Bactericidal effects of amoxycillin/clavulanic acid against Legionella pneumophila

The antibacterial activities of amoxycillin, clavulanic acid and the combination of both agents against Legionella spp. were compared in serial-dilution tests, time-kill curve studies and in turbidimetric studies in a continuous recording biophotometer. Both beta-lactam compounds showed high levels of activity against L. pneumophila in serial dilution tests, clavulanic acid (MIC 0.1-0.25 mg/l) being two-fold more active than amoxycillin. The combination of amoxycillin and clavulanic acid was more effective than either of the constituents and was two to four times more active than erythromycin. Clavulanic acid was shown to reduce the extent of inactivation of amoxycillin by L. pneumophila and amoxycillin/clavulanic acid was rapidly bactericidal against the organism in tests in which amoxycillin was ineffective. Microscopical examination showed distinctive morphological effects produced by amoxycillin and by clavulanic acid and synergy between the compounds could be attributed to beta-lactamase inhibition, or by binding to different penicillin binding proteins, or both. These results warrant further studies in vitro and in vivo to elaborate the bactericidal effects demonstrated by amoxycillin and clavulanic acid against Legionella spp.