Antimicrobial susceptibility and synergistic activity of meropenem against gram-negative non-fermentative bacteria isolated from cystic fibrosis patients

The objective of the present study was to evaluate the activity of meropenem (a beta-lactam carbapenem with good bactericidal potency and a very wide spectrum of activity) and of ticarcillin, ceftazidime ciprofloxacin, tobramycin, cefepime, which are the most commonly used antimicrobial agents for treatment of pulmonary infections associated with CF. The effect of these antibiotics was tested on 27 multiresistant strains isolated from 24 CF patients during 2000 and 2001. Furthermore, the in vitro synergistic effect of meropenem in association with the other antibiotics was evaluated. Ciprofloxacin, ticarcillin, meropenem and ceftazidime had the most activity and inhibited 66%, 37%, 36% and 33% of strains respectively. The addition of a second antibiotic to meropenem resulted in a synergistic effect on 5 (18.5%) isolates; on average 2.8 synergistic combinations where determined per strain. Of these 27 isolates, antagonism was observed in 3 (11%) strains (1 antagonistic combination per strain). Our study suggests that selecting effective double antibiotic therapy cannot be made empirically for CF patients infected with Gram-negative multiresistant bacilli. Therefore in vitro methods for testing double antibiotic combinations are mandatory.