Daptomycin exerts rapid bactericidal activity against Bacillus anthracis without disrupting membrane integrity

Abstract

To examine whether the novel cyclic lipopeptide antibiotic daptomycin could be used to treat anthrax and to study the mechanisms underlying its bactericidal action against Bacillus anthracis. Spore-forming B anthracis AP422 was tested. MIC values of antibiotics were determined. Cell membrane potential was measured using flow cytometric assays with membrane potential-sensitive fluorescent dyes. Cell membrane integrity was detected using To-Pro-3 iodide staining and transmission electron microscopy. K(+) efflux and Na(+) influx were measured using the fluorescent probes PBFI and SBFI-AM, respectively. Daptomycin exhibited rapid bactericidal activity against vegetative B anthracis with a MIC value of 0.78 μg/mL, which was comparable to those of ciprofloxacin and penicillin G. Furthermore, daptomycin prevented the germinated spores from growing into vegetative bacteria. Daptomycin concentration-dependently dissipated the membrane potential of B anthracis and caused K(+) efflux and Na(+) influx without disrupting membrane integrity. In contrast, both ciprofloxacin and penicillin G did not change the membrane potential of vegetative bacteria or spores. Penicillin G disrupted membrane integrity of B anthracis, whereas ciprofloxacin had no such effect. Daptomycin exerts rapid bactericidal action against B anthracis via reducing membrane potential without disrupting membrane integrity. This antibiotic can be used as an alternate therapy for B anthracis infections.