Serological Techniques of Salmonella Isolates

Salmonella is a genus of bacteria; certain subspecies of this genus are common culprits in food poisoning. Salmonella classification is complex, not least because different systems of nomenclature have been used in the past. In general, however, salmonella are usually classified into one of two different species, which are further subdivided by subspecies and by serotype (also called serovar). Serological techniques play a key role in classifying salmonella.

Identification

• Antibodies are Y-shaped molecules produced by the immune system that are designed to stick to highly specific targets. A given antibody will typically bind to only one antigen or target molecule. Scientists can use this property to differentiate between different strains of bacteria in the same genus or species. The methods used to serotype bacteria are called serological techniques; there are several different methods in common use.

Slide Agglutination

• Slide agglutination is the simplest test. A drop of bacteria suspended in saline solution is placed in two separate locations on a slide and the antibodies for a specific antigen are added to only one of the two sites. If the antigen for the antibody is present in the sample the antibodies will bind to it; since each antibody is Y-shaped and can bind two antigens simultaneously, the bacteria and antibodies become cross-linked and form clumps that are usually large enough to be visible to the naked eye. If the antigen is absent, however, or is only present in very small quantities, no visible change will take place.

Tube Agglutination

• Tube agglutination is similar to slide agglutination, except that the procedure is performed in a test tube (or in multiple test tubes) to determine whether the antigen is present. Antibodies that are specific to the bacterial flagella (the long whip-like tails the bacteria use to propel themselves) will exhibit a characteristic pattern of agglutination where the bacteria are only loosely clumped together and can be shaken apart.

ELISA

• Enzyme-linked immunosorbent assays (ELISAs) form a powerful technique to rapidly identify bacteria from an unknown sample by their antigens. The double antibody sandwich assay is the most common technique for salmonella. First, antibodies for a known antigen are affixed to the walls of the wells in a microtiter plate (a plate with multiple small wells or containers for different samples). The unknown antigen is added and the wells are then washed to remove any unbound or free-floating antigen. If the antigen matched the antibody, it will remain behind when the wells are washed. Next, more antibodies are added; these antibodies are different, however, in that they have an enzyme (a protein that can catalyze a reaction) linked to them. If the antigen was present in the sample and remains stuck to the antibodies on the walls of the well, these enzyme-linked antibodies will stick to the antigen. Finally, a special chemical is added to the well. If the enzyme-linked antibodies are still there, the enzyme will alter the test chemical so that it changes color. So if the color of the test chemical changes, the matching antigen was present in the sample.

Types

• There are more than 2,000 different serotypes of salmonella. The bacteria are grouped into different serotypes using their O antigens and H antigens. The O antigen is a molecule that constitutes part of the bacteria's cell wall, while the H antigen is part of the flagella or tail. Two of the best-known serotypes are typhi and typhimurium; the former is responsible for typhoid fever, while the latter causes food poisoning. Serotyping salmonella is a way to tell the medically harmful subtypes of this genus apart.