Binding to faeces and influence on human anaerobes of antimicrobial agents used for selective decontamination

The degree of binding of ampicillin, cephradine, co-trimoxazole, gentamicin, nalidixic acid, neomycin, polymyxin B and tobramycin by faecal substance as well as the influence of these antibiotics on human intestinal obligate anaerobes was investigated. In contrast to ampicillin, cephradine, co-trimoxazole and nalidixic acid, the nonabsorbable antibiotics polymyxin B and neomycin were bound to a considerable degree by human faeces. The binding of tobramycin and gentamicin to the solid part of faeces was less effective. The inhibitory effect of co-trimoxazole, gentamicin, nalidixic acid, neomycin, polymyxin B and tobramycin on the human obligate anaerobes was weak as compared with ampicillin and cephradine. Drugs which effectively eliminate Enterobacteriaceae from the gastrointestinal tract and which have a moderate effect on obligate anaerobes, like polymyxin B, are particularly suitable for selective decontamination of the gastrointestinal tract. The strong inactivating binding of aminoglycosides and polymyxin B to faeces accounts for the relatively high oral dose needed for a suitable faecal concentration.