Abstract
Since Legionella species, the causative organisms of Legionnaires' disease, are known to multiply in amoebal cells and survive disinfection treatments, it is necessary for biocides in order to control Legionella species to also be lethal against the host-amoeba. 4, 4'-(1, 6-Hexamethylenedithio) bis (1-octylpyridimium iodide) (4DTBP-6, 8) and N, N'-1, 6-hexamethylenebis (1-decyl-4-carbamoylpyridinium bromide) (D-38) are the novel bis-quaternary ammonium compounds (bis-QACs), which show a wide and effective antimicrobial activity against both bacteria and fungi and exhibit low toxicity, compared with N-dodecylpyridinium iodide (P-12), a mono-QAC with a typical structure, and 2-bromo-2-nitropropane-1, 3-diol (Bronopol), which has been used as a major organic biocide against Legionella species in cooling towers. The biocidal effect of these bis-QACs was investigated against Legionella species, L. pneumophila AQ 998, L. pneumophila GIFU 9134 and L. bozemanii GIFU 9140, and Acanthamoeba 96. The bis-QACs showed higher bactericidal activity against Legionella species than the comparative biocides. Amoebicidal activity was estimated by the direct cellcounting method and ATP assay based on luciferase-catalyzed reaction to assess the biocidal ability against both trophozoites and cysts. The bis-QACs, 4DTBP-6, 8 and D-38, had amoebicidal activity at the concentration of 10mg/l, while the mono-QAC, P-12, had cysticidal activity above the concentration of 100mg/l, and Bronopol had no amoebicidal activity but had growth-inhibitory effect below the concentration of 100mg/l.
