Citywide emergence of Pseudomonas aeruginosa strains with reduced susceptibility to polymyxin B

Objectives: To determine the prevalence of Pseudomonas aeruginosa isolates with reduced suscepti-bility to polymyxin B, and to assess the in vitro activity of antibiotic combinations. Methods: All unique patient isolates of P. aeruginosa were collected from 11 Brooklyn, NY hospitals during a three month period in 2003. Isolates with reduced susceptibility to polymyxin B (MIC> 2 mg/L) underwent ribotyping. The activity of polymyxin B combined with rifampicin, azithromycin and/or imipenem was tested by the chequerboard and time–kill methods against a subset of isolates. Results: Of 527 isolates, only 61 % were susceptible to imipenem. Twenty-five isolates (5%), from 8/11 hospitals, had reduced susceptibility to polymyxin B (MICs 4–8 mg/L), compared with 0/691 isolates collected in 2001. Ten of 25 were resistant to multiple other antibiotic classes. Ribotyping of the iso-lates revealed 19 unique types. Chequerboard testing of the 10 multiresistant isolates demonstrated synergy for the combinations of polymyxin B with azithromycin, imipenem and rifampicin in 6, 2, and 1 isolates, respectively. Time–kill studies revealed bactericidal activity for the following antibiotics when combined with polymyxin B: imipenem plus rifampicin against all 10 isolates, rifampicin in 9/10 iso-lates, imipenem in 8/10 isolates and azithromycin in 4/10 isolates. MICs of bacteria surviving incubation in polymyxin B alone rose for 4/9 isolates (MIC range 12–48 mg/L). Conclusions: P. aeruginosa with reduced susceptibility to polymyxin B have emerged in multiple strains in Brooklyn, NY. Combinations of polymyxin B with rifampicin and/or imipenem are bacteri-cidal. The clinical utility of these combinations remains to be determined