Host resistance to Serratia marcescens infection: serum bactericidal activity and phagocytosis by normal blood leukocytes.

Serratia marcescens strains isolated from clinical specimens can be divided into those which are sensitive or resistant to the bactericidal activity of normal human serum. Serum bactericidal activity is heat labile, cation dependent, and is absorbable by whole, serum-sensitive Serratia or ethanol-insoluble extracts of these organisms. Bacteremic Serratia infection is invariably caused by the serum-resistant strains. Serum resistant Serratia are ingested and killed by normal human leukocytes and fresh normal serum. Heating or preabsorption of serum with whole, heat-killed, or ethanol-insoluble antigen extracts of the serum-resistant Serratia diminishes opsonization and phagocytosis. Serratia opsonins in the serum of healthy individuals are type-specific IgM globulins which combine with the organism and activate complement by the alternate pathway.