Salmonella Group B Defined

Many cases of food poisoning result from eating undercooked eggs contaminated with salmonella.

Salmonella is a genus of bacteria that are common in a variety of environments. Salmonella can cause illness in humans, and salmonella bacteria are currently the most frequently diagnosed cause of foodborne disease in the United States. As is often true in bacterial taxonomy, the species concept is difficult to apply with salmonella, so the classification system is somewhat complex. The group B category describes salmonella bacteria that share a specific feature in common.

Serotypes

Your immune system protects you from microbial invaders by recognizing and destroying intruders. An antigen is a molecule the immune system recognizes as the mark of a foreign or unknown entity like a bacteria or virus. Antibodies produced by the immune system are highly target-specific in that each antibody will only bind a molecule with a specific shape. This property of antibodies is the basis of a technique called serotyping, a system of classification that distinguishes bacteria based on the antigens they present--so if two bacteria have the exact same antigens, then they are the same serotype. There are more than 2,500 different serotypes of salmonella bacteria.

O Antigens

Salmonella and many other bacteria contain molecules called lipopolysaccharides in their cell walls; these molecules consist of a fat molecule (a lipid) attached to a long chain of sugars (a polysaccharide). The polysaccharide chain varies between different serotypes of salmonella bacteria to the point where the immune system can tell them apart. This polysaccharide chain is called the O antigen.

Identification of Group B

Salmonella bacteria are assigned to different groups depending on the O antigens they present. All salmonella bacteria that exhibit O antigen 4 are considered part of Group B. Another common group is Group A (salmonella that share O antigen 2).

Considerations

Even though there are technically only a couple species of bacteria in the genus, salmonella are often described as if the serotype name were a species name. The infectious agent that causes typhoid fever, for example, is often called Salmonella typhi, although the correct name is actually Salmonella enterica enterica serotype Typhi. Given that the concept of separate species and subspecies isn't really well defined for bacteria like salmonella, however, both systems of nomenclature are still in common use.

Significance

Some of the more dangerous salmonella serotypes are Group B. Serotype Typhimurium, for example, is the most common serotype in the United States, responsible for an estimated 15 to 20 outbreaks of food poisoning a year. Serotype Paratyphi is another dangerous germ and a Group B member. Group B is therefore one of the most medically relevant groups of salmonella serotypes.

References

• Centers for Disease Control: Congressional Testimony 2009
• John L's Bacteriology Pages: Bacteriology 102, Salmonella
• Todar's Online Textbook of Bacteriology: Endotoxins
• "Journal of Clinical Microbiology"; Salmonella nomenclature; Brenner, Villar et al.; 2000.
• University of Maryland Life Sciences: Pathogenic Microbiology, Salmonella summary