Host response to mixed anaerobic infection with Bacteroides melaninogenicus and Fusobacterium necrophorum.

Abstract

This study characterizes the acute phase of the host response to mixed infection withBacteroides melaninogenicus andFusobacterium necrophorum. Female NMRI mice were injected intraperitoneally with 0.5 ml of 1:1 mixtures ofB. melaninogenicus andF. necrophorum (total inoculum = 1 × 109 bacteria). This infectious dose was sufficient to produce septicemia within 24 hours, with subsequent hepatic abscess formation in survivors at one to two weeks. A 25–30% mortality rate was seen during the acute phase (0–96 h) of infection. Blood, liver and spleen samples were obtained at various intervals after infection; quantitative anaerobic bacteriology was performed. Laked blood BHI agar roll tubes were used to differentiate black-pigmentingB. melaninogenicus fromF. necrophorum. Other tests performed included: the limulus amebocyte lysate (LAL) assay for endotoxin, total white blood cell (WBC) count, differential WBC, hematocrit, blood glucose levels, and serum glutamic-pyruvic transaminase (SGPT) activity. An acute to chronic infection progressed in mice infected with this mixture of anaerobic bacteria, whereas no infection resulted when either organism was injected alone. The first 96 hours of infection demonstrated a typical acute inflammation, documented by blood chemistry, hematology and histology. Quantitativein vivo growth patterns were obtained for both of the infecting bacteria. The liver and spleen were the tissues containing the most bacteria. The modified LAL assay revealed circulating endotoxin during the early phase of the infection. In nearly all of the survivors, chronic intrahepatic abscesses developed two to six weeks after the acute phase. The results suggest thatFusobacterium, which contains the more potent endotoxin, predisposes the animals to chronic infection withBacteroides which predominates during the chronic phase. Bacterial endotoxins probably play a more central role in this type of infection than was previously thought.