Rapid and reliable identification of Streptococcus pneumoniae isolates by pneumolysin-mediated agglutination

Electrochemical Detection for Isothermal Loop-Mediated Amplification of Pneumolysin Gene of Streptococcus pneumoniae Based on the Oxidation of Phenol Red Indicator

A highly sensitive electrochemical methodology for end-point detection of loop-mediated isothermal nucleic acid amplification reactions was developed. It is based on the oxidation process of phenol red (PR), commonly used as a visual indicator. The dependence of its redox process on pH, which changes during amplification, allows performing quantitative measurements. Thus, the change in the oxidation potential of PR during the amplification is used, for the first time, as the analytical signal that correlates with the number of initial DNA copies. As a proof-of-concept, the amplification of the pneumolysin gene from Streptococcus pneumoniae, one of the main pathogens causing community-acquired pneumonia, is performed. Combination of isothermal amplification with electrochemical detection, performed on small-size flexible electrodes, allows easy decentralization. Adaptation to the detection of other pathogens causing infectious diseases would be very useful in the prevention of future epidemics.

Magnetic Nanoclusters Increase the Sensitivity of Lateral Flow Immunoassays for Protein Detection: Application to Pneumolysin as a Biomarker for Streptococcus pneumoniae

Lateral flow immunoassays for detecting biomarkers in body fluids are simple, quick, inexpensive point-of-care tests widely used in disease surveillance, such as during the coronavirus disease 2019 (COVID-19) pandemic. Improvements in sensitivity would increase their utility in healthcare, food safety, and environmental control. Recently, biofunctional magnetic nanoclusters have been used to selectively label target proteins, which allows their detection and quantification with a magneto-inductive sensor. This type of detector is easily integrated with the lateral flow immunoassay format. Pneumolysin is a cholesterol-dependent cytolysin and one of the most important protein virulence factors of pneumonia produced by Streptococcus pneumoniae. It is recognized as an important biomarker for diagnosis in urine samples. Pneumonia is the infectious disease that causes the most deaths globally, especially among children under five years and adults over 65 years, most of them in low- and middle-income countries. There especially, a rapid diagnostic urine test for pneumococcal pneumonia with high sensitivity and specificity would be helpful in primary care. In this work, a lateral flow immunoassay with magnetic nanoclusters conjugated to anti-pneumolysin antibodies was combined with two strategies to increase the technique's performance. First, magnetic concentration of the protein before the immunoassay was followed by quantification by means of a mobile telephone camera, and the inductive sensor resulted in detection limits as low as 0.57 ng (telephone camera) and 0.24 ng (inductive sensor) of pneumolysin per milliliter. Second, magnetic relocation of the particles within the test strip after the immunoassay was completed increased the detected signal by 20%. Such results obtained with portable devices are promising when compared to non-portable conventional pneumolysin detection techniques such as enzyme-linked immunosorbent assays. The combination and optimization of these approaches would have excellent application in point-of-care biodetection to reduce antibiotic misuse, hospitalizations, and deaths from community-acquired pneumonia.

Detection of Streptococcus pneumoniae in Urine by Loop-Mediated Isothermal Amplification

Objective To assess the loop-mediated isothermal amplification (LAMP) to detect cell-free DNA from Streptococcus pneumoniae in urine samples from children with pneumococcal pneumonia.

Methods LAMP reactions using four primers (backward inner primer, forward inner primer, B3, and F3) targeting conserved regions of the S. pneumoniae ply gene and DNA from the recombinant plasmid pTrc99A-ply were optimized for temperature (65°C) and MgSO4 concentration (8 mM) conditions. Urine samples from 71 patients with symptoms of pneumonia and from 17 healthy children were tested side by side using the isothermal methodology LAMP and the commercial urinary antigen test, BinaxNOW S. pneumoniae assay. Percentages of sensitivity, specificity, positive predictive value (PPV), negative predictive value, and positive (LR) were calculated to compare both tests.

Results The specificity of the LAMP reaction was confirmed against several species of bacteria and yeast that can cause pneumonia or urine infections. The suitability of the LAMP assay was evaluated in urine samples from 71 patients and 17 healthy children. All patients (100%) with confirmed pneumococcal pneumonia were positive for the LAMP assay. Among patients with possible/probable pneumonia, 74.1% were identified as positive using the LAMP test. Notably, a higher specificity (95.4%), PPV (94.1%) and positive LR (21.7) were found compared with the urinary antigen test.

Conclusion The presence of S. pneumoniae cell-free DNA in urine samples of pediatric patients can be used as a specific diagnostic biomarker for community-acquired pneumonia by using the LAMP methodology.

Protein Expression Analysis by Western Blot and Protein-Protein Interactions

Western blot analysis is widely used for detecting protein expression, analysis of protein-protein interactions, and searching for new biomarkers. Also, it is a diagnostic tool used for detection of human diseases and microorganism infections.Some Streptococcus pneumoniae proteins are important virulence factors and a few of them are diagnostic markers. Here, we describe the detection of two pneumococcal proteins, pneumolysin and PpmA, in human urine by using monoclonal and polyclonal antibodies.

Interpreting assays for the detection of Streptococcus pneumoniae

Streptococcus pneumoniae is both an aggressive pathogen and a normal part of the human respiratory microbiome. Clinicians and microbiologists have struggled to develop tests that can identify pneumococcal respiratory infection and accurately distinguish colonization from invasive disease. Molecular methods hold the promise of an improved ability to rapidly detect microorganisms in respiratory secretions and to make an accurate diagnosis; however, interpretation of diagnostic testing for S. pneumoniae remains problematic. Molecular assays, such as those targeting the pneumolysin gene, may cross-react with other streptococcal species, confounding detection and quantification. Assays that target the autolysin gene appear to be more specific. Even when accurately identified, however, the significance of S. pneumoniae DNA detected in clinical samples is difficult to determine. Here we will discuss the challenges faced in the interpretation of molecular testing for S. pneumoniae, and some strategies that might be used to improve our ability to diagnose pneumococcal respiratory infection.

Streptococcus pneumoniaeProtein Vaccine Candidates: Properties, Activities and Animal Studies

Streptococcus pneumoniae is a causative agent for community acquired pneumonia, bacteremia, acute otitis media, and meningitis. Recent emergence of multi-drug resistant clinical isolates prompts the need of effective vaccine for the prevention of disease. The licensed polysaccharide-based pneumococcal vaccines only elicit protective antibodies against the infection of serotypes that are included in the vaccine. To broaden the protection, the use of pneumococcal proteins will be a feasible and preferable alternative. This communication provides a review on the biochemical properties of these protein candidates, their immunization results in animal studies, and perspectives on the development of protein-based pneumococcal vaccine.

Oriented immobilisation of anti-pneumolysin Fab through a histidine tag for electrochemical immunosensors

Orientation of reagents is a key step in the construction of immunosensors. When the immunoreagent is a recombinant protein, this can be achieved by employing hexahistidine tags. The orientation of recombinant histidine-tagged Fab fragments of monoclonal anti-pneumolysin antibodies on gold films is evaluated. Using histidine as a chelator of Ni or employing an anti-polyhistidine antibody for capturing the His6 residue is considered. Measurements are based in the signal of indigo, which comes from the hydrolysis of 3-indoxylphosphate by alkaline phosphatase (AP). The attachment of the enzyme occurs through the interaction of biotin with AP-labelled streptavidin or employing AP-conjugated immunoreagents. In the case of the interaction Ni-histidine, for the study of the self-assembling process a His-tagged and biotinylated protein (His6-GST-B) was employed. General conditions were studied and non-specific adsorption was avoided with the use of 1-hexanethiol. Improvements of the signal compared with the direct adsorption were only achieved by the use of histidine capturing antibodies. With an optimised ratio anti-polyhis:His6-Fab the signal increases approximately a 100%. Precision is adequate and the response is linear with the concentration of pneumolysin between 0.1 and 10 ng/mL.

Streptococcus pneumoniae virulence factors and their clinical impact: An update

The morbidity and mortality rates associated with Streptococcus pneumoniae remain very high worldwide. The virulence of this bacterium is largely dependent on its polysaccharide capsule, which is quite heterogeneous and represents a serious obstacle for designing effective vaccines. However, it has been demonstrated that numerous protein virulence factors are involved in the pathogenesis of pneumococcal disease. An important related finding from experimental animal models is that non-capsulated strains of pneumococci are protective against capsulated ones. Hence, new vaccine designs are focused on the surface proteins (e. g., PspA and PspC) and on the cytolysin, pneumolysin. Moreover, several virulence factors have potential value for pneumococcal diagnosis by urinalysis. In this paper, we review the virulence factors involved in bacteria-host interactions, and the new developments in vaccines and diagnostic methods.

Role of pneumolysin for the development of acute lung injury in pneumococcal pneumonia

OBJECTIVE

Acute respiratory failure is a significant complication of severe pneumococcal pneumonia. In a mouse model, we observed early-onset lung microvascular leakage after pulmonary infection with Streptococcus pneumoniae, and we hypothesized that the important virulence factor pneumolysin may be the direct causative agent.

DESIGN

Controlled, in vivo, ex vivo, and in vitro laboratory study.

SETTING

Laboratory.

SUBJECTS

Female mice, 8-12 wks old.

INTERVENTIONS

Ventilated and blood-free perfused murine lungs were challenged with recombinant pneumolysin via the airways as well as via the vascular bed. In addition, we analyzed the impact of pneumolysin on electrical cell impedance and hydraulic conductivity of human umbilical vein endothelial cell (HUVEC) and alveolar epithelial cell (A549) monolayers.

MEASUREMENTS AND MAIN RESULTS

Aerosolized pneumolysin dose-dependently increased capillary permeability with formation of severe lung edema but did not affect pulmonary vascular resistance. Intravascular pneumolysin caused an impressive dose-dependent increase in pulmonary vascular resistance and in lung microvascular permeability. By immunohistochemistry, pneumolysin was detected mainly in endothelial cells of pulmonary arterial vessels, which concomitantly displayed strong vasoconstriction. Moreover, pneumolysin increased permeability of HUVEC and A549 monolayers. Interestingly, immunofluorescence of endothelial cell monolayers exposed to pneumolysin showed gap formation and moderate stress fiber generation.

CONCLUSIONS

Pneumolysin may play a central role for early-onset acute lung injury due to severe pneumococcal pneumonia by causing impairment of pulmonary microvascular barrier function and severe pulmonary hypertension.

Identification, prevalence and population structure of non-typable Streptococcus pneumoniae in carriage samples isolated from preschoolers attending day-care centres

The authors aimed to get insights into the population structure of non-(sero)typable pneumococci (NTPn), a specific group of natural atypical pneumococci whose identification is often difficult, and which has remained insufficiently studied. A total of 265 presumptive NTPn, isolated between 1997 and 2003 from the nasopharynx of children, were characterized. Strains were confirmed to be pneumococci on the basis of bile solubility, and PCR detection or Southern blotting hybridization of lytA and psaA, genes ubiquitous in this species. Multilocus sequence typing (MLST) was used to exclude two isolates that gave ambiguous results. Non-typability was confirmed by the Quellung reaction using Omniserum. A total of 213 isolates were considered to be true NTPn. The molecular analysis of the true NTPn by PFGE and MLST showed that this population was genetically diverse, although a dominant cluster, accounting for 66 % of the isolates, was identified. Antimicrobial resistance was observed in most genetic backgrounds, and multidrug resistance to penicillin, erythromycin, clindamycin, tetracycline and sulfamethoxazole-trimethoprim was associated with strains belonging to the dominant cluster. Comparison with PFGE fingerprints and sequence types of large collections of serotypable strains showed that the genetic backgrounds of all but two NTPn were different from those found in serotypable strains. In addition, we found that NTPn strains with similar genetic backgrounds to those identified in our study had been isolated from disease sources in other countries. These observations seem to indicate that NTPn have diverse genetic backgrounds and may have evolved as a distinct group of pneumococcal isolates.

Accuracy of phenotypic and genotypic testing for identification of Streptococcus pneumoniae and description of Streptococcus pseudopneumoniae sp. nov

We have identified an unusual group of viridans group streptococci that resemble Streptococcus pneumoniae. DNA-DNA homology studies suggested that a subset of these isolates represent a novel species that may be included in the S. oralis-S. mitis group of viridans group streptococci. We suggest that this novel species be termed Streptococcus pseudopneumoniae. A combination of phenotypic and genetic reactions allows its identification. S. pseudopneumoniae strains do not have pneumococcal capsules, are resistant to optochin (inhibition zones, less than 14 mm) when they are incubated under an atmosphere of increased CO2 but are susceptible to optochin (inhibition zones, >14 mm) when they are incubated in ambient atmospheres, are not soluble in bile, and are positive by the GenProbe AccuProbe Pneumococcus test. The bile solubility test is more specific than the optochin test for identification of S. pneumoniae. Genetic tests for pneumolysin (ply) and manganese-dependent superoxide dismutase (sodA) and identification tests with a commercial probe, AccuProbe Pneumococcus, do not discriminate between the new species and S. pneumoniae.

Pneumolysin is a key factor in misidentification of macrolide-resistant Streptococcus pneumoniae and is a putative virulence factor of S. mitis and other streptococci

We evaluated the applicability of ply PCR for confirmation of the identification of Streptococcus pneumoniae. lytA PCR, 16S rRNA sequencing, and amplified-fragment length polymorphism were used as reference methods. In contrast to the lytA gene, the ply gene proved to be not specific for S. pneumoniae. The presence of the ply gene in other streptococci, in particular Streptococcus mitis, suggests that pneumolysin plays a pathogenic role.

Protection against pneumococcal pneumonia in mice by monoclonal antibodies to pneumolysin

Pneumolysin (PLY) is an important virulence factor of Streptococcus pneumoniae. We examined the ability of three murine monoclonal antibodies (MAbs) to PLY (PLY-4, PLY-5, and PLY-7) to affect the course of pneumococcal pneumonia in mice. The intravenous administration of antibodies PLY-4 and PLY-7 protected the mice from the lethal effect of the purified toxin. Mice treated with PLY-4 before intranasal inoculation of S. pneumoniae type 2 survived longer (median survival time, 100 h) than did untreated animals (median survival time, 60 h) (P < 0.0001). The median survival time for mice treated with a combination of PLY-4 and PLY-7 was 130 h, significantly longer than that for mice given isotype-matched indifferent MAbs (P = 0.0288) or nontreated mice (P = 0.0002). The median survival time for mice treated with a combination of three MAbs was significantly longer (>480 h) than that for mice treated with PLY-5 (48 h; P < 0.0001), PLY-7 (78 h; P = 0.0007), or PLY-4 (100 h; P = 0.0443) alone. Similarly, the survival rate for mice treated with three MAbs (10 of 20 mice) was significantly higher than the survival rate obtained with PLY-5 (1 of 20; P = 0.0033), PLY-4 (2 of 20; P = 0.0138), or PLY-7 (3 of 20; P = 0.0407) alone. These results suggest that anti-PLY MAbs act with a synergistic effect. Furthermore, MAb administration was associated with a significant decrease in bacterial lung colonization and lower frequencies of bacteremia and tissue injury with respect to the results for the control groups.

Characterisation of mouse monoclonal antibodies for pneumolysin: fine epitope mapping and V gene usage

Pneumolysin (PLY) is a cholesterol-dependent cytolysin (CDC) produced by Streptococcus pneumoniae, the main cause of community-acquired pneumonia. We have applied a set of diverse molecular methodologies (PCR-derived PLY peptides, biopanning of a library of phage-displayed random nonapeptides, indirect ELISA and competition tests with soluble peptides) to achieve concordant complementary observations in order to obtain a fine epitope mapping of three mouse monoclonal antibodies (PLY-4, PLY-7 and PLY-8) for PLY. PLY-4 seems to recognise a conformation-dependent epitope with a core reactivity involving R232. The epitopes recognised by PLY-7 and PLY-8 are within the sequences (401)GQDLTAH(407) and (450)KRTISIWGT(458), respectively. PLY-7 also recognises suilysin (SLY), in which the homologous reactive amino acid stretch is (429)GVNLTSH(435). In a homology model of PLY with the crystal structure of perfringolysin O (PFO), R232 is part of a well-exposed contorted loop on the edge of the concave and convex faces of domain 1. The sequences reactive with PLY-7 and PLY-8 would conform one of the loops at the bottom of domain 4 and a beta strand of one of the two beta sheets of this domain, respectively. Western blot analyses carried out with anti-PLY rabbit IgG and polyclonal mouse serum identified stretches comprising residues 40-98, 199-248, 352-414 and 415-471 of PLY as immunogenic and antigenic; altogether with their recognition by the monoclonal antibodies herein considered, these results stress the immunological significance of domains 1 and 4 of the PLY molecule. PLY-4, PLY-7 and PLY-8 share the same Vkappa chain; this chain and that of the PLY-5 monoclonal antibody are essentially in germline configuration, whereas the VH regions of these monoclonals come from diverse gene segments and are mutated.

Immunodetection of pneumolysin in human urine by ELISA

An ELISA test has been employed for the detection of pneumolysin (PLY) in urine from 14 pneumococcal pneumonia patients and from 11 healthy adult volunteers. The urines of all the 11 healthy adult volunteers developed signals around the mean of the blanks, whereas all the pneumococcal pneumonia patient urines rendered signals at least five times this mean. Chemiluminescent Western blot analyses of these urines, carried out with the PLY-specific rabbit polyclonal IgG preparation used in ELISA, were negative. The 30-kDa filtrates of three high-signal urines were ELISA negative, suggesting that this ELISA test mainly detected high molecular weight forms in urine rather than free PLY-derived antigenic fragments. The urine sample, which rendered the highest ELISA signal, was then concentrated by filtration through a 10-kDa filter. When this concentrate was subjected to Western blot with the ELISA-capture monoclonal antibody, a major band was developed. Its relative molecular mass was similar to that of recombinant PLY and its peptide mass fingerprinting showed peptides corresponding to amino acid stretches from the four domains of the PLY molecule. When the pool of PLY-negative urines was sham-contaminated with purified recombinant pneumolysin, a conspicuous matrix effect was observed; nevertheless, this ELISA test was still reproducible and highly sensitive, detecting pneumolysin in the order of picograms per milliliter. A comparison was also made between this PLY-ELISA and the Binax NOW Streptococcus pneumoniae Urinary Antigen Test in analysing bacterial isolates. On the basis of the minimum number of pneumococci examined, both tests were shown to have similar potency, but strain-dependent discrepancies were observed. This ELISA could provide an alternative to the Binax NOW Streptococcus pneumoniae Urinary Antigen Test in the diagnosis of pneumococcal pneumonia.

Evaluation of gene-technological and conventional methods in the identification of Streptococcus pneumoniae

To find reliable methods able to identify the "difficult" Streptococcus pneumoniae isolates, an in-house polymerase chain reaction (PCR) for pneumolysin gene (Ply-PCR) and a commercial RNA hybridisation test (AccuProbe) were evaluated. Selected isolates of suspected pneumococci, sent for confirmation of identification and for serotyping, were classified into four groups based on their optochin sensitivity and capsule reaction. All isolates in Group 1, which consisted of 24 typical, optochin-sensitive, encapsulated pneumococcal strains, were positive in the Ply-PCR and AccuProbe tests. In Group 2, which consisted of 25 optochin-sensitive, but unencapsulated pneumococcal strains, all the isolates were positive in the Ply-PCR test, and 23 were positive in the AccuProbe test. In Group 3, which consisted of 15 atypical, optochin-resistant but encapsulated pneumococci, 12 of the isolates were positive in the Ply-PCR and 12 in the AccuProbe test, and 11 of these 12 strains were positive in both tests. In Group 4, which consisted of 36 equivocal optochin-resistant, unencapsulated isolates, 15 strains were positive in the Ply-PCR test and 8 strains in the AccuProbe test. As a conclusion, the Ply-PCR and AccuProbe tests identified similarly typical optochin-sensitive pneumococci, but gave partly controversial results about atypical pneumococci. Thus, they did not reliably help in the identification of suspected pneumococcal isolates lacking the conventional characteristics of pneumococcus.

A specific and ultrasensitive chemiluminescent sandwich ELISA test for the detection and quantitation of pneumolysin

A chemiluminescent sandwich ELISA test has been developed for the detection and quantitation of pneumolysin. The test is based on a mouse monoclonal as the capture antibody and on rabbit polyclonal IgGs as detection antibodies, in combination with an anti-rabbit IgG alkaline phosphatase conjugate. The estimated detection limit of the purified recombinant toxin in phosphate-buffered saline with 0.05% Triton X-100 is around 5 pg ml(-1), with averaged intra- and inter-assay variation coefficients of 7% and 13.5%, respectively. The assay has been applied to the quantitation of pneumolysin in pneumococcal isolates, providing, for the first time, a direct measurement of the amount of the toxin produced by different strains, a variation has been found in their pneumolysin content. The test is highly specific as no other purified toxins or human pneumonia- or meningitis-associated bacteria yielded false-positive results. This specific and highly sensitive method could help in the diagnosis of human infections.