The antistreptolysin O (ASO) titre is used as evidence of preceding streptococcal infection. The mechanism by which ASO is measured depends on:

• A Complement fixation with group A carbohydrate antigen
• B Agglutination of sheep red blood cells by heterophile antibody
• C Neutralisation of the oxygen-labile haemolysin produced by group A streptococci ✓
• D Precipitation with M protein in an agar gel system

Explanation

Streptolysin O is an oxygen-labile thiol-activated haemolysin that lyses red cells unless neutralised by specific antibody. In the ASO test patient serum is incubated with a fixed amount of streptolysin O and residual haemolytic activity indicates the titre. Streptolysin S is oxygen stable and not antigenic, which is why ASO does not detect skin infection well where S predominates.

Reference: Ananthanarayan and Paniker's Textbook of Microbiology, 11th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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