Abstract

The inhibitory effect of Bacteroides fragilis on the in vitro killing of Escherichia coli by polymorphonuclear leucocytes was studied with two pairs of E coli and B fragilis isolated from human wound infections. Both B fragilis strains behaved similarly: they inhibited the killing of one E coli strain, while the killing of the other E coli strain was not affected. The different behaviour of the two E coli strains depended on their need for fresh serum in the killing by the polymorphonuclear leucocytes. The inhibitory effect of the B fragilis strains could be completely accounted for by their effect on complement.