Effect of anaerobic bacteria on killing of Proteus mirabilis by human polymorphonuclear leukocytes

Killing of Proteus mirabilis by human polymorphonuclear leukocytes was tested in the presence of different Bacteroides species. In vitro experiments showed that anaerobic bacteria interfered with the killing of aerobic bacteria. However, this inhibitory effect was not a property of all Bacteroides species. Bacteroides gingivalis W83 showed the greatest inhibitory effect of the five Bacteroides strains tested. Killing of P. mirabilis was inhibited by the culture supernatant of B. gingivalis but not by washed cells. Two factors were found in the supernatant of B. gingivalis to account for the inhibitory effect. One was heat stable with a molecular weight of less than 3,500 and inhibited the killing activity of leukocytes, and the other was heat labile and partly inactivated the complement system. The killing experiments paralleled chemiluminescence measurements.