Polymyxin B and Colistin: Activity, Resistance and Crossresistance in vitro.

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Abstract
Polymyxin B and colistin. 2 closely related polypeptide antibiotics, had similar antibacterial activity against a large number of pathogenic bacteria. Polymyxin was somewhat more active than colistin against 55% of susceptible strains. on the average 3.4 and 4 times as active against most strains of N. gonorrhoeae and H. influenzae, respectively. Highly resistant variants developed on repeated subculture of a strain of A. aerogenes and one of Ps. aeruginosa on agar containing either of these antibiotics; complete crossresistance to the other antibiotic developed in each instance. Single-step mutants of the same strain of A. aerogenes, highly resistant to each antibiotic, were readily obtained by seeding a large inoculum on the surface of agar containing 100 or 200 μg/ ml of that antibiotic; these mutants were completely cross-resistant to the heterologous antibiotic. Antibiotic-dependent variants were not encountered.