Objectives: To comparatively evaluate the antimicrobial activities of colistin and polymyxin B with those of other antimicrobials against a worldwide collection of 40 625 Gram-negative bacilli.
Methods: Antimicrobial susceptibility testing was performed and interpreted using the CLSI broth microdilution method except for colistin against Enterobacteriaceae.
Results: The polymyxins showed potent in vitro activities (MIC₉₀, ≤ 0.5-1 mg/L) against this large collection of clinical isolates, with very low resistance rates (< 0.1%-1.5%). Resistance to the polymyxins remained stable among organisms tested except for Klebsiella spp. isolates collected from the Asia-Pacific and Latin American regions, where a trend towards greater resistance was observed (P ≤ 0.05). In addition, an important reduction in imipenem susceptibility among Acinetobacter spp. and Klebsiella spp. was demonstrated in most geographical regions.
Conclusions: Although the polymyxins showed excellent in vitro activity against the vast majority of Gram-negative bacilli evaluated, a trend to greater resistance was observed in the Asia-Pacific and Latin American regions. Therefore, the clinical use of polymyxins must be cautious and surveillance monitored.