Opsonins in normal mouse serum for the phagocytic killing of Proteus mirabilis by murine neutrophils.

An assay of phagocytic killing by murine neutrophils in homologous serum was used to determine the nature of the opsonins in normal mouse serum for phagocytic killing of Proteus mirabilis. Leucocytes from the peritoneal cavities of mice given an intraperitoneal inoculation of brain-heart infusion broth 3 h previously, phagocytosed and killed P. mirabilis in a 2-h assay in the presence of 10% serum from normal mice. The serum factors supporting phagocytic killing were heat-labile (50 degrees C or 56 degrees C for 30 min) and could be absorbed at 37 degrees C but not 4 degrees C by three different species of Gram-negative bacteria. The tested species of Gram-positive bacterium did not absorb the activity. At the end of the assays, greater than 90% of leucocyte-associated bacteria were associated with neutrophils. Leucocytes from unstimulated peritoneal cavities (less than I% neutrophils) did not kill bacteria in this assay, in contrast to leucocyte suspensions containing up to 98% neutrophils. These findings indicated that the phagocytic killing of P. mirabilis in this assay was mediated by neutrophils, and that complement fixation by the alternative pathway provided necessary opsonins in normal mouse serum.