Impairment by Bacteroides species of opsonisation and phagocytosis of enterobacteria.

The ability of human polymorphonuclear leucocytes to phagocytose and kill Proteus mirabilis was impaired in vitro when the human serum, used to opsonise the target bacteria, was pretreated with cultures of various Bacteroides species. Live and dead, either heat-killed or clindamycin-treated, bacteroides cells elicited the same phenomenon. When bacteroides-treated serum was used to opsonise different Proteus species, the subsequent uptake of all strains by polymorphonuclear leucocytes was inhibited, whereas bacteroides-treated serum inhibited the uptake of some but not all of the test strains of Escherichia coli. The opsonic activity of untreated human serum was reduced when the classical complement pathway was inhibited by ethyleneglycol-bis-(beta-aminoethyl ether)N,N'-tetra-acetic acid (EGTA); subsequent treatment with bacteroides did not further reduce the opsonic activity of the serum for P. mirabilis.