Effect of atmosphere and duration of incubation on primary isolation of group A streptococci from throat cultures

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Abstract
Throat swabs (human) submitted to the clinical laboratory were inoculated onto duplicate 5% sheep blood agar plates, incubated aerobically or anaerobically (GasPak jar) at 35.degree. C and examined semiquantitatively after 24 and 48 h. Group A streptococci were identified by the fluorescent-antibody technique. Of 1040 specimens , 506 (48.6%) grew .beta.-hemolytic streptococci, including 200 (19.2%) group A streptococci. Group A streptococci were recovered significantly more often with anaerobic incubation than with aerobic incubation after 24 h (182 vs. 138; P < 0.001) and after 48 h (193 vs. 174; P < 0.05). Non-group A .beta.-hemolytic streptococci were also recovered significantly more often with anaerobic incubation after 24 and 48 h (P < 0.001). Colony counts were not affected by the incubation atmosphere. Thus, incubation of throat cultures in an anaerobic atmosphere is superior to incubation in air for detection of group A streptococci. The greater sensitivity of anaerobic incubation may not justify the extra laboratory effort and cost required to differentiate group A streptococci from non-group A streptococci detected as a result of anaerobic incubation. Throat cultures should be examined after 24 and 48 h, especially if plates are incubated aerobically.