Streptococcus pneumoniae = pneumococcus
Around the turn of the century (1900) German physician and bacteriologist Friedrich Neufeld discovered that antibodies against specific pneumococcal capsular antigens could be produced and used for typing Streptococcus pneumoniae organisms. These antibodies, when bound to the cell wall antigen produced a clearing around each individual cell with the appearance of the capsule having swollen. The word “Quellung” is German for ‘swelling’ however this is a misnomer as the capsule does not swell but simply appears enlarged with the clear zone produced by the bound antibodies. The clearing is best visualized by using a stain to enhance contrast between the clear zone of bound antibody and the surrounding material. Methelyne Blue is commonly used as in the photo below.
A patient’s specimen, often sputa, cerebral spinal fluid (CSF) or colony isolate, is placed on a clean microscope slide and allowed to dry. A drop of Polyvalent(1) Quellung antisera is applied to the specimen/slide and mixed with a drop of methyene blue stain. A coverslip is place onto the mixture and allowed to incubate/react at room temperature for 15-20 minutes. (can be placed in a humidity chamber to prevent evaporation). The slide is then examined under the microscope for distinct clearing around the cells. One may have to search for a microscopic field which best shows the clearing. A distinct zone of clearing indicates a positive Quellung reaction and confirms the identification as Streptococcus pneumoniae.
The price of Quellung antisera has escalated in recent years (~$600/ml CND). A limited shelf life and prohibitive cost make its use rather restrictive. Molecular techniques may soon replace this classic test.
Specific antibodies can be produced against the capsular antigens of organisms from other genera that have capsules -eg. Klebsiella pneumoniae.
1) Polyvalent Antisera- contains all known serotypes to identify any pneumococcus. Specific or individual antisera can be employed for scientific study of individual serotypes.