Opsonization requirements of Serratia marcescens

Dextran-separated human peripheral blood leukocytes (55 vol%) from healthy adults served to delineate the opsonic requirements of various serologically defined assay strains of Serratia marcescens. 'Natural' antibodies of three commercial IgG immunoglobulin preparations (25 vol%), in the absence of complement (C), did not opsonize all 19 test strains, in particular certain strains carrying O-antigens O6 and/or O14, despite documented (ELISA technique) contents of anti-O6 and -O14 IgG antibodies. The 'natural' IgG opsonins proved O-antigen specific. Several of 13 selected assay strains of S. marcescens differed in opsonic requirements for 'classically' or 'alternatively' activated human C; opsonization was optimal with intact fresh human serum (25 vol%). All 13 tested strains activated both pathways of C. Anti-live cell rabbit immune sera (RIS) surpassed conventional anti-O and anti-H RIS (25 vol%) in terms of opsonizing activity, which proved partially heat-labile and O-antigen specific for two, but not for two other test strains. Isolated IgM immunoglobulin fractions of anti-live cell RIS either totally lacked or displayed inferior opsonizing activity in the absence of C, although O-agglutinin activity was only from 4- to 8-fold lower than that of unfractionated anti-live cell RIS. Human and rabbit 'natural' IgM antibodies opsonized moderately. Added guinea pig C (10 vol%) failed to enhance opsonizing activity of 'natural' and 'immune' rabbit IgM and 'natural' human IgG and IgM antibodies.