In vitro activity of daptomycin in combination with low-dose colistin against a diverse collection of Gram-negative bacterial pathogens

The activity of colistin in combination with daptomycin was assessed using 30 Gram-negative type strains and multidrug-resistant isolates with defined mechanisms of resistance. Daptomycin minimum inhibitory concentrations (MICs) were determined with and without sub-inhibitory concentrations of colistin. The activity of daptomycin was not affected with respect to Escherichia coli, Klebsiella pneumoniae or Pseudomonas aeruginosa. For colistin-susceptible Acinetobacter baumannii, sensitisation factors ranged from 8 to 128 (median 32), with the daptomycin MIC being reduced to the Clinical and Laboratory Standards Institute (CLSI) enterococci susceptibility breakpoint of 4 mu g/ml for the ATCC 19606 type strain. A combination of daptomycin and colistin may be useful for the treatment of A. baumannii but not infections due to other Gram-negative species.