Laboratory and clinical studies of polymyxin B

been described in the recent literature. Their anti-bacterial spectra are similar. They differ from one another in amino acid content. It is the pur-pose of this presentation to describe our studies of the antibacterial activity, pharmacology, un-toward reactions, and clinical experience with polymyxin B and polymyxin E.5 IN VITRO ACTION We found that the susceptibility to polymyxin of 78 strains of ten different genera by the tube dilution test (Table I) is marked against Salmo-nella, Shigella, Klebsiella, microorganisms of the coli aerogenes groups, and most importantly, Pseu-domonas. Brucella and many staphylococci are moderately sensitive, while Proteus and hemolytic streptococci are refractory. It is of interest that a strain of Pseudomonas, which developed high resistance to streptomycin in three daily transfers, did not become resistant to polymyxin after 27 transfers. Size of inoculum and human serum re-duced but slightly polymyxin activity. The con-centration of polymyxin required to inhibit Kleb-siella and Pseudomonas is not reduced significantly by the addition of subinhibitory amounts of strep-tomycin, aureomycin, sulfadiazine or penicillin, either singly or in several combinations. Poly-myxin antagonizes the cumulative streptomycin-penicillin action on Proteus