Detection and quantification of Panton-Valentine leukocidin in Staphylococcus aureus cultures by ELISA and Western blotting: diethylpyrocarbonate inhibits binding of protein A to IgG

A concise detection strategy of Staphylococcus aureus using N-Succinyl-Chitosan-dopped bacteria-imprinted composite film and AIE fluorescence sensor

Staphylococcus aureus is one of the major foodborne pathogens. Efficient detection and isolation of Staphylococcus aureus from complex samples are crucial. Herein, we report a concise strategy to detect of Staphylococcus aureus with high sensitivity and specificity, based on N-Succinyl-Chitosan doping bacteria-imprinted composite film and aggregation-induced emission (AIE)-featuring fluorescence sensor. The good shaping and mechanical properties of polydimethylsiloxane provide a specific recognition site suitable for Staphylococcus aureus. For the first time, chitosan derivatives is combined with polydimethylsiloxane to prepare a two-component composite film, which possesses a remarkable absorption performance of Staphylococcus aureus using the natural excellent absorption property of chitosan. The positive charged AIE-featuring Au(I)-disulfide nanoparticles realized the quantitative characterization of Staphylococcus aureus without cooperation with bio-recognition elements. To conclude, this study provides new possibilities for the manufacture of highly efficient bacterial separators with superior performance and facilitates the application of unlabeled nanoparticles in quantitative analysis.

Nusbiarylins Inhibit Transcription and Target Virulence Factors in Bacterial Pathogen Staphylococcus aureus

The emergence of multidrug resistance in the clinically significant pathogen Staphylococcus aureus is a global health burden, compounded by a diminishing drug development pipeline, and a lack of approved novel antimicrobials. Our previously reported first-in-class bacterial transcription inhibitors “nusbiarylins” presented a promising prospect towards the discovery of novel antimicrobial agents with a novel mechanism. Here we investigated and characterised the lead nusbiarylin compound, MC4, and several of its chemical derivatives in both methicillin-resistant S. aureus (MRSA) and the S. aureus type strains, demonstrating their capacity for the arrest of growth and cellular respiration, impairment of RNA and intracellular protein levels at subinhibitory concentrations. In some instances, derivatives of MC4 were also shown to attenuate the production of staphylococcal virulence factors in vitro, such as the exoproteins α-toxin and Panton–Valentine Leukocidin (PVL). Trends observed from quantitative PCR assays suggested that nusbiarylins elicited these effects possibly by acting via but not limited to the modulation of global regulatory pathways, such as the agr regulon, which coordinates the expression of S. aureus genes associated with virulence. Our findings encourage the continued development of more potent compounds within this novel family of bacterial transcription inhibitors.

Three-dimensional origami paper-based device for portable immunoassay applications

In this study, we demonstrate a three-dimensional surface-modified origami-paper-based analytical device (3D-soPAD) for immunoassay applications. The platform enables the sequential steps of immunoassays to be easily performed using a folded, sliding paper design featuring multiple pre-stored reagents, allowing us to take advantage of the vertical diffusion of the analyte through the different paper layers. The cellulose substrate is composed of carboxymethyl cellulose modified with N-(3-dimethylaminopropyl)-N'-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide, which provide covalent bonding sites for bio-recognition molecules. After the optimization of the operation parameters, we determined the detection limit of the 3D-soPAD for human immunoglobulin G (HIgG) which can be as low as 0.01 ng mL-1, with a total turnaround time of 7 min. In order to study the long-term storage of the platform, anti-HIgG horseradish peroxidase (aHIgG-HRP) conjugates were stored by freeze-drying in sugar matrices composed of 10% sucrose/10% trehalose (w/w%) on the paper device, retaining 80% of their activity after 75 days of storage at 4 °C. To evaluate the performance of the paper device using real samples, we demonstrated the detection of protein A (a biomarker for Staphylococcus aureus infection) in highly viscous human synovial fluid. These results show that the proposed 3D-soPAD platform can provide sensitive, high-throughput, and on-site prognosis of infection in resource-limited settings.

Evidence for the evolutionary steps leading to mecA-mediated β-lactam resistance in staphylococci

The epidemiologically most important mechanism of antibiotic resistance in Staphylococcus aureus is associated with mecA–an acquired gene encoding an extra penicillin-binding protein (PBP2a) with low affinity to virtually all β-lactams. The introduction of mecA into the S. aureus chromosome has led to the emergence of methicillin-resistant S. aureus (MRSA) pandemics, responsible for high rates of mortality worldwide. Nonetheless, little is known regarding the origin and evolution of mecA. Different mecA homologues have been identified in species belonging to the Staphylococcus sciuri group representing the most primitive staphylococci. In this study we aimed to identify evolutionary steps linking these mecA precursors to the β-lactam resistance gene mecA and the resistance phenotype. We sequenced genomes of 106 S. sciuri, S. vitulinus and S. fleurettii strains and determined their oxacillin susceptibility profiles. Single-nucleotide polymorphism (SNP) analysis of the core genome was performed to assess the genetic relatedness of the isolates. Phylogenetic analysis of the mecA gene homologues and promoters was achieved through nucleotide/amino acid sequence alignments and mutation rates were estimated using a Bayesian analysis. Furthermore, the predicted structure of mecA homologue-encoded PBPs of oxacillin-susceptible and -resistant strains were compared. We showed for the first time that oxacillin resistance in the S. sciuri group has emerged multiple times and by a variety of different mechanisms. Development of resistance occurred through several steps including structural diversification of the non-binding domain of native PBPs; changes in the promoters of mecA homologues; acquisition of SCCmec and adaptation of the bacterial genetic background. Moreover, our results suggest that it was exposure to β-lactams in human-created environments that has driven evolution of native PBPs towards a resistance determinant. The evolution of β-lactam resistance in staphylococci highlights the numerous resources available to bacteria to adapt to the selective pressure of antibiotics.

The emergence and rise of mecA-mediated β-lactam resistance in staphylococci has been one of the greatest concerns of the scientific and medical communities worldwide. However, little is known regarding the origin of the mecA gene determinant. In this study we demonstrate that antibiotic pressure in the human environment and in food additives used in livestock was the major driving force of the evolution and spread of resistance to β-lactams. Furthermore, we confirm the previous findings suggesting that the development of resistance occurs in primitive species of staphylococci through diversification of a native penicillin binding protein involved in cell wall synthesis. We also demonstrate that resistance was achieved through four distinct mechanisms: accumulation of substitutions in a specific domain of the protein; diversification of the promoter of the gene; acquisition of SCCmec, and adaptation of the genetic background. Our results highlight the resources that primitive bacteria used to thrive in a changing environment that has led to the methicillin-resistant Staphylococcus aureus (MRSA) pandemics.

A novel IgY-Aptamer hybrid system for cost-effective detection of SEB and its evaluation on food and clinical samples

In the present study, we introduce a novel hybrid sandwich-ALISA employing chicken IgY and ssDNA aptamers for the detection of staphylococcal enterotoxin B (SEB). Cloning, expression and purification of the full length recombinant SEB was carried out. Anti-SEB IgY antibodies generated by immunizing white leg-horn chickens with purified recombinant SEB protein and were purified from the immunized egg yolk. Simultaneously, ssDNA aptamers specific to the toxin were prepared by SELEX method on microtiter well plates. The sensitivity levels of both probe molecules i.e., IgY and ssDNA aptamers were evaluated. We observed that the aptamer at 250 ngmL(-1) concentration could detect the target antigen at 50 ngmL(-1) and the IgY antibodies at 250 ngmL(-1), could able to detect 100 ngmL(-1) antigen. We further combined both the probes to prepare a hybrid sandwich aptamer linked immune sorbent assay (ALISA) wherein the IgY as capturing molecule and biotinylated aptamer as revealing probe. Limit of detection (LOD) for the developed method was determined as 50 ngmL(-1). Further, developed method was evaluated with artificially SEB spiked milk and natural samples and obtained results were validated with PCR. In conclusion, developed ALISA method may provide cost-effective and robust detection of SEB from food and environmental samples.

Application of monoclonal antibodies generated against Panton-Valentine Leukocidin (PVL-S) toxin for specific identification of community acquired methicillin resistance Staphylococcus aureus

Panton-Valentine Leukocidin (PVL) produced by community acquired methicillin Staphylococcus aureus (CA-MRSA) involved in skin and soft-tissue infections and necrotizing pneumonia comprised of two fractions, namely PVL S and PVL F. In the present study, three monoclonal antibodies designated as MAb1, MAb9 and MAb10 were generated against recombinant PVL-S (35kDa) protein of S. aureus. All the three MAbs specifically reacted to confirm PVL-S positive strains of S. aureus recovered from clinical samples in Western blot analysis. Similarly all the three MAbs did not show any binding to other tested 14 different pathogenic bacteria belonging to other genera and species in Western blot analysis. Furthermore, a simple dot-ELISA method was standardized for the identification of PVL-S toxin containing S. aureus strains. Initially in dot-ELISA, Protein A (Spa) of S. aureus posed background noise problems due to the non-specific binding of antibodies resulting in false positive reactions. With the addition of 10mM diethylpyrocarbonate (DEPC) along with 5% milk in PBS in the blocking step prevented this non-specific binding of Spa to mouse anti-PVL monoclonal antibodies in dot-ELISA. Once standardized, this simple dot-ELISA was evaluated with nine PVL positive strains recovered from food, environmental and clinical samples and the results were compared with PCR assay for the presence of PVL toxin genes and also with Western blot analysis. A 100% correlation was found between dot-ELISA, PCR assay and Western blot analysis. Collectively our results suggest the newly developed simple dot-ELISA system can be of immense help in providing, rapid detection of the PVL toxin containing S. aureus strains at a relatively low cost and will be a valuable tool for the reliable identification of CA-MRSA.

Hyperexpression of α-hemolysin explains enhanced virulence of sequence type 93 community-associated methicillin-resistant Staphylococcus aureus

BACKGROUND

The community-associated methicillin-resistant S. aureus (CA-MRSA) ST93 clone is becoming dominant in Australia and is clinically highly virulent. In addition, sepsis and skin infection models demonstrate that ST93 CA-MRSA is the most virulent global clone of S. aureus tested to date. While the determinants of virulence have been studied in other clones of CA-MRSA, the basis for hypervirulence in ST93 CA-MRSA has not been defined.

RESULTS

Here, using a geographically and temporally dispersed collection of ST93 isolates we demonstrate that the ST93 population hyperexpresses key CA-MRSA exotoxins, in particular α-hemolysin, in comparison to other global clones. Gene deletion and complementation studies, and virulence comparisons in a murine skin infection model, showed unequivocally that increased expression of α-hemolysin is the key staphylococcal virulence determinant for this clone. Genome sequencing and comparative genomics of strains with divergent exotoxin profiles demonstrated that, like other S. aureus clones, the quorum sensing agr system is the master regulator of toxin expression and virulence in ST93 CA-MRSA. However, we also identified a previously uncharacterized AraC/XylS family regulator (AryK) that potentiates toxin expression and virulence in S. aureus.

CONCLUSIONS

These data demonstrate that hyperexpression of α-hemolysin mediates enhanced virulence in ST93 CA-MRSA, and additional control of exotoxin production, in particular α-hemolysin, mediated by regulatory systems other than agr have the potential to fine-tune virulence in CA-MRSA.

Genome sequencing unveils a novel sea enterotoxin-carrying PVL phage in Staphylococcus aureus ST772 from India

Staphylococcus aureus is a major human pathogen, first recognized as a leading cause of hospital-acquired infections. Community-associated S. aureus (CA-SA) pose a greater threat due to increase in severity of infection and disease among children and healthy adults. CA-SA strains in India are genetically diverse, among which is the sequence type (ST) 772, which has now spread to Australia, Europe and Japan. Towards understanding the genetic characteristics of ST772, we obtained draft genome sequences of five relevant clinical isolates and studied the properties of their PVL-carrying prophages, whose presence is a defining hallmark of CA-SA. We show that this is a novel prophage, which carries the structural genes of the hlb-carrying prophage and includes the sea enterotoxin. This architecture probably emerged early within the ST772 lineage, at least in India. The sea gene, unique to ST772 PVL, despite having promoter sequence characteristics typical of low expression, appears to be highly expressed during early phase of growth in laboratory conditions. We speculate that this might be a consequence of its novel sequence context. The crippled nature of the hlb-converting prophage in ST772 suggests that widespread mobility of the sea enterotoxin might be a selective force behind its 'transfer' to the PVL prophage. Wild type ST772 strains induced strong proliferative responses as well as high cytotoxic activity against neutrophils, likely mediated by superantigen SEA and the PVL toxin respectively. Both proliferation and cytotoxicity were markedly reduced in a cured ST772 strain indicating the impact of the phage on virulence. The presence of SEA alongside the genes for the immune system-modulating PVL toxin may contribute to the success and virulence of ST772.

Staphylococcus aureus isolated from endophthalmitis are hospital-acquired based on Panton-Valentine leukocidin and antibiotic susceptibility testing

PURPOSE

Staphylococcus aureus (SA) endophthalmitis is generally a postsurgical infection with an undefined source of entry. Hospital-acquired (HA) SA infections are associated with multi-antibiotic resistance and absence of the Panton-Valentine Leukocidin (PVL) toxin. Community-acquired (CA) SA infections are not associated with multi-antibiotic resistance and possess the PVL toxin. We hypothesize that CA infection is more common than HA for SA endophthalmitis.

METHODS

Twenty de-identified SA isolates, collected from the vitreous and/or aqueous of clinical endophthalmitis, were tested for the presence of PVL toxin and antibiotic susceptibility. PVL testing was performed using a kit to detect the Staphylococcal toxin by reversed passive latex agglutination (PVL-RPLA "Seiken," Denka Seiken Co., LTD). SA isolates were tested for antibiotic susceptibility using disk diffusion at the time of isolation. Multi-antibiotic resistance was defined as resistance to at least 3 classes of antibiotics.

RESULTS

Of the 20 isolates, 15 were multi-antibiotic resistant and PVL-negative consistent with HA, and 1 was not multi-antibiotic resistant and PVL-positive, consistent with CA. Two isolates tested positive for PVL with one demonstrating both methicillin and fluoroquinolone (FQ) resistance. Of the 18 PVL-negative SA isolates, 15 (83%) were multi-antibiotic resistant (12 methicillin-resistant SA, 14 FQ resistant).

CONCLUSIONS

Our results reject the hypothesis that SA isolated from endophthalmitis is consistent with CA sources due to the lack of the PVL toxin and multiple resistant patterns of the SA. PVL does not appear to be a key virulence factor for the development of SA endophthalmitis.

Development of an immunochromatographic strip for simple detection of penicillin-binding protein 2'

Infections with methicillin-resistant Staphylococcus aureus (MRSA) and methicillin-resistant coagulase-negative Staphylococcus (MR-CNS) are a serious problem in hospitals because these bacteria produce penicillin-binding protein 2' (PBP2' or PBP2a), which shows low affinity to β-lactam antibiotics. Furthermore, the bacteria show resistance to a variety of antibiotics. Identification of these pathogens has been carried out mainly by the oxacillin susceptibility test, which takes several days to produce a reliable result. We developed a simple immunochromatographic test that enabled the detection of PBP2' within about 20 min. Anti-PBP2' monoclonal antibodies were produced by a hybridoma of recombinant PBP2' (rPBP2')-immunized mouse spleen cells and myeloma cells. The monoclonal antibodies reacted only with PBP2' of whole-cell extracts and showed no detectable cross-reactivity with extracts from other bacterial species tested so far. One of the monoclonal antibodies was conjugated with gold colloid particles, which react with PBP2', and another antibody was immobilized on a nitrocellulose membrane, which captures the PBP2'-gold colloid particle complex on a nitrocellulose strip. This strip was able to detect 1.0 ng of rPBP2' or 2.8 × 10(5) to 1.7 × 10(7) CFU of MRSA cells. The cross-reactivity test using 15 bacterial species and a Candida albicans strain showed no detectable false-positive results. The accuracy of this method in the detection of MRSA and MR-CNS appeared to be 100%, compared with the results obtained by PCR amplification of the PBP2' gene, mecA. This newly developed immunochromatographic test can be used for simple and accurate detection of PBP2'-producing cells in clinical laboratories.