiometric strategy and LFIA and has great potential to be used as an effective tool for rapidly detecting S. suis serotype 2 in clinical samples.

Establishment and Application of an Indirect ELISA for the Detection of Antibodies to Porcine Streptococcus suis Based on a Recombinant GMD Protein

S. suis is an important zoonotic pathogen from sick and recessive carrier pigs that poses a serious threat to animal husbandry production and public health. It usually causes horizontal transmission among pigs. The morbidity and mortality of this disease are very high. Human infection is caused through direct or indirect contact with sick pigs. The two large-scale outbreaks in China were due to the outbreak of S. suis on pig farms, which spread to human infection; thus, detecting S. suis in pig herds is crucial. At present, the commercial S. suis ELISA type 2 kits on the market can only detect single serotypes, high probabilities of interaction reactions, and biosafety risks when using inactivated S. suis as an antigen. Phosphate-3-glyceraldehyde dehydrogenase (GAPDH), muramidase-released protein (MRP), and dihydrolipoamide dehydrogenase (DLDH) are important S. suis type 2, S. suis type 7, and S. suis type 9 protective antigens. This study purified the GMD protein (B-cell-dominant epitopes of GAPDH, MRP, and DLDH antigens) and used a diverse combination of dominant epitopes of the multiple different antigens as coated antigens, improving the sensitivity and safety of the indirect ELISA experiments. An indirect ELISA method (GMD-ELISA) was developed for detecting S. suis antibodies. The antigen-antibody response was optimized using checkerboard titration. The results of testing using ELISA for Salmonella enterica (S. enterica), Escherichia coli (E. coli), Staphylococcus aureus (SA), and Streptococcus pyogenes (S. pyogenes) were all negative, indicating that this method had strong specificity. The results were still positive when the dilution ratio of S. suis-positive serum reached 1:6, 400, thus indicating that the method had high sensitivity. The results of the reproducibility assay for indirect ELISA showed that the intra-assay coefficient of variation and the inter-assay coefficient of variation were less than 10%, indicating that the method had good repeatability. We investigated the seroprevalence of S. suis in 167 serum samples collected in East China, and 33.5% of the samples were positive for antibodies against S. suis, indicating that the prevalence of S. suis is high in pig farms in Eastern China. The novel GMD-ELISA is a convenient, sensitive, and specific diagnostic method that provides technical support for rapid diagnosis and epidemiological investigation.

Specific quantitative detection of Streptococcus suis and Actinobacillus pleuropneumoniae in co-infection and mixed biofilms

Respiratory infections seriously affect the swine industry worldwide. Co-infections of two vital pathogenic bacteria Streptococcus suis (S. suis) and Actinobacillus pleuropneumoniae (A. pleuropneumoniae), colonizing the respiratory tract often occurs in veterinary clinical practice. Moreover, our previous research found that S. suis and A. pleuropneumoniae can form biofilm in vitro. The formation of a mixed biofilm not only causes persistent infections, but also increases the multiple drug resistance of bacteria, which brings difficulties to disease prevention and control. However, the methods for detecting S. suis and A. pleuropneumoniae in co-infection and biofilm are immature. Therefore, in this study, primers and probes were designed based on the conservative sequence of S. suis gdh gene and A. pleuropneumoniae apxIVA gene. Then, a TaqMan duplex real-time PCR method for simultaneous detection of S. suis and A. pleuropneumoniae was successfully established via optimizing the reaction system and conditions. The specificity analysis results showed that this TaqMan real-time PCR method had strong specificity and high reliability. The sensitivity test results showed that the minimum detection concentration of S. suis and A. pleuropneumoniae recombinant plasmid was 10 copies/μL, which is 100 times more sensitive than conventional PCR methods. The amplification efficiencies of S. suis and A. pleuropneumoniae were 95.9% and 104.4% with R² value greater than 0.995, respectively. The slopes of the calibration curves of absolute cell abundance of S. suis and A. pleuropneumoniae were 1.02 and 1.09, respectively. The assays were applied to cultivated mixed biofilms and approximately 10⁸ CFUs per biofilm were quantified when 10⁸ CFUs planktonic bacteria of either S. suis or A. pleuropneumoniae were added to biofilms. In summary, this study developed a TaqMan real-time PCR assay for specific, accurate quantification of S. suis or A. pleuropneumoniae in mixed biofilms, which may help for the detection, prevention and control of diseases caused by a bacterial mixed infection involving S. suis and A. pleuropneumoniae.

Inactivation of the htpsA gene affects capsule development and pathogenicity of Streptococcus suis

Streptococcus suis

serotype 2 (S. suis 2) is an important swine pathogen and also an emerging zoonotic agent. HtpsA has been reported as an immunogenic cell surface protein on the bacterium. In the present study, we constructed an isogenic mutant strain of htpsA, namely ΔhtpsA, to study its role in the development and virulence of S. suis 2. Our results showed that the mutant strain lost its typical encapsulated structure with decreased concentrations of sialic acid. Furthermore, the survival rate in whole blood, the anti-phagocytosis by RAW264.7 murine macrophage, and the adherence ability to HEp-2 cells were all significantly affected in the ΔhtpsA. In addition, the deletion of htpsA sharply attenuated the virulence of S. suis 2 in an infection model of mouse. RNA-seq analysis revealed that 126 genes were differentially expressed between the ΔhtpsA and the wild-type strains, including 28 upregulated and 98 downregulated genes. Among the downregulated genes, many were involved in carbohydrate metabolism and synthesis of virulence-associated factors. Taken together, htpsA was demonstrated to play a role in the morphological development and pathogenesis of the highly virulent S. suis 2 05ZYH33 strain.

How Streptococcus suis serotype 2 attempts to avoid attack by host immune defenses

Streptococcus suis (S. suis) type 2 (SS2) is an important zoonotic pathogen that causes swine streptococcosis, a widespread infectious disease that occurs in pig production areas worldwide and causes serious economic losses in the pork industry. Hosts recognize pathogen-associated molecular patterns (PAMPs) through pattern recognition receptors (PRRs) to activate both innate and acquired immune responses. However, S. suis has evolved multiple mechanisms to escape host defenses. Pathogenic proteins, such as enolase, double-component regulatory systems, factor H-combining proteins and other pathogenic and virulence factors, contribute to immune escape by evading host phagocytosis, reactive oxygen species (ROS), complement-mediated immune destruction, etc. SS2 can prevent neutrophil extracellular trap (NET) formation to avoid being trapped by porcine neutrophils and disintegrate host immunoglobulins via IgA1 hydrolases and IgM proteases. Currently, the pathogenesis of arthritis and meningitis caused by SS2 infection remains unclear, and further studies are necessary to elucidate it. Understanding immune evasion mechanisms after SS2 infection is important for developing high-efficiency vaccines and targeted drugs.

Methods for the detection and characterization of Streptococcus suis: from conventional bacterial culture methods to immunosensors

One of the most important zoonotic pathogens worldwide, Streptococcus suis is a swine pathogen that is responsible for meningitis, toxic shock and even death in humans. S. suis infection develops rapidly with nonspecific clinical symptoms in the early stages and a high fatality rate. Recently, much attention has been paid to the high prevalence of S. suis as well as the increasing incidence and its epidemic characteristics. As laboratory-acquired infections of S. suis can occur and it is dangerous to public health security, timely and early diagnosis has become key to controlling S. suis prevalence. Here, the techniques that have been used for the detection, typing and characterization of S. suis are reviewed and the prospects for future detection methods for this bacterium are also discussed.

Molecularly imprinted polymer nanoparticle-based assay (MINA): application for fumonisin B1 determination

The enzyme-linked immunosorbent assay (ELISA) has been used as a standard tool for monitoring food and animal feed contamination from the carcinogenic fumonisin B1 (FB1).