The in vitro inactivation of thirteen beta-lactam antibiotics by other mechanisms than adsorption to faecal substance.

We have investigated the antibiotic inactivating capacity of intestinal contents in vitro in faeces. In the presently reported study the influence of beta-lactamase catalyzed hydrolysis on the antimicrobial activity of 13 commonly used beta-lactam antibiotics was investigated, while the influence of non-specific adsorption of antibiotics to faecal compounds was also taken into account. The following antibiotics were tested: benzylpenicillin, amoxicillin, amoxicillin/clavulanate, cloxacillin, piperacillin, temocillin, cefuroxime, cefamandole, cephradine, cefotaxime, ceftazidime, aztreonam and imipenem. Faecal samples were obtained from 30 healthy volunteers. Six different concentrations of each antibiotic were added to 1 g of faeces. After 24 h of incubation at 37 degrees C the remaining amount of active antibiotic was determined by means of a "growth inhibition assay". The contribution to the test results of non-specific adsorption to macromolecules was calculated by means of a model and the inactivation data were subsequently corrected. The amount of antibiotic non-specifically bound to faecal macromolecules varied from 0% to 80% of the amount of antibiotic initially added to the faeces. A considerable difference was found in the degree of inactivation of several antibiotics. However, in contrast to earlier investigations, the results of this study show that in a normal population the influence of beta-lactamase catalyzed hydrolysis on the activity of beta-lactam antibiotics is apparently very small when compared to the influence of non-specific adsorption of beta-lactam antibiotics to faecal compounds.