Treatment of Murine Coccidioidomycosis with Polymyxin B

An agar dilution method was employed to test the susceptibility of 12 strains ofCoccidioides immitis to polymyxin B (PB). After 3 days of incubation, eight strains were markedly inhibited by 5.0 μg of PB per ml and four strains did not grow. PB at 10 μg/ml inhibited the growth of all strains through 20 days of incubation. To determine whether PB has anticoccidioidal activity in vivo, mice in groups of 30 were infected intraperitoneally with a mean lethal dose (LD50 ) or 20 LD50 of arthrospores ofC. immitis ATCC 28868 (Silveira). Treatment by intraperitoneal injection of PB (2.5 mg/kg) was begun 2 or 5 days after challenge. By 40 days after infection, 47 and 7% of the untreated mice challenged with an LD50 , respectively, were alive. Of mice infected with an LD50 or 20 LD50 and treated with PB beginning 2 days after the challenge, 90% of each group were alive by day 40. Initiation of PB therapy 5 days after infection permitted survival of 84% of the mice infected with an LD50 ; however, only 27% of the mice infected with 20 LD50 survived by day 40. In this latter group there was evidence that PB prolonged life since 56% of the treated mice were alive by day 15 as compared with 30% of the controls. PB in vivo was fungistatic since the majority of treated mice hadC. immitis in the liver, lungs, and spleen.