Quelling reaction - Introduction, History, Principle, Procedure, Recent use

Introduction to Quelling reaction

Quelling reaction, also known as Neufeld reaction, is used to visualize/detect bacterial capsules, which if positive, appear enlarged.

In this biochemical reaction, specific antibodies bind to the bacterial capsule resulting in the species being able to be visualized under a microscope. Some organisms which show positive quelling reactions are Streptococcus pneumoniae, Klebsiella pneumoniae, Neisseria meningitidis, Bacillus anthracis, Haemophilus influenzae, Escherichia coli, and Salmonella.

History of Quelling reaction

Historically, scientist Fred Neufeld was the first one to describe the quelling reaction in 1902. He had observed both microscopic capsular swelling and macroscopic agglutination in Streptococcus pneumonia. At the time, it could distinguish only the three known pneumococcal serotypes.

In the 1920s, with the arrival of serum therapy, the quelling reactions became clinically important to be used to treat certain types of pneumococcal pneumonia. This method was particularly advantageous because to treat an individual patient, correct identification of the infecting pneumococcal serotype was a must and this method was the one that was just able to deliver.

As decades went on, Dr. Albert Sabin, the developer of the oral polio vaccine, modified this procedure to obtain rapid results. Other researchers also contributed to his technique to identify 29 additional serotypes.

Principle of Quelling reaction

The microorganism-specific antibody is derived from the serum of a laboratory animal that has been immunized. The antigen binds with the bacterial polysaccharide antigen.

As a result of this biochemical reaction, and subsequent precipitation of the large, complex molecule, the bacterial capsule appears to swell. Due to the increased surface tension, the outlines of the capsule become clearly distinguished.

Procedure of Quelling reaction

The steps/procedure of quelling reaction is as follows:

1. Antibiotics specific to suspected (test) organisms are mixed with test colony

2. capsule is stained

3. Microscopy

4. If capsule(s) seems enlarged, then the quelling reaction positive

Recent use of Quelling reaction

After the introduction of antibiotics in the 1940s, serum therapy was largely discontinued. However, the identification of specific serotypes remained important, mostly for epidemiological as well as determining variable invasiveness of pneumococci. Researching the prevalence of various serotypes was an important factor development and distribution of pneumococcal vaccines.

In recent years, the quelling reaction has been gradually replaced by latex agglutination method and molecular typing techniques including polymerase chain reaction (PCR).