Susceptibility testing of the polymyxins: where are we now?

Antimicrobial susceptibility testing for the polymyxins-colistin and polymyxin B-is fraught with technical challenges. Key among these is the propensity of the polymyxins to adsorb to polystyrene, a material often used for in vitro minimum inhibitory concentration testing devices. This effect may be mitigated by the addition of a surfactant such as polysorbate 80; however, concern exists that polysorbate 80 may act synergistically with the polymyxins and artificially lower minimum inhibitory concentrations. Furthermore, the polymyxins diffuse poorly through agar, compromising the performance of both disk diffusion and Etest methods. Very few peer-reviewed studies have investigated in vitro susceptibility test methods for the polymyxins, and it is clear that an in vitro test that reliably predicts the activity of the polymyxins in vivo has yet to be defined. This review describes the methods available and challenges associated with susceptibility testing of colistin and polymyxin B and discusses the current breakpoints for both agents.