BACTERIA LACKING A MULTIDRUG PUMP - A SENSITIVE TOOL FOR DRUG DISCOVERY

Microorganisms express multidrug-resistance pumps (MDRs) that can conf ound antibiotic discovery. We propose the use of mutants deficient in MDRs to overcome this-problem. Sensitivity to quinolones and to amphip athic cations (norfloxacin, benzalkonium chloride, cetrimide, pentamid ine, etc.) was increased 5- to 30-fold in a Staphylococcus aureus muta nt with a disrupted chromosomal copy of the NorA MPR. NorA was require d both for increased sensitivity to drugs in the presence of an MDR in hibitor and for increased rate of cation efflux. This requirement sugg ests that NorA is the major MDR protecting S. aureus from the antimicr obials studied. A 15- to 60-fold increase in sensitivity to antimicrob ials also was observed in wild-type cells at an alkaline pH that favor s accumulation of cations and weak bases. This effect was synergistic with a norA mutation, resulting in an increase up to 1,000-fold in sen sitivity to antimicrobials. The usefulness of applying MDR mutants for natural product screening was demonstrated further by increased sensi tivity of the norA(-) strain to plant alkaloid antimicrobials, which m ight be natural MDR substrates.