This study describes the selection of single-stranded DNA (ssDNA) aptamers against Salmonella enterica serovar Typhimurium using a modified whole cell systematic evolution of ligands by exponential enrichment (whole cell SELEX). For evolving specific aptamers, ten rounds of selection to live Salmonella cells, alternating with negative selection against a cocktail of related pathogens, were performed. The resulting highly enriched oligonucleotide pools were sequenced and clustered into eight groups based on primary sequence homology and predicted secondary structure similarity. Fifteen sequences from different groups were selected for further characterization. The binding affinity and specificity of aptamers were determined by fluorescence binding assays. Aptamers (SAL 28, SAL 11, and SAL 26) with dissociation constants of 195 ± 46, 184 ± 43, and 123 ± 23 nM were used to develop a nanogold-based colorimetric detection method and a sedimentation assay. The former showed a better sensitivity limit of 10(2) CFU/mL using aptamer SAL 26. This approach should enable further refinement of diagnostic methods for the detection of Salmonella enterica serovar Typhimurium and of other microbial pathogens.

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A simple, sensitive and selective colorimetric biosensor for the detection of Staphylococcal enterotoxin B (SEB) was developed using SEB-binding aptamer (SEB2) as recognition element and unmodified gold nanoparticles (AuNPs) as colorimetric probes. The assay is based on color change from red to purple due to conformational change of aptamer in the presence of SEB, and the phenomenon of salt-induced AuNPs aggregation which could be monitored by naked eye or UV–vis spectrometer. Results showed that the AuNPs can effectively differentiate the SEB induced conformational change of the aptamer in the presence of a given high salt concentration. A linear response in the range of 50 μg/mL to 0.5 ng/mL of SEB concentration was obtained. The assay was highly specific to SEB as compared to other related toxins. The limit of detection (LOD) of SEB achieved within few minutes was 50 ng/mL visually and spectrometric method improved it to 0.5 ng/mL. Robustness of the assay was tested in artificially spiked milk samples and cross-checked using in house developed sandwich ELISA (IgY as capturing and SEB specific monoclonal as revealing antibody) and PCR. This colorimetric assay could be a suitable alternative over existing methods during biological emergencies due to its simplicity, sensitive and cost effectiveness.

Aflatoxins are naturally occurring mycotoxins that contaminate food and agro commodities, leading to acute and chronic health conditions in human and animals. In the present work, an attempt was made to generate high-affinity single stranded DNA aptamers that specifically bind to Aflatoxin B1 (AFB1) by a modified Systemic Evolution of Ligands by Exponential Enrichment (SELEX) procedure with the aid of Immunoaffinity columns. Ten rounds of SELEX and alternating three counter SELEX rounds with a cocktail of related and other mycotoxins were performed to enhance the specificity. Resultant 105 aptamers were clustered into 12 groups according to their primary sequence homology. Candidates with lowest Gibbs free energy (dG value) and unique stem loop structures were selected for further characterization. Aptamers, AFLA5, AFLA53, and AFLA71 exhibiting lower Kd values (50.45 ± 11.06, 48.29 ± 9.45, and 85.02 ± 25.74 nM) were chosen for development of ELONA and determination of purification ability of toxin. The detection limit (LOD) of AFLA5 and AFLA71 was 20 and 40 ng/ml, respectively. HPLC analysis implied that selected aptamers were able to recover and quantify 82.2 to 96.21% (LOQ – 53.74 ng) and 78.3 to 94.22% (LOQ – 66.75 ng) of AFB1 from spiked corn samples, respectively. These findings indicate, immunoaffinity based SELEX can pave an alternative approach to screen aptamers against mycotoxin detection and purification.

Aptamers are synthetic DNA recognition elements which form unique conformations that enable them to bind specifically to their targets. In the present study, an attempt was made to standardize a new modified combinatorial method comprising of Ni-NTA affinity Systematic Evolution of Ligands by Exponential Enrichment (SELEX; based on affinity between His tag protein and Ni-NTA), membrane SELEX (based on immobilization of protein on nitrocellulose membrane), and microtiter plate based SELEX (to monitor affinity and to enrich the selected aptamers) for protein targets. For experimental evaluation, staphylococcal interotoxin B was the molecule chosen. The new combinatorial method enhanced selection ability up to 51.20 % in comparison with individual conventional procedures. Employing this method following six rounds of selection, high-affinity aptamers with very different properties could be obtained with a dissociation constant (K d) value as low as 34.72 ± 25.09 nM. The optimal aptamers could be employed in fluorescence binding assay, enzyme-linked oligonucleotide assays, and aptamer-based Western blot assay for characterization and detection. These results pave a potential path without using of any robotics for high-throughput generation of aptamers with advantages in terms of rapidity, simplicity, and ease in handling.

In the present study, a colorimetric DNAzymes biosensor strategy was devised in combination with immunomagnetic separation for rapid and easy detection of enterotoxin B harboring Staphylococcus aureus from food and clinical samples. The method employs immunocapture of S. aureus and amplification of seb gene by DNAzyme complementary sequence integrated forward primer and with specific reverse primer. The DNAzyme sequence integrated dsDNA PCR products when treated with hemin and TMB (3,3',5,5'-tetramethylbenzidine) in the presence of HO produce colorimetric signal. A linear relationship of optical signal with the initial template of seb was obtained which could be monitored by visually or spectrophotrometrically for qualitative and quantitative detection. The limit of detection for the assay was approximately 10 CFU/mL of seb gene harboring target. This method is convenient compared to gel based and ELISA systems. Further, spiking studies and analysis on natural samples emphasized the robustness and applicability of developed method. Altogether, the established assay could be a reliable alternative, low-cost, viable detection tool for the routine investigation of seb from food and clinical sources.

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