Effect of selective media on recovery of obligately anaerobic gram-negative rods from human faeces

Total numbers and gross composition of the anaerobic human faecal flora were compared using non-selective and selective media. Combinations of selective agents to suppress the gram-negative part of the flora such as vancomycin and neomycin, vancomycin and kanamycin, or kanamycin and bile were found to reduce total numbers of recovered obligately anaerobic gram-negattive rods by 50–75%. With reference to experiments with penicillin as selective agent, underlying mechanisms for this phenomenon are discussed. It is concluded that selective media should not be used for quantitative enumeration of anaerobic gram-negative rods from the faecal flora.