Production of Dense Red Band Around Growth of Staphylococcus aureus on Blood Agar Plates

Summary 1. When certain strains of S. aureus are grown on human blood agar, a dense broad red band develops around the growth. This band surrounds a hemolytic area produced by strains that are hemolytic for human erythrocytes (e.g., Guy Strain) and surrounds directly the growth of strains that are not hemolytic (e.g., Oxford Strain). 2. Production of the red bands appears to depend upon a substance that diffuses through agar. 3. When sterile agar sections from plates previously inoculated with S. aureus are transferred onto blood plates, red bands develop around the transplanted section. 4. When plain agar is inoculated with the Guy Strain at a point near blood agar, a circular dense red band develops around the hemolytic zone in the blood medium; when inoculated with the Oxford Strain, a dense solid red band (half-moon) develops without hemolytic zone.