Biological Inactivation by Faeces of Antimicrobial Drugs Applicable in Selective Decontamination of the Digestive Tract

The in-vitro inactivation of seven antimicrobial drugs by human faecal substance has been investigated. Nalidixic acid, colistin, neomycin, tobramycin, temocillin, trimethoprim, and aztreonam were separately mixed in graded concentrations with faecal suspensions prepared from faeces of eight healthy volunteers. Each was mixed separately with a sample of each of the eight suspensions. All seven antimicrobial drugs appeared to be rapidly biologically inactivated by intestinal contents in a dose-dependent fashion, although not all to the same extent. The results may explain why with some unabsorbed or minimally absorbed drugs higher oral doses are required to achieve eradication of Gram-negative bacilli from the digestive tract than others, when selective decontamination (SD) is attempted in clinical practice.