The Quellung reaction is a biochemical reaction

Chemical reaction

A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...

 in which antibodies

Antibody

An antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...

 bind to the Bacterial capsule of Streptococcus pneumoniae

Streptococcus pneumoniae

Streptococcus pneumoniae, or pneumococcus, is Gram-positive, alpha-hemolytic, aerotolerant anaerobic member of the genus Streptococcus. A significant human pathogenic bacterium, S...

, Klebsiella pneumoniae

Klebsiella pneumoniae

Klebsiella pneumoniae is a Gram-negative, non-motile, encapsulated, lactose fermenting, facultative anaerobic, rod shaped bacterium found in the normal flora of the mouth, skin, and intestines....

, Neisseria meningitidis

Neisseria meningitidis

Neisseria meningitidis, often referred to as meningococcus, is a bacterium that can cause meningitis and other forms of meningococcal disease such as meningococcemia, a life threatening sepsis. N. meningitidis is a major cause of morbidity and mortality during childhood in industrialized countries...

and Haemophilus influenzae

Haemophilus influenzae

Haemophilus influenzae, formerly called Pfeiffer's bacillus or Bacillus influenzae, Gram-negative, rod-shaped bacterium first described in 1892 by Richard Pfeiffer during an influenza pandemic. A member of the Pasteurellaceae family, it is generally aerobic, but can grow as a facultative anaerobe. H...

and thus allow them to be visualized under a microscope

Microscope

A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...

. If the reaction is positive, the capsule becomes opaque

Opacity (optics)

Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...

 and appears to enlarge.

Quellung is the German word for "swelling" and describes the microscopic appearance of pneumococcal or other bacterial capsules after their polysaccharide

Polysaccharide

Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...

 antigen

Antigen

An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...

 has combined with a specific antibody. The antibody usually comes from a bit of serum

Serous fluid

In physiology, the term serous fluid is used for various bodily fluids that are typically pale yellow and transparent, and of a benign nature, that fill the inside of body cavities. Serous fluid originates from serous glands, with secretions enriched with proteins and water. Serous fluid may also...

 taken from an immunized laboratory animal. As a result of this combination, and precipitation of the large, complex molecule formed, the capsule appears to swell, because of increased surface tension, and its outlines become clearly demarcated.

The pneumococcal Quellung reaction was first described in 1902 by the scientist Fred Neufeld

Fred Neufeld

Fred Neufeld was a physician and bacteriologist who discovered the pneumococcal types. This discovery led Fred Griffith to show that one pneumococcal type could be transformed into another . Subsequently, Oswald Avery demonstrated that the transforming substance was DNA...

, both as microscopic capsular swelling and macroscopic agglutination (clumping visible with the naked eye). It has been used to identify the 90 known capsular serotypes of Streptococcus pneumoniae in a diagnostic setting, but in recent years has been challenged by molecular typing techniques which target genetic differences.

Further reading

• Park, I. H., Pritchard, D., Cartee, R., Brandao, A., Brandileone, M. & Nahm, M. (2007). Discovery of a new capsular serotype (6C) within serogroup 6 of Streptococcus pneumoniae. J Clin Microbiol 45, 1225–1233.
• Pai R, R. E. Gertz, B. Beall. 2006. Sequential multiplex PCR approach for determining capsular serotypes of Streptococcus pneumoniae isolates. J. Clin. Microbiol. 44:124-31.