Plastic dipstick

Chapters 31 and 32 deal with similar concepts, in that probes are hybridized to a nucleic acid extract, derived by bacterial lysis. The probes have a label that can be detected in a nonradioactive assay, and the probe plus target complex can be specifically captured by magnetic beads (Chapter 31) or a plastic dipstick (Chapter 32). A positive signal means that a sequence, and hence a particular bacterium, is present in the tested sample. These protocols are useful in clinical and food microbiological contexts, where diagnosis of bacterial presence must be made quickly, and can be used to formulate a decision whether or not other confirmatory tests should be carried out. 

Materials commonly use as solid supports include polystyrene, polyvinyl, nylon, glass, nitrocellulose, silica, polyacrylamide, or polystyrene beads. Separation of the bound from the free reagents can be achieved through either filtration for particulate solid supports such as agarose, polyacrylamide, and polystyrene beads, or centrifugation. For disposable forms of solid supports such as multiwell plates, plastic tubes, cuvettes, balls, and dipsticks, separation can be performed through simple rinsing steps. 

The simplest way of fraction separation, however, is when one of the immunoreactants (i.e., either the antigen or the antibody) is immobilized on a solid phase, i.e., immunoreagent modified solid-phase separations, such as plastic tubes, microtitre plate, latex-, glass- or magnetic beads, dipsticks or nitrocellulose membranes [121]. The efl ciency of separation is determined by the nature of the solid surface. Unspecific binding of the label to the solid phase can occur, especially when the latter has hydrophobic properties [96]. The adsorption of the antibody to the solid surface causes its partial inactivation, which can explain some unexpected effects when comparing the same antibody used in liquid and solid-phase systems (see also Section 9.3.4.4). 

When the 15-min incubation is complete, remove the cover from the tubes and add a plastic poly-dT coated dipstick to each tube. Incubate the dipsticks m the tubes for 1 h at 65 C. 

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