Effect of bacterial competition on the opsonization, phagocytosis, and intracellular killing of microorganisms by granulocytes.

The ingestion of Escherichia coli by human granulocytes in vitro was reduced in the presence of Bacteroides fragilis or Staphylococcus aureus. This reduction of ingestion proved to be mainly attributable to the absence of opsonization of E. coli, which was due to complement consumption by B. fragilis and S. aureus. The intracellular killing of E. coli was decreased in the presence of B. fragilis and S. aureus because of consumption of complement components required for extracellular stimulation of granulocytes to kill intracellular bacteria. Decreased intracellular killing of E. coli by granulocytes containing either B. fragilis or S. aureus is due to the limited killing capacity of granulocytes. These interactions between E. coli and B. fragilis or S. aureus found for phagocytosis and intracellular killing were also observed in in vivo studies: in an experimental thigh lesion infection in mice, E. coli showed stronger proliferation after coinoculation with B. fragilis or with S. aureus than after injection of E. coli alone. These in vitro and in vivo findings indicate that bacterial interactions, not only between aerobic and anaerobic bacteria but also between two species of aerobic microorganisms, compete for host defense mechanisms (i.e., opsonization, phagocytosis, and intracellular killing).