Comparative activities of aztreonam and cefotaxime against Escherichia coli and Bacteroides spp. in pure and mixed cultures.

The activities of aztreonam and cefotaxime against one Escherichia coli strain and three Bacteroides strains have been studied by using pure and mixed cultures of aerobic and anaerobic bacteria. A fixed concentration (8 micrograms/ml) of cefotaxime or aztreonam killed E. coli that was exposed in pure culture, but the pressure of Bacteroides spp. in the same system deteriorated cefotaxime, allowing the full growth of E. coli after 24 h. On the other hand, the presence of Bacteroides spp. produced less deterioration of aztreonam, which reduced the initial E. coli inoculum. This observation could be useful in clinical practice when treating mixed aerobic and anaerobic infections.