Curing antibiotic resistance in Bacteroides species by aminoacridines and ethidium bromide

Three clinical isolates of Bacteroides fragilis and one faecal isolate of B. thetaiotaomicron resistant to one or more of the antibiotics chloramphenicol, erythromycin, clindamycin and tetracycline, were cured of their resistance markers by treatment with subinhibitory levels (16 micrograms/ml) of acriflavine, acridine orange and ethidium bromide. Chloramphenicol, erythromycin and clindamycin resistance markers were cured en bloc after exposure to the agents for 24 hr but elimination to tetracycline resistance markers required longer incubation (17-21 days) with the reagents. The minimum inhibitory concentrations of the antibiotics for the Bacteroides strains before and after elimination are compared. Elimination of these antibiotic resistances indicates that the resistance markers are located as extrachromosomal plasmids (R-factors). The emergence of plasmid-mediated antibiotic resistances may compromise treatment of bacteroides infections and may also provide a reservoir of antibiotic resistance in the intestinal flora.