2.5. Specific immune response
2.5.1. Enzyme linked immunosorbant assay (ELISA)
The serum antibody response to A. hydrophila was measured using an indirect ELISA method described by Delamare et al. [19] with minor modifications [20]. Flat-bottomed microtiter plate was coated with (100 μL/well) whole bacterial cells. The bacteria were earlier harvested from overnight liquid culture, washed thrice with 0.15 mol/L NaCl solution and adjusted to 3 × 106 CFU in 0.05 M carbonate bicarbonate buffer (pH 9.6). The plate was incubated overnight at 4 °C, washed thrice with PBS containing 0.05% (v/v) Tween 20 (PBS-T). Non-specific binding sites were blocked with (100 μL/well) 1% (w/v) BSA in PBS. The plate was incubated at 28 °C for 1 h and washed thrice with PBS-T. In each well, 100 μL of test serum sample (diluted to 1:30 in PBS-T) was added and incubated at 28 °C for 1 h; the plate was washed thrice with PBS-T. In each well, 100 μL rabbit anti-catla polyclonal antibody (produced in our laboratory, diluted 1:100 using PBS) was added. The plate was allowed to stand for 1 h at 37 °C; washed thrice with PBS-T. Then 100 μL of goat anti-rabbit IgG antibody conjugated with horseradish peroxidase (GeNei, India, diluted 1:2000 in PBS) was added to each well and incubated at 37 °C for 1 h. The plate was again washed thrice with PBS-T. 100 μL of 3, 3′, 5, 5′- tetramethyl benzidine (GeNei, India) mixed with 0.1% (v/v) of hydrogen peroxide was added for colour development. The reaction was stopped by adding 25 μL of 1 M sulphuric acid. The optical density was measured at 450 nm. The mean absorbance value of each well was used to express serum antibody level.
2.5. Specific immune response
2.5.1. Enzyme linked immunosorbant assay (ELISA)
The serum antibody response to A. hydrophila was measured using an indirect ELISA method described by Delamare et al. [19] with minor modifications [20]. Flat-bottomed microtiter plate was coated with (100 μL/well) whole bacterial cells. The bacteria were earlier harvested from overnight liquid culture, washed thrice with 0.15 mol/L NaCl solution and adjusted to 3 × 106 CFU in 0.05 M carbonate bicarbonate buffer (pH 9.6). The plate was incubated overnight at 4 °C, washed thrice with PBS containing 0.05% (v/v) Tween 20 (PBS-T). Non-specific binding sites were blocked with (100 μL/well) 1% (w/v) BSA in PBS. The plate was incubated at 28 °C for 1 h and washed thrice with PBS-T. In each well, 100 μL of test serum sample (diluted to 1:30 in PBS-T) was added and incubated at 28 °C for 1 h; the plate was washed thrice with PBS-T. In each well, 100 μL rabbit anti-catla polyclonal antibody (produced in our laboratory, diluted 1:100 using PBS) was added. The plate was allowed to stand for 1 h at 37 °C; washed thrice with PBS-T. Then 100 μL of goat anti-rabbit IgG antibody conjugated with horseradish peroxidase (GeNei, India, diluted 1:2000 in PBS) was added to each well and incubated at 37 °C for 1 h. The plate was again washed thrice with PBS-T. 100 μL of 3, 3′, 5, 5′- tetramethyl benzidine (GeNei, India) mixed with 0.1% (v/v) of hydrogen peroxide was added for colour development. The reaction was stopped by adding 25 μL of 1 M sulphuric acid. The optical density was measured at 450 nm. The mean absorbance value of each well was used to express serum antibody level.
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