Classification Levels of Salmonella

Salmonella is the name of a genus that includes two species and many subspecies of bacteria. Since some of these bacteria are a common cause of food poisoning, salmonella is of great interest to public health officials; unfortunately, several inconsistent methods have been used to categorize salmonella bacteria in the past, so the classification system has become fairly complicated. The nomenclature now adopted by the Centers for Disease Control is described below.

Species

• All salmonella bacteria are by definition members of the genus Salmonella. At one point in the past, some scientists defined each serotype (a variant of the bacteria that exhibits molecules that are recognized by a specific antibody) as a separate species, often named depending on where they were first isolated or other features. If this system of nomenclature were still in use today, there would be over 2400 species of salmonella. Fortunately, genetic tests performed on salmonella in 1973 revealed that most of the serotypes were closely related enough genetically that they could be subdivided into only a few separate species. Initially scientists assigned all serotypes to one of three different species; as of 2000, the CDC recognizes only two species, Salmonella enterica and Salmonella bongori.

Subspecies

• The species Salmonella is is further divided into six separate subspecies, namely, S. enterica enterica, S. enterica salamae, S. enterica arizonae, S. enterica diarizonae, S. enterica houtenae and S. enterica indica.

Serotypes

• Antibodies are produced by the immune system and are highly target-specific: each antibody only binds to a given molecule or antigen. You can think of an antibody as being somewhat like a guided missile that's engineered to recognize a specific target. Salmonella bacteria are divided into serotypes based on the antigens they exhibit. There are two antigens scientists use to classify them into serotypes--the O antigen, a molecule in the bacterial cell wall, and the H antigen, a molecule in the bacteria's flagellum, the "tail" it uses to propel itself. There are over 2400 known serotypes of salmonella. Each is designated with an O-# and an H-#, which denote the antigens it presents. Many common serotypes have names as well; serotype typhi and serotype typhimurium are two examples.

Serogroups

• Serotypes can also be classified into serogroups using their O antigens. For example, serotypes which share O antigen 2 are group A while serotypes which share antigens 6 and 7 are group C.

Misconceptions

• The name of a given serotype should be written as genus species subspecies serotype name; for example, serotype typhimurium is Salmonella enterica enterica serotype typhimurium. The serotype names, however, are often written as if they were species names out of simple convenience; this shorthand sometimes creates confusion. For the sake of clarity--and if you're in doubt--it's preferable to specify that you're using a serotype; for example, writing Salmonella serotype Typhimurium is clearer than Salmonella typhimurium.