Previous work has shown that certain cooling tower amoebae and ciliated protozoa are resistant to several cooling tower biocides, even at the manufacturer?s recommended dosages. For the present study, an Acunthumoeba species was isolated from a cooling tower in Australia. Suspensions of the trophozoites (feeding stages) were exposed to isothiazolones. Cysts were tested separately. The minimum lethal concentration (MLC) for trophozoites was between 31-62 ppm of the biocide product, which is slightly less than the MLC for an amoebae species from the United States; and cyst forms were twofold more resistant than those of the U.S. species, with a MLC of 62,500 ppm. A ciliate and an amoeba species were also exposed to bromochlorodimethylhydantoin. The MLC for the ciliate species was 1 ppm of the biocide product, and the MLC was 30- 40 ppm for the amoeba trophozoites. Since amoebae can expel vesicles containing live Legionella, experiments were conducted to determine whether exposure of Acunthamoebu polyphugu to biocides influenced release of such potentially infectious particles. Vesicle release was not inhibited by any of the three biocides: quatemary ammonium compounds (QACs), isothiazolones, and a thiocarbamate compound. These results suggest that amoebae from various sources are resistant to recommended levels of biocides, and the amoebae may continue to release potentially infectious vesicles in the presence of biocides.
Most biocide studies detail efficacy against planktonic bacteria, especially Legionella pneumophilu.?J Protozoa may harbor and protect Legionella from cooling tower biocides. Cooling tower protozoa have been shown to be more resistant to biocides than species not originally isolated from cooling towers.4 In addition, Srikanth and Berk4 demonstrated that at certain biocide concentrations, amoeba population growth is actually stimulated; and amoebae not originally isolated from cooling towers can adapt to become as resistant as cooling tower isolates in less than 3 days.5 To examine further the response of protozoa to cooling tower biocides, one species of Acunthumoeba was isolated from a cooling tower on the campus of Sydney University in Australia and exposed to one of the biocides used in the tower. In addition, a ciliated protozoan species and Acunthumoebu hatchet2 were isolated from a cooling tower on the campus of Tennessee Technological University and exposed to bromochlorodimethylhydantoin (BCDMH), which was not used in the tower. Results of these tests were compared with biocide tests previously reported for other cooling tower protozoa isolated in the United States.
Berk et aL6 have also demonstrated that amoebae feeding on Legionella produce membrane-bound vesicles containing live Legionella, and that the vesicles are small enough to fit into the lower alveolar spaces of the lungs, where Legionnaire?s disease begins. Since biocides may affect the rate at which such vesicles may be released from the amoebae, three biocides were tested to determine whether they increased or inhibited the rate of release of respirable sized vesicles from an amoeba species. This research may lead to new disinfection strategies for cooling towers.
