FIG. 1.

The effect of environmental oxygen and carbon dioxide on colony morphology and capsule size. Opaque (O) and transparent (T) variants of a type 6A isolate were grown in nutrient broth at 37°C to mid-log phase under the following conditions: atmospheric (A), microaerophilic (B), or anaerobically with supplemental carbon dioxide (C). The upper panels shows colony size and opacity with oblique, transmitted illumination (magnification, ×45). The lower panels show the refractile zone occupied by the capsule as visualized using the quellung reaction with group 6 antiserum (magnification, ×3,200).