Influence of neomycin and metronidazole on colonic microflora of volunteers

The influence on colonic microflora of neomycin and metronidazole alone, or in combination, was studied in volunteers taking a normal diet. Bacterial counts on daily faecal samples revealed that metronidazole had no influence on anaerobic faecal bacteria. Neomycin on the other hand, was effective against sensitive aerobes. However, the combination of neomycin and metronidazole profoundly reduced both aerobic and bacteria. Assay of faecal antibiotic concentrations showed that neomycin achieved high intraluminal levels while metronidazole was undetectable or present at low concentrations.