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Postantibiotic Effect of Meropenem and Ciprofloxacin in the Presence of 5-Fluorouracil

Background: The postantibiotic effect (PAE) of meropenem and ciprofloxacin was studied in the presence of the antineoplastic agent 5-fluorouracil (5-FU). The purpose of the study was to investigate whether the PAEs of the combinations differed from the PAEs of the antibiotics alone. Methods: The PAEs of the combinations of 5-FU plus meropenem or ciprofloxacin were determined with viable counts against four reference strains of Staphylococcus aureus, Staphylococcus epidermidis, Pseudomonas aeruginosa, Escherichia coli and two clinical isolates of S. epidermidis. The results were compared with the PAEs of the antibiotics drugs and 5-FU alone. The gram-positive strains were tested for slime production, both alone and in the presence of 5-FU. Results: Against two of the three tested strains of S. epidermidis, the combination of ciprofloxacin and 5-FU gave a synergistic prolongation of the PAE in comparison with the PAEs induced by the drugs alone. The combinations showed indifference against the other bacteria. The combination of meropenem and 5-FU had a synergistic PAE against one of the three tested strains of S. epidermidis and an additive effect against E. coli but showed indifference against the rest of the strains. Conclusions: The presence of 5-FU did not influence the PAEs of the antibiotics against most of the tested strains, but caused a synergistic prolongation of the PAEs induced by ciprofloxacin and meropenem against some of the tested strains of S. epidermidis. 5-FU inhibited slime production in the same S. epidermidis strains, which might have contributed to the longer PAE.