Co-culture of three bacterial strains: Staphylococcus aureus, Enterococcus fecalis, and Escherichia coli. Anti- Enterococcus fecalis antibodies incubated with the co-culture bound to some cocci but not to the other cocci or rods (white arrows). This interaction was visualized by electron microscopy by adding gold labled anti human IgG antibodies to the medium