Particle Agglutination

Numerous procedures have been developed to detect antigen via the agglutination (clumping) of an artificial carrier particle such as a latex bead with antibody bound to its surface.

Latex Agglutination

Antibody molecules can be bound in random alignment to the surface of latex (polystyrene) beads (Figure 9-5). Because the number of antibody molecules bound to each latex particle is large, the potential number of antigen-binding sites exposed is also large. Antigen present in a specimen being tested binds to the combining sites of the antibody exposed on the surfaces of the latex beads, forming cross-linked aggregates of latex beads and antigen. The size of the latex bead (0.8 jim or larger) enhances the ease with which the agglutination reaction is recognized. Levels of bacterial polysaccharides detected by latex agglutination have been shown to be as low as 0.1 ng/mL. Because the pH, osmolality, and ionic concentration of the solution influence the amount of binding that occurs, conditions under which latex agglutination procedures are carried out must be carefully standardized. Additionally, some constituents of body fluids, such as rheumatoid factor, have been found to cause false-positive reactions in the latex agglutination systems available. To counteract this problem, it is recommended that all specimens be pretreated by boiling or with ethylenediaminetetraacetic acid (EDTA) before testing. Commercial test systems are usually performed on cardboard cards or glass slides; manu-

Particle Agglutination Test

Figure 9-7 Streptex, Remel, Inc., Lenexa, Kan. Colony of beta-hemolytic Streptococcus agglutinates with group B Streptococcus (Streptococcus agaladiae) latex suspension.

Figure 9-6 Cryptococcal Antigen Latex Agglutination System (CALAS), Meridian Diagnostics, Inc., Cincinnati, Ohio. Patient 1 shows positive agglutination; patient 2 is negative.

Figure 9-7 Streptex, Remel, Inc., Lenexa, Kan. Colony of beta-hemolytic Streptococcus agglutinates with group B Streptococcus (Streptococcus agaladiae) latex suspension.

facturer's recommendations should be followed precisely to ensure accurate results. Reactions are graded on a 1+ to 4+ scale, with 2+ usually the minimum amount of agglutination seen in a positive sample. Control latex (coated with antibody from the same animal species from which the specific antibody was made) is tested alongside the test latex. If the patient specimen or the culture isolate reacts with both the test and control latex, the test is considered nonspecific and therefore uninterpretable.

Latex tests are very popular in clinical laboratories to detect antigen to Cryptococcus neoformans in cerebrospinal fluid or serum (Figure 9-6) and to confirm the presence of beta-hemolytic Streptococcus from culture plates (Figure 9-7). Latex tests are also available to detect Streptococcus agaladiae, Clostridium difficile toxins A and B, and rotavirus.

Coagglutination

Similar to latex agglutination, coagglutination uses antibody bound to a particle to enhance visibility of the agglutination reaction between antigen and antibody. In this case the particles are killed and treated S. aureus organisms (Cowan I strain), which contain a large amount of an antibody-binding protein, protein A, in their cell walls. In contrast to latex particles, these staphylococci bind only the base of the heavy-chain portion of the antibody, leaving both antigen-binding ends free to form complexes with specific antigen (Figure 9-8). Several commercial suppliers have prepared coagglutination reagents for identification of streptococci, including Lancefield groups A, B, C, D, F, G, and N; Streptococcus pneumoniae; Neisseria meningitidis; N. gonorrhoeae; and Haemophilus influenzae types A to F grown in culture. The coagglutination reaction is highly

152 Partll GENERAL PRINCIPLES IN CLINICAL MICROBIOLOGY

Protein A

Staphylococcus aureus Specific antibody

Was this article helpful?

0 0
Bacterial Vaginosis Facts

Bacterial Vaginosis Facts

This fact sheet is designed to provide you with information on Bacterial Vaginosis. Bacterial vaginosis is an abnormal vaginal condition that is characterized by vaginal discharge and results from an overgrowth of atypical bacteria in the vagina.

Get My Free Ebook


Responses

  • Maxima
    How is particle agglutination carried out?
    3 years ago

Post a comment