Gram Positive Cocci
Classification of Streptococci into groups A, B, C, D is based on; (AIIMS Nov 2009)
|A||Colony morphology on Blood agar|
|B||Antigenicity of cell wall carbohydrate|
|C||Presence or absence of M protein|
|D||Presence or absence of capsule|
a. Lancefield classified all beta hemolytic catalase negative gram positive cocci into 20 serogroups A to W on the basis of C carbohydrate antigen. This group specific carbohydrate: is an integral part of the cell wall therefore has to be extracted for grouping by precipitation test with group antisera.
b. This carbohydrate is contained in the cell wall of many streptococci and forms the basis of serologic grouping into Lancefield groups A–H and K–U.
c. The serologic specificity of the group-specific carbohydrate is determined by an amino sugar. For group A streptococci, this is rhamnose-N-acetylglucosamine; for group B, it is rhamnose-glucosamine polysaccharide; for group C, it is rhamnose-N-acetylgalactosamine; for group D, it is glycerol teichoic acid containing d-alanine and glucose; and for group F, it is glucopyranosyl-N-acetylgalactosamine.
d. There are various methods of antigen extraction e.g. Lancefield’s acid extraction method (using hydrochloric acid), Fuller’s method (using formamide), Maxted’s method (using enzyme produced by Streptomyces albus) and Rantz and Randall’s method (using autoclaving). On the basis of group specific carbohydrate antigen in the cell wall, -hemolytic streptococci are divided into 21 serological groups also called as lancefield groups (A-H, K-V).
e. Colony morphology on Blood agar has been used by Brown to classify catalase negative gram positive cocci into alpha-, beta-hemolytic and non-hemolytic.
f. Griffith used M protein to type Group A beta hemolytic streptococci.