Determination of the activity on Legionella of eight macrolides and related agents by comparative testing on three media.

It has proved difficult to develop a standard method for the determination of minimum inhibitory concentrations (MICs) of antibiotics against the Legionellaceae. A major obstacle has been the inactivation of certain antibiotics by components of Charcoal Yeast Extract agar, especially charcoal. To determine the MICs of eight macrolides and related agents (erythromycin, spiramycin, oleandomycin, josamycin, midecamycin, lincomycin, clindamycin and pristinamycin) for 36 strains of Legionella, we used two charcoal-free media: Buffered Yeast Extract agar and Buffered Antibiotic Medium no. 1 (BAM1), the latter having been developed in our own laboratory. The inhibitory effect of charcoal was most marked on josamycin and pristinamycin. This effect was absent on the charcoal-free media, which both, however, inhibited spiramycin. BAM1 agar seemed the better of the two charcoal-free media as it gave more consistent growth. The most active agents were josamycin (0.06-0.25 mg/l), pristinamycin (0.06-0.5 mg/l) and erythromycin (0.12-0.5 mg/l). Midecamycin (0.12-1 mg/l) and spiramycin (1-5 mg/l) also had useful activity.