Laboratory diagnosis of Streptococci Infection
Streptococci are readily cultured from blood agar plates where a or B haemolysis may be seen. The colonies may be mucoid, matt, or glossy. Pour plates should be used in order to be certain of detecting B-haemolysis. Lancefield typing may be carried out on extracted carbohydrate C Ags classically by the precipitin test with specific rabbit antisera. However, a simple method may be used to identify group A since group A strains are exquisitely sensitive to bacitracin.
The patientís sera may be tested for the presence of anti-M antibodies by a variety of serological tests. However, test results may be often misleading because of cross-reacting Ags. Serological tests for Abs to extracellular products are much easier to perform and are widely used to obtain evidence of recent streptococcal infection. The antistreptolysin test measures Abs against streptolysin O. Antibodies against streptokinase, hyaluronidase, or DNAse may be also be used.
B-haemolytic streptococci are among the most susceptible pathogens to antimicrobial agents. Penicillin is the drug of choice in the treatment of S. pyogenes infection. Where a history of hypersensitivity to penicillin exits, erythrocmycin may be used instead. Prophylactic penicillin is often given to patients with rheumatic fever in order to prevent a recurrence.†