Uso del sistema E-test per lo studio di combinazioni antibiotiche verso batteri Gram-negativi multiresistenti in Fibrosi Cistica

Objectives: Cystic Fibrosis patients are prone to infection by Gram-negative bacteria, such as Pseudomonas aeruginosa and Burkholderia cepacia, which become very resistant with recurrent antibiotic treatments.The purpose of this study was to evaluate the susceptibility patterns of 12 isolates of Burkholderia cepacia and 8 isolates of Pseudomonas aeruginosa, isolated from Cystic Fibrosis patients to five individual antibiotics (ceftazidime, ciprofloxacin, piperacillin/tazobactam, levofloxacin and trimethoprim-sulfamethoxazole) and to four antibiotic combinations (ceftazidime associated with one of the other antibiotics). Methods: Susceptibility tests were carried out using an agar diffusion method, the E-test (AB Biodisk, Solna, Sweden). Results: Strains were selected because of their resistance to individual antimicrobial agents, tested with automated system (Phoenix, BD), which ranged from 41.6% for ceftazidime to 83.3% for ciprofloxacin for Burkholderia cepacia and from 25% for ceftazidime to 100% for trimethoprim-sulfamethoxazole for Pseudomonas aeruginosa. By using E-test,we were able to demonstrate synergy against 2 strains of Pseudomonas aeruginosa (25%) with ceftazidime- piperacillin/tazobactam. No synergy was detected against all strains of Burkholderia cepacia. Conclusions:These results suggest that the E-test offers a simple, labour-efficient and accurate method for MIC determination on agar medium and the susceptibility to antibiotic combinations greatly improves the guide to antibiotic therapy for infections to Gram-negative bacteria in Cystic Fibrosis patients.