An eight-laboratory study addressed the urgent need for quality control (QC) ranges for susceptibility determination when testing colistin (polymyxin E) and polymyxin B, two polycationic peptide antimicrobial agents, against multidrug-resistant gram-negative bacilli. For Escherichia coli ATCC 25922l, the QC ranges were as follows: for colistin, 0.25 to 1 g/ml (11 to 17 mm), and for polymyxin B, 0.25 to 2 g/ml (13 to 19 mm). For Pseudomonas aeruginosa ATCC 27853, the QC ranges were as follows: for colistin, 0.25 to 2 g/ml (11 to 17 mm), and for polymyxin B, 0.25 to 2 g/ml (14 to 18 mm). More than 97% of all reported QC results were within these proposed ranges.The polymyxin class antimicrobial agents (colistin or polymyxin E and polymyxin B) are polycationic peptides that were originally synthesized from Bacillus polymyxus (1, 13). The mechanism of action for the polymyxins has been determined to be secondary to surfactant-like properties that produce enhanced permeability of the bacterial cytoplasmic membrane, leading to bacterial death (1, 13, 15). These agents were first described more than five decades ago and were initially applied to therapy for gram-negative bacillary infections before the discovery of other broad-spectrum agents, such as the aminoglycosides, carboxypenicillins, and cephalosporins (14). Toxicity issues (12) and the emergence of alternative antimicrobial regimens resulted in the elimination of colistin and polymyxin B from National Committee for Clinical Laboratory Standards (NCCLS) interpretive category and quality control (QC) tables in the early 1980s, although polymyxin B has continued to be widely used in topical over-the-counter, triple-antibiotic ointment (neomycin-polymyxin B-bacitracin) preparations (6, 7). Recently the occurrence of multidrug-resistant Pseudomonas aeruginosa and Acinetobacter spp. in several nations in epidemic proportions has necessitated the reconsideration of polymyxin therapies (2,4,5,12,14) with the subsequent need for accurate susceptibility testing by reference and standardized methods (3, 9, 10). Contemporary updates on polymyxin pharmacokinetics and pharmacodynamics have also been published (5). This report describes results from a multilaboratory trial designed to establish colistin and polymyxin B QC ranges for disk diffusion and the broth microdilution MIC method (9, 10), using a study design published in the NCCLS M23A2 document (11).
