Influence of Five Antianaerobic Antibiotics on Endotoxin Liberation by Gram-Negative Anaerobes

Endotoxin, a lipopolysaccharide (LPS), has for many years been recognized as a key effector molecule in the pathogenesis of gram-negative sepsis and septic shock. Seven strains of the Bacteroides fragilis group were studied for their ability to liberate endotoxin upon exposure to five anti-anaerobic antibiotics, trovafloxacin, cefoxitin, imipenem, meropenem and piperacillin/tazobactam, in an in-vitro experiment. The minimum inhibitory concentrations (MICs) of the antibiotics were determined by using the broth macrodilution technique. Thereafter, endotoxin liberation was detected in the filtered broth cultures of the anaerobic bacteria by the limulus amebocyte lysate (LAL) assay after exposing the organisms to four different concentrations of the antibiotics in supplemented Brucella broth. Aliquots of the broth cultures were also taken at intervals of 0, 6, 24 and 48 h for viable counts. All seven gram-negative anaerobic bacteria investigated liberated induced cell-free endotoxin in filtered broth culture many times higher than the control. There was noticeable variation in the propensity of some antibiotics to induce endotoxin liberation. At four times the MICs, cefoxitin and piperacillin/tazobactam induced negligible quantities of endotoxin after 48 h exposure, whereas the others induced high levels of endotoxin release. After exposure to all concentrations for 48 h, endotoxin activity in the test system was many times higher with the Bacteroides fragilis sensu stricto than with the rest of the species in the Bacteroides group. To varying degrees, all five antibiotics had the capacity to induce endotoxin liberation by gram-negative anaerobic bacteria. This differential endotoxin release by the B. fragilis group may, in part, explain why B. fragilis sensu stricto, more than the other Bacteroides spp., is usually associated with clinical infections and higher morbidity.