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Bacteroides fragilis strains formed much smaller colonies on most types of blood agar plates than they did on the same media without blood. Blood inhibited strains of B. fragilis subsp. distasonis the most and B. fragilis subsp. fragilis the least. The inhibition could be eliminated by adding hemin to the blood agar. The inhibitory component of the blood was inside the erythrocytes and appeared to be the hemin-free globin of hemoglobin.
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Wilkins et al. (1976) studied this question.
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