Genetic Diversity of the Pneumococcal Capsule: Implications for Molecular-based Serotyping

Mary Catherine McEllistrem

Disclosures

Future Microbiol. 2009;4(7):857-865. 

In This Article

Quellung Reaction & the Need for Additional Serotyping Methods

The classic quellung reaction, or Neufeld test, uses rabbit antisera to detect a capsular reaction observed with light microscopy.[7–9] In 2004, the Statens Serum Institute developed a latex agglutination test, which allowed even those without expertise in microscopy to accurately serotype pneumococci. Specifically, 10 µl of latex suspension is mixed with 10 µl of an overnight bacterial culture. A positive reaction is indicated by an agglutination appearing within 30 s. The checkerboard system using 14 latex pools minimizes how many agglutination tests are required to serotype each strain.[10]

However, the cost of the serotyping reagents may make the classical serological typing method prohibitive for most clinical laboratories. With the exception of nine serotypes, the latex pools cannot determine the exact serotype; rather, the latex pools restrict the possible serotype to three to nine choices. Therefore, an additional 110 antisera or so must be procured to determine the specific serotype with the quellung reaction. While the initial expense is significantly less than a PCR thermocycler, the ongoing cost of replacing these numerous reagents may make this method unaffordable. In addition to expenditure concerns, the latex agglutination test cannot always differentiate Streptococcus mitis, Streptococcus oralis, or nontypeable pneumococci from typeable pneumococci. As a result of these limitations, molecular capsular-based serotyping methods have been pursued. The genotypic assays have the added potential benefit of being able to detect pneumococci directly from the clinical specimen rather than from a pneumococcal culture.

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