Interplay between efflux pumps may provide either additive or multiplicative effects on drug resistance

The effects of simultaneous expression of several efflux pumps on antibioti c resistance were investigated in Escherichia coli and Pseudomonas aerugino sa. Several combinations of efflux pumps have been studied: (i) simultaneou s expression of a single-component efflux pump, which exports antibiotics i nto the periplasm, in combination with a multicomponent efflux pump that ac complishes efflux directly into the external medium; (ii) simultaneous expr ession of two single-component pumps; and (iii) simultaneous expression of two multicomponent pumps. It was found that when efflux pumps of different structural types were combined in the same cell (the first case), the obser ved antibiotic resistance was much higher than that conferred by each of th e pumps expressed singly. Simultaneous expression of pairs of single-compon ent or multicomponent efflux pumps (the second and third cases) did not pro duce strong increases in antibiotic resistance.