In vitro activities of gemifloxacin versus five quinolones and two macrolides against 271 Spanish isolates of Legionella pneumophila: Influence of charcoal on susceptibility test results

The MICs at which 90% of isolates are inhibited for gemifloxacin, trovaflox acin, and grepafloxacin were low (less than or equal to 0.01 mu g/ml) for 2 71 Legionella isolates when they were determined by the broth microdilution method but increased (greater than or equal to 6 dilutions) when they were determined by the agar dilution method. This was due to the charcoal in th e agar dilution medium, as shown by the progressive decrease in the MICs wh en the charcoal concentrations decreased. As free drug is the active fracti on, charcoal binding should be considered.