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We studied the requirements for opsonization of Listeria monocytogenes type 4b with chemiluminescence and bactericidal assays and electron microscopy. Preopsonization with 3% adult serum had good opsonic activity (27,300 +/- 11,000 [standard deviation] counts, chemiluminescence assay), while 3% newborn cord serum was not opsonically active (820 +/- 530 counts, P less than 0.001 versus adult serum). In addition, organisms opsonized with cord serum were not killed (0% bacterial killing) and were less frequently visualized intracellularly on electron micrographs (0 to 4 bacteria per cell) than organisms opsonized with adult serum (70% killing and 10 to 20 bacteria per cell). Opsonic requirements for L. monocytogenes type 4b at low concentrations of serum were studied in detail with Sepharose-protein A-treated adult serum to obtain immunoglobulin G (IgG) and IgM fractions and zymosan-absorbed and C4 inactivator-treated serum to obtain alternative and classical complement pathway-deficient sera, respectively. In the presence of complement, IgM was opsonically active (59% of control) while IgG was not (6% of control). In addition, classical complement activity was required for efficient opsonization (greater than 100% of control) while the alternative complement pathway was unnecessary (3% of control). Since IgM is absent and classical complement activity is low in neonatal serum and at the common sites of neonatal Listeria infection, the requirement for IgM and classical complement activity for efficient opsonization of L. monocytogenes type 4b at low serum concentrations may be a factor in the pathogenesis of neonatal disease.

Opsonization of Listeria monocytogenes type 4b by human adult and newborn sera