Low concentrations of hemoglobin (0.1 µg./ml. or less) exert a lethal action on some Gram-negative bacteria under certain conditions in vitro. Hemoglobins from various mammals and the distinct genetic types of human hemoglobin all manifest similar bactericidal activity.

The bactericidal effect is a function of the globin moiety of the molecule; native and acid or acid-alcohol denatured globins have the same degree of activity.

Hemoglobins kill enterobacteriaceae only under precisely controlled conditions. The test medium must be low in ionic concentration and acid in reaction. Various strains of Escherichia and Salmonella are susceptible to the lethal effect of hemoglobin, while the few strains of Shigella, Klebsiella, and Proteus examined were resistant.

Certain acid polysaccharides and basic amines or proteins block the bactericidal effect when incorporated in the test in low concentration. Present evidence also suggests that exposure of the microorganisms to certain cations such as magnesium renders them resistant to the lethal action of globin. Hemoglobin loses its bactericidal power when complexed with haptoglobin, and serum fractions rich in free haptoglobin protect otherwise susceptible bacteria from killing by hemoglobin.

The reaction appears to be a bactericidal rather than a bacteriostatic one. At 38°C. maximal killing requires approximately 30 minutes; at temperatures of 28°C. or 0°C. the bactericidal action does not take place. The minimal concentration of hemoglobin required to kill 50 per cent of the microorganisms in the test is unrelated to the size of the bacterial inoculum.

Under conditions suitable for bactericidal action, hemoglobin is adsorbed onto heat-killed susceptible strains of coliform bacteria; material possessing bactericidal activity can be eluted with dilute mineral acid.

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