IN-VITRO SYNERGY TESTING OF MACROLIDE QUINOLONE COMBINATIONS AGAINST 41 CLINICAL ISOLATES OF LEGIONELLA

Combination antimicrobial therapy against Legionella species has not b een well studied, Several quinolones have activity against Legionella strains, which prompted this in vitro search fora synergistic combinat ion with the macrolides. By a checkerboard assay, erythromycin, clarit hromycin, and azithromycin, each in combination with ciprofloxacin and levofloxacin, were tested for synergy against 46 isolates of Legionel la. The agar dilution method was employed using buffered charcoal-yeas t extract media, A final inoculum of 10(4) CFU per spot was prepared f rom 24-h growth of each isolate. Plates were incubated at 35 degrees C for 48 h, Synergy, partial synergy, additive effect, or indifference was observed for all combinations of antibiotics tested, There was no antagonism observed. Synergy occurred to a significantly greater exten t for the clarithromycin-levofloxacin (P = 0.0001) and azithromycin-le vofloxacin (P = 0.003) combinations versus erythromycin-levofloxacin. The azithromycin-ciprofloxacin combination demonstrated significantly greater synergy than did either erythromycin-ciprofloxacin (P = 0.003) or clarithromycin-ciprofloxacin (P = 0.001), The newer macrolides cla rithromycin and azithromycin may be more active in combination with a fluoroquinolone than is erythromycin.