Studies on Pseudomonas Aeruginosa Infections

II. Susceptibility of Pseudomonas Aeruginosa to various Antibiotics, especially to Colistin

Abstract

1. Susceptibility (minimal inhibitory concentration M.I.C.) of 132 strains of Pseudomoas aeruginosa to Colistin sulfate by serial plate dilution method was 2.92±1.61 mcg/ml. More. than half of Pseudomonas aeruginosa strains were inhibited in the concentration of 12.5 mcg/ml of Colistin methanesulfonate. Polymyxin B was as or slightly less active than Colistin sulfate.
Cross-resistance was noticed between Colistin (sulfate and methanesulfonate) and Polymyxin B. Natural resistant strain of Pseudomonas aeruginosa to Colistin and Polymyxin B was quite difficult to be found out in present study by the usual measuring method for suscehtibility, but this method must be improved.
2. In vitro development of resistance of Pseudomonas aeruginosa to Colistin was, readily demonstrated, but the in vitro induced resistant forms were not stable and reverted in the absence of the antibiotic. In vivo development of resistance of Pseudomonas aeruginosa to Colistin was supposed to occur in clinical practice, too. It was believed that the resistance was developed by the ability of adaptation of Pseudomonas aeruginosa to Colistin, but the mechanism of the adaptation to the antibiotic was not made sure in present study.
3. Titration end point of M.I.C. depended on the size of inoculum: in vitro activity of Colistin was decreased by increasing the size of the inoculum. This phenomenon was seen not only in Colistin and Polymyxin B, but also in Streptomycin, Kanamycin, Tetracycline and Chloramphnicol