16.2.3: Serological Typing of Streptococci

Last updated
Save as PDF

Page ID: 122731

\( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

The Clearview® Strep A Exact II Dipstick is an example of a lateral flow immunologic assay. It is is a qualitative serologic test for detecting Group A Streptococcal antigen (the unknown antigen) directly from throat swabs and is used as an aid in diagnosing streptococcal pharyngitis caused by Streptococcus pyogenes (Group A Beta Streptococci).

The test consists of a membrane strip that is precoated with rabbit anti-Strep A monoclonal antibody-red latex conjugate (known antibodies made in rabbits against strep A antigen with red latex particles attached) located in a pad at the beginning of the strip. It is also precoated with rabbit anti-Strep A monoclonal antibody (known antibodies made in rabbits against the strep A antigen but without attached red latex particles) that is immobilized at the test line where the test results are read (see Fig. \(\PageIndex{1A}\)). The red latex particles attached to the rabbit anti-Strep A antibodies are what ultimately causes the “positive” red band.

Illustration of the test strip for detecting Group A Streptococcus antigen. — Figure \(\PageIndex{1A}\): **Serologic Testing to Identify Group A Streptococcus Antigen: The Test Strip.** ( Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0 )

The throat swab is placed in an extraction solution that lyses the Streptococcus pyogenes, if present, and exposes the strep A antigen in the bacterial cell wall. When the test strip is immersed in the extracted sample, the Group A Streptococcal antigen extracted from the Streptococcus pyogenes on the throat swab of a person with strep throat begins to move chromatographically up the membrane and binds to the known antibody-red latex conjugate in the pad located at the beginning of the strip, forming a Strep A antigen-antibody complex (see Fig. \(\PageIndex{1B}\)). This Strep A antigen-antibody complex continues to moves up the membrane to the test line region where the immobilized rabbit anti-Strep A antibodies are located.

Illustration of cell wall fragments containing Strep A antigen binding to known anti-Strep A antibodies bound to red latex particles. — Figure \(\PageIndex{1B}\): **Serologic Testing to Identify Group A Streptococcus Antigen: Known Antibodies Against Strep A Antigen binding to Strep A Antigens**. Cell wall fragments containing Strep A antigen binds to known anti-Strep A antibodies bound to red latex particles. ( Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0 )

If Group A Streptococcal antigen is present in the throat swab, a red-colored sandwich of known antibody/Strep A antigen/red latex conjugate forms in the test line region of the strip (see Fig. 6C). The control region of the strip has immobilized anti-rabbit antibodies, that is, antibodies made in a different animal against rabbit antibodies. The red color at the control line region appears when molecules of the rabbit anti-Strep A antibody-red latex conjugate not trapped at the test line reach the control area and are stopped by binding to the anti-rabbit antibodies. This indicates that the test is finished. As a result, a positive test for Group A Strep antigen appears as a red band in the test result area and a red band in the control area (see Fig. \(\PageIndex{1C}\)).

Illustration of a positive test for Group A Strepyococcus showing twp red bands. — Figure \(\PageIndex{1C}\): **Serologic Testing to Identify Group A Streptococcus Antigen: A Positive Test**. Note two red stripes. ( Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0 )

If there is no Group A Streptococcal antigen present in the throat swab no red band appears in the test result region of the strip and a single red band appears in the control line region, indicating a negative test for Group A Strep antigen (see Fig. \(\PageIndex{1D}\)).

Illustration of a negative test for Group A Streptoccus antigen showing only one red stripe in the test control area. — Figure \(\PageIndex{1D}\): **Serologic Testing to Identify Group A Streptococcus Antigen: A Negative Test**. Note only one red stripe in the test control area. ( Copyright; Gary E. Kaiser, Ph.D. The Community College of Baltimore County, Catonsville Campus CC-BY-3.0 )

Animations

Contributors and Attributions

Dr. Gary Kaiser (COMMUNITY COLLEGE OF BALTIMORE COUNTY, CATONSVILLE CAMPUS)