Gram Positive Cocci
A child develops pustule on front of his leg. Which on culture is found bacitracin sensitive beta hemolytic streptococci. A similar stain is found by school doctor in students causing pharyngitis. Which test can differentiate between two strains?
|B||mec A gene|
|D||C carbohydrate antigen|
a. Beta hemolytic streptococci are classified according to their C carbohydrate antigen into 20 Lancefield groups. The great majority of human infections due to hemolytic streptococci are caused by group A, (Streptococcus pyogenes).
b. Griffith further subdivided Group A into serotypes on the basis of M, T and R proteins. The M protein is an important virulence factor by inhibiting phagocytosis and intracellular killing by leukocytes. It is heat stable and trypsin sensitive and can be extracted by Lancefield method used for extraction of C carbohydrate. Group A has about 120 M protein types
c. Most common infections due to Group A Streptococcus are suppurative -respiratory tract infections like pharyngitis, otitis media, quinsy, mastoiditis and sinusitis and skin infections like erysipelas, impetigo and cellulitis. Non-suppurative diseases include acute rheumatic fever and acute glomerulonephritis.
d. Suppurative infections of skin due to S. pyogenes are usually due to higher numbered M types, i.e 25, 49, 52-55 and 57-61 whereas respiratory tract infections due to S. pyogenes are usually due to lower numbered M types, i.e 1, 3, 5, 6, 12,14. Hence as in this question, the two strains, one from pyoderma and other from pharyngitis will be differentiated by M protein.
e. The emm gene codes for the M protein. Hence sequencing of this gene would be superior to M protein typing.
f. M Type-49 is called Red lake strain