All the polymyxins have a similar antibacterial spectrum, although there are slight quantitative differences in their activity in vitro. They are inactive against Gram-positive organisms, but nearly all enterobacteria, except Proteus spp., Burkholderia cepacia and Ser. marcescens, are highly susceptible. The MIC of polymyxin B or colistin sulfate for Esch. coli and Klebsiella spp. is 0.01–1 mg/L; the corresponding concentration for Ps. aeruginosa is 0.03–4 mg/L. Bacteroides fragilis is resistant, but other Bacteroides spp. and fusobacteria are susceptible. Resistance of V. cholerae eltor to polymyxin B distinguishes it from the classic vibrio.
The sulfomethyl derivatives are generally 4–8 times less active than the sulfates, but their activity is difficult to measure precisely since on incubation they spontaneously decay to the parent compound, with a corresponding progressive increase in antibacterial activity.
Binding of polymyxins to the bacterial cell membrane can increase permeability to hydrophilic compounds, including sulfonamides and trimethoprim, producing significant synergy. Synergy with ciprofloxacin is also described. Calcium ions exert a strong pH-dependent competition for membrane binding sites, and the presence of calcium and magnesium ions in certain culture media adversely affects the bactericidal activity, notably against Ps. aeruginosa.
There is complete cross-resistance between the polymyxins, but stable acquired resistance in normally susceptible species is very rare. Adaptive resistance, probably due to changes in cell-wall permeability, is readily achieved by passage of a variety of enterobacteria in the presence of the agents in vitro.
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