Nadezhda A. Byzova

Rapid pretreatment-free immunochromatographic assay of chloramphenicol in milk.

A pretreatment-free immunochromatographic assay for detection of chloramphenicol (CAP) in milk was developed. The assay is based on competition between CAP molecules in the sample and immobilized CAP-protein conjugate for binding to monoclonal anti-CAP antibodies conjugated with colloidal gold particles (average diameter 30nm). The assay is carried out in the course of sample flowing along test strip with immobilized reactants, and its results can be detected by the naked eye or by a photometric device. Effect of the concentration of immunoreactants on assay characteristics was studied. The assay protocol with maximal sensitivity and reliability was optimized using measured values of brightness of lines. Detection limit for CAP is 10ngmL(-1). Assay duration is 10min, and it can be carried out at room temperature without any additional devices and reactants. The developed test strip has been applied to CAP detection in dairy products.

Pretreatment-free immunochromatographic assay for the detection of streptomycin and its application to the control of milk and dairy products

A rapid pretreatment-free immunochromatographic assay was developed for the control of the streptomycin (STR) content in milk and dairy products. The assay is based on the competition between an immobilized STR-protein conjugate and STR in a sample to be tested for the binding to monoclonal anti-STR antibodies conjugated to colloidal gold during the flow of the sample along a membrane strip with immobilized reactants. It is possible to improve the cut-off level of positive and negative samples distinguished by a change in the molar STR to protein ratio in the immobilized conjugate. The cut-off level (500 ng mL(-1)) thus achieved corresponds to the stated MRL of STR in milk and dairy products. For STR concentrations in the range of 16-250 ng mL(-1) its content can be quantitatively measured based on the degree of binding of a colloidal gold label in the test strip zone with the immobilized STR-protein conjugate. The duration of the assay is 10 min. The selected sizes of membrane pores and colloidal gold particles allow the assay to be carried out at room temperature without additional reactants and pretreatment. The applicability of the assay for milk, whole milk, sour clotted milk, and kefir with different fat content (from 0.5% to 6%) was confirmed. The results of quantitative immunochromatographic assay show good correlation with traditional ELISA (r was equal to 0.935 and 0.940 for the series tested).

Rapid immunochromatographic assay for ofloxacin in animal original foodstuffs using native antisera labeled by colloidal gold

An immunochromatographic assay was developed to detect fluoroquinolone antibiotic ofloxacin based on the competitive binding of ofloxacin and the membrane-immobilized ofloxacin-protein conjugate to colloidal gold-labeled antibodies in the course of the labeled antibodies, and to test sample flow through the membrane. The specific feature of labeling by colloidal gold is that native antiserum is used instead of purified immunoglobulins or specific antibodies. This makes the synthetic procedure easier, with no sacrifice in the detection limit. The proposed test makes it possible to detect down to 30 ng mL(-1) of ofloxacin, which corresponds to the demands of food safety assessment. The assay time is 10 min. The assay provides reliable information on the ofloxacin content in milk without the sample preparation and in chicken and pork meat with the minimum sample preparation (the separation of the insoluble fraction of the homogenate by centrifugation). The high degree of detection of ofloxacin in foodstuffs by the proposed assay (70-112%) was shown by a comparison with the data obtained with the use of a commercial immunoenzymatic kit.

Interaction of Plum Pox Virus with Specific Colloidal Gold-Labeled Antibodies and Development of Immunochromatographic Assay of the Virus

Two monoclonal antibodies (mABs) raised against plum pox virus (PPV) were shown to recognize its D, M, and C strains. Conjugates of the antibodies with colloidal gold (CG) nanoparticles averaging 26 nm in diameter were synthesized. The binding constants of PPV with both the native and conjugated mABs were determined using a Biacore X device. The complexes between the CG-mAB conjugates and plum pox virions were examined by means of transmission electron and atomic force microscopy. Using the conjugates with optimal component ratio, an express immunochromatographic assay of PPV was developed with a detection limit of 3 ng/ml and duration of 10 min. The assay was tested for PPV detection in sam- ples of stone fruit tree leaves and demonstrated a good compatibility with the data obtained by “sandwich”-ELISA. The developed assay can be used in the field and applied for monitoring viral infection and for quarantine purposes.

Development of immunochromatographic test systems for express detection of plant viruses

Express immunochromatographic test-strip assays were developed for detection of five plant viruses varying in shape and size of virions: spherical carnation mottle virus, bean mild mosaic virus, rodshaped tobacco mosaic virus, and filamentous potato viruses X and Y. Multimembrane composites (test strips) with immobilized polyclonal antibodies against viruses and colloidal gold-conjugated antibodies were used for the analysis. The immunochromatographic test strips were shown to enable the detection of viruses both in purified preparations and in leaf extracts of infected plants with a sensitivity from 0.08 to 0.5 μg/ml for 10 min. The test strips may be used for express diagnostics of plant virus diseases in field conditions.

Rapid polyelectrolyte-based immunofiltration technique for testosterone detection in serum samples

A new immunofiltration assay for testosterone is proposed. During the first step of the assay, testosterone molecules in serum samples compete in solution with the testosterone-peroxidase conjugate for interaction with anti-testosterone antibodies pre-bound to the conjugate between staphylococcal protein A and polymethacrylate polyanion. The reaction mixture is then filtered through a membrane charged with immobilized poly(N-ethyl-4-vinylpyridinium) polycation. The filtration is accompanied by a rapid separation of the polyanion containing complexes due to high-affinity electrostatic interactions. Following removal of unbound compounds the immobilized peroxidase is detected using a substrate that produces an insoluble coloured product. The proposed assay has been shown to combine high speed (20 min) and sensitivity (0.1 ng ml(-1)), and to be applicable for out-of-laboratory conditions. Based on densitometric measurements, the RSD of the assay is calculated to be 3.2-5.1% (n = 4). The proposed assay is 4 times faster than the microplate enzyme immunoassay (ELISA) based on the same immunoreagents. Pre-incubation of the antibody and the polyanion-protein A conjugate at a certain ratio excludes the influence of immunoglobulins from the tested serum samples on the assay results. The polyanion-protein A conjugate can be used as a universal reagent, eliminating the necessity to modify specific antibodies for each immunoassay.

Cut-off on demand: adjustment of the threshold level of an immunochromatographic assay for chloramphenicol

Immunochromatographic assay of small molecules is carried out in a competitive format and is based on the disappearance of analytical zone colouration, as the analyte concentration exceeds a certain threshold level. Therefore, the assay of toxic contaminants has the highest information content if this threshold level (cut-off between negative and positive samples) corresponds to the maximum residue limit of the analyte. In this study, adjusting the threshold level was investigated for an immunochromatographic assay for chloramphenicol, which is a veterinary drug of concern in the food industry because of its toxic effects. Test strips were produced using hapten–(protein carrier) and antibody–(gold nanoparticle) conjugates, and the effects of the compositions and concentrations of these conjugates on the threshold level of the assay were investigated. Changing all of these parameters at once shifted the threshold level to values that could reach more than two orders of magnitude. Thus, varying the composition of chloramphenicol–bovine serum albumin conjugates causes a shift of the threshold level from 1500 to 2 ng mL−1, and varying the composition of chloramphenicol–soybean trypsin inhibitor conjugates causes a shift from 930 to 9 ng mL−1. Besides, the threshold shift is accomplished by the shift of the working range for quantitative immunochromatography based on measurements of the analytical zone colouration. In this way, the limit of instrumental detection could also be varied by more than two orders of magnitude. The proposed approach can be applied to different low-molecular-weight compounds and it allows the adoption of an immunochromatographic test of the desired levels to control the target analyte without screening of a series of antibodies.

Immunochromatographic technique for express determination of ampicillin in milk and dairy products

An immunochromatographic method for determination of β-lactam antibiotic ampicillin has been developed. The method is based on the competitive interaction between antibiotic molecules contained in the sample and protein conjugate of penicillin immobilized on a membrane for binding with specific antibodies labeled with colloidal gold, which occurs during movement of the sample to be tested and reagents along the membrane. The completion of the test system ensures control of exceeding the maximum permissible content of the antibiotic in milk and dairy products (10 ng/mL). The possibility of testing milk, raw milk, and dairy products for 10 minutes at room temperature without sample pretreatment has been demonstrated.

Express Immunochromatographic Detection of Antibodies against Brucella Abortus in Cattle Sera Based on Quantitative Photometric Registration and Modulated Cut-Off Level

An immunochromatographic test system was developed for rapid detection of the levels of specific IgG antibodies to Brucella abortus lipopolysaccharide, as a tool for diagnosis of brucellosis in cattle. The pilot test strips were examined using blood sera from sick (78 samples) and healthy (35 samples) cows. The results obtained by immunochromatographic assay, using a portable optical densitometer for digital video detection, correlate well with the results obtained by immunoenzyme assay and are in agreement with the results of the disease diagnosis. The new test system allows detection of antibodies within 10 min and can be proposed as an alternative to the methods available for serodiagnosis of brucellosis.

Development of an immunochromatographic test system for the detection of Helicobacter pylori antigens

A test system based on immunochromatography in the sandwich format and intended for express detection of Helicobacter pylori antigens has been developed. Contact of a sample with a test strip coated with immunochemical reagents triggers the movement of the liquid along the membrane components of the test strip, immunochemical interactions, and the formation of detection zones stained by gold nanoparticles. The concentration and kinetic dependences of the immunochemical interactions have been characterized. The reagent and membrane composition of the test system has been selected to provide a minimal detection limit. The detection of H. pylori cell wall antigens at concentrations as low as 0.3 μg/mL in aqueous solution and a suspension of a clinical sample of feces has been demonstrated; the assay duration was 10 min. Staining enhancement by the addition of silver salts allowed for a further reduction of the detection limit to 0.03 μg/mL. The developed test system can be used for field diagnostics.