Throat cultures for group A beta-hemolytic Streptococcus. Importance of anaerobic incubation.

The bacteriologic detection of group A Streptococcus in pharyngitis is vital in everyday practice to prevent serious potential sequelae. The purposes of this study were to determine whether throat cultures should be incubated in anaerobic atmosphere and whether an increased recovery rate could be obtained by stabbing of the plates (partial anaerobiosis) and by using a sulfamethoxazole-trimethoprim disk to enhance growth and identification. We examined 243 throat cultures, in duplicate, which were incubated in room air and in anaerobiosis (carbon dioxide, 10%). We found that, in aerobic incubation, the recovery rate of group A streptococci was 5.7%; in anaerobic incubation it was 19.8%. Stabbing of the agar to create a partial anaerobiosis was useless. When directly placed on the plate, the sulfamethoxazole-trimethoprim disk facilitated the identification of beta-hemolysis areas. To achieve maximum detection of group A streptococci in specimens obtained from the throats of infected children, we found that anaerobic incubation should be used.