Identification of Efflux Pump-mediated Multidrug-resistant Bacteria by the Ethidium Bromide-agar Cartwheel Method

Background/aim: Bacterial multidrug resistance may be mediated by the overexpression of efflux pumps. Conventional evaluation of efflux activity using efflux pump substrates, such as ethidium bromide, requires specialised instrumentation. The agar-based method, previously reported, has been modified to evaluate as many as twelve bacterial strains and has been termed the ethidium bromide-agar cartwheel method.
Materials and methods: Agar plates containing different concentrations of ethidium bromide were swabbed with bacterial cultures. The cell efflux capacity increased with increasing ethidium bromide concentration, which produced fluorescence of the bacterial mass.
Results: The method was shown to be useful for the detection of efflux activity among multidrug-resistant Gram-negative and Gram-positive clinical isolates, as confirmed by the determination of minimum inhibitory concentration for several antibiotics in the presence of known efflux pump inhibitors.
Conclusion: This method may be adapted to the clinical laboratory for the presumptive identification of multidrug-resistant isolates that overexpress efflux pump systems.