Binding to and antibacterial effect of ampicillin, neomycin and polymyxin B on human faeces

Neomycin and polymyxin B, used during selective decontamination of the gastrointestinal tract, were studied for their effect on the human faecal flora in vitro. The selective effect was found to be associated with a relative insusceptibility of the obligate anaerobic flora as compared with the facultatively anaerobic Gram-negative rods (Escherichia coli). Both neomycin and polymyxin B were bound by human faeces, in contrast to ampicillin. The results may explain the selective effect of neomycin and polymyxin B on the human flora in vivo.