Protective effect of subinhibitory polymyxin B alone and in combination with ampicillin for overwhelming Haemophilus influenzae type B infection in the infant rat: Evidence for in Vivo and in Vitro release of free endotoxin after ampicillin treatment

The potential endotoxin modifying effects of subinhibitory doses of polymyxin B were evaluated in an animal model of overwhelming septicemia. Five to six day old Sprague-Dawley rats were infected intraperitoneally with 10⁶-10⁷ cfu of Haemophilus influenzae type b. At 12 h after infection, at which time mortality was 18%, subinhibitory doses of polymyxin b (0.0125 mg/kg x 3 q 3 h) either alone or in combination with 500 mg/kg ampicillin significantly increased survival at 17 and 20 h (p=0.009, 0.01 and p=0.003, 0.01) compared to animals treated with 0.5 mg/kg of ampicillin alone. Prolonged survival at 36 h (p=0.009) was seen in animals receiving both ampicillin and low dose polymyxin compared to either ampicillin dose alone. Ampicillin significantly reduced the number of bacteria in blood of survivors (p<0.023 at 30 min) compared to untreated animals but increased the activity of free endotoxin at 30 min compared to controls (p=0.006). In vitro endotoxin release from H. influenzae type b increased 5-fold after addition of 100 μg/ml of ampicillin, whereas a six-fold reduction in endotoxin activity was measured after the addition of 7 μg/ml of polymyxin B. Subinhibitory doses of polymyxin B modulate the ethal effects of overwhelming H. influenzae type b infection in infant rats and might be beneficial as adjunct treatment in gram-negative septicemia.